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I noticed two flaws in this explanations. Arvin uses the explanation of light as he just freely chooses at his convenience to explain the speed of light and its limit. Once he says that light is an electromagnetic wave and once that it is photons. When Arvin explains the fields that are the root of forces he actually uses two electrons nagatively charged. And sure enough in such situation the two containing mass and charge objects will cause forces to act. Any mass object can be made to exert a force. But suddenly Arvin switches to light being an electomagnetic wave and then again to photons as he pleases. So, if Arvin chooses light to be photons and he further explains that photons are massless then, how can something massless exert any force? Photons have no charge either. So, again, how can photons create a force? But Arvin in a snide way swithes to waves. Arvin! Make up your mind! You can't have massless and chargeless photons exert a force. Another point is special relativity; there is one big difference in Alice and Bob situations. Bob flying away with the 0.87c experiences ACCELERATION whereas Alice does not. So, by twisting the time axle is wrong for Alice because she did not experience acceleration. I do not agree with Arvin; Alice or Bob can never receive a message before it was sent. It is just the manipulations with the graph which does work one way when Bob is accelerating but does not work for Alice because she is at stand still relative to Bob. Arvin uses a bit of sophistry here and a slight trick in mathematics. More on it here in a related correspondent: It is all fine and dandy. As it was stated here: light carries energy. Well, not so fast. If light is a wave then we can consider that as energy. But if we consider light as photons then we have a problem. Photons are massless. Therefore, how can a massless photon knock a free electron from a metal? I think this rather now has something to do with the observer who ''controls'' if he/she wants the light to be a particle or a wave. And therefore, also we come up to the paradox of Schrodinger's cat. Something massless cannot exert force and therfore cannot have energy and cannot do work. Massless photons cannot exert force either. How do they create photoelectric effect? That just does not make sense. Unless we have an active observer/experiment conducting person which leads us to the duality of light and the delayed choice experiment in Alain Aspect method. E=hf only pertains to frequency of light and the Einstein's KE for electrons is dealing with, well, electrons which have mass. So, the Einstein's equations using electrons with mass cannot be applied to kinetic energy of photons which have no mass. Something that has no mass cannot have kinetic nor potential energies.
Thats the most unscientist explaination i can imagine. And more then totaly wrong. That are FAKENEWS!!!! And also stupid bullshit. Each photon of that LASER still moves with the speed of light. Light is not a beam. Dont spread such lies. Dont speak when you do not know what you tell. Liar who put there lies in public should get punished for making mankind into idiots who do not even care to give criminals control about nuklearweapons. Sry, but dont thank someone who just fools you.
@@ericephemetherson3964 Why should photon not carry pure energy without being a mass-body? I'd say a foton IS energy and nothing else, else it wouldn't travel with the speed of light. E=mc² says in which relation mass can be converted into energy and the other way round. So lots of energy are needed to create mass: 2,176 x 10^59 times.
Man, I would say the typical "I wish I had teachers like Arvin when I was in school", but nah - I'm glad Arvin's precisely where he is right now. He reaches way more people this way, and his miraculous ability to put complex topics into plain English as well as these mad useful graphics couldn't exist so completely in any other medium. The fact that anyone can access it, for free, at any time, from any place, and pause/fast-forward/rewind or watch at 2x speed or 1/2 speed - this - everything right here - it's just perfect. If you're reading this, thank you Arvin (:
The problem here lies in time dilation. If the message is sent at 2 seconds and gets to the target at 2 seconds, that's true instantaneous.Same if the response is sent at 3 seconds and gets back at 3 seconds. Causality and paradoxes occur because what's described here is a message traveling back in time itself in the first place. But that's an issue of perspective with time dilation taken into account. If the message travels instantaneously between two points that are not experiencing time dilation, then there is no paradox. So the answer to this paradox is that even though the transmission to bob was instantaneous and bob appears to have received it in the past according to his clock (but only his clock) the response still comes back instantaneously at 4 seconds and is received at 4 seconds by Alice. Remember, we're not talking about actual time travel, just perceived time. The universe itself isn't changing, only the way the people involved experience it.
Yeah, Alice is sending the message when the ship has traveled 4 sec at 0.87C. Thus Bob cannot receive the message before the ship he is on has actually traveled the distance from Alice perspective. Bobs perspective of Alice might say one thing, but for actual physics? Bob is on a ship that accelerated to 0.87C. No idea if this view on it actually holds water or not, but the explanation in the video is lacking.
@@alichi101 I also don't understand why would this be a message sent back in time. We (but also Bob and Alice) know who accelerated and whose time is slower. Why is it a problem for the universe to know this? :) What if Bob slows down to normal speed for a microsecond (after he travelled 2 sec his time), will he be able to correctly observe that his time was the slower one? Will he see that 2 seconds passed for him and 4 for Alice? I assume yes and in this case I don't see the paradox.
@@csmcs2 My understanding is that theres is no such thing as knowing which person "really" accelerated. As mentioned, the laws of physics are the same everywhere in the universe, and both of the two of them would perceive their physical reality to be the true one.
You keep using the word instantaneous as though two events could be instantaneous in different reference frames. The reality is that instantaneity itself is relative even after accounting for time dilation. Two events that occur at different points in space cannot be said to have happened at the same time for all references frames. Instead, different observers traveling at different velocities will disagree as to how much time has elapsed between the two events. And if there is at least one reference frame in which both events happened at the same time then different observers will disagree as to which event happened first. Your understanding of the problem wrongly assumes that you can simply give a timestamp as “2 seconds” without specifying the reference frame.
11:40 I don't understand why the case isn't: Alice sends a signal at 2 seconds, from Bob's perspective it arrives at 2 seconds but it _appears_ as though Alice doesn't send it until 4 seconds relative to Bob, and when Bob sends a reply at 4 seconds, from Alice's perspective it arrives at 4 seconds but it _appears_ as though Bob doesn't send it until 8 seconds relative to Alice.
I never intuitively understood what light was or how it propagated until I saw this. I've known logically for ages, but never really 'got' it. For some reason this description really clicked with me, thank you so much for this video.
Einstein's idea of space and time not being absolute, but the speed of light being absolute is nothing short of groundbreaking. This simple yet complex idea has opened so many doors to the answers to the nature of the universe.
There is another factor to consider here... a ship within an Alcubierre style field exists in normal spacetime, and is in fact relatively stationary. There would be no time dilation between the inside of the bubble and the outside observer. The contraction and expansion of space is what moves the field at relativistic speeds, but information in and out of that field would still be constrained by C. If person A sends a message to person B who is moving at high warp speeds, then person B will either outrun the message or intercept it head on, but no matter the relative direction of travel, the message would always be received after person A sends it, because there is no time dilation involved. You could even send a message in one direction, warp past it, and then receive it yourself. This still doesn't break causality, because you cannot reply to the message before you send it, because, again, at no point is time dilated.
Any travel faster than light will break causality. Take the example given of Alice and Bob. If an Alcubierre style travel method exists, then a third person C could travel between Alice and Bob faster than light and send messages between them. You then run into the same grandfather paradox situation
@@nirmalasokan1687 Even if you travel instantly, you still cannot intercept a message before it is sent. You can tell Bob that Alice sent a message, even its contents, and you can tell Alice of Bob's reply before he receives Alice's message, but this is no different than comparing physical mail to email. You can't instantly email someone to tell them not to send a letter after they've already posted it.
@@DanGovier Any faster than light travel will appear to go back in time from a different inertial observer's point of view This video explains it better than I can th-cam.com/video/an0M-wcHw5A/w-d-xo.html
Time dilation is caused by Light moving the same speeds in all reference frames. If there's no dilation, wouldn't light then move far beyond C within the Alcubierre field? And In the video He talks about the issues with infinite speed of light
i agree there are many garbage explanations & Arvin Ash is a gem among with a few others such as ScienceClic, DrPhysicsA, DrBecky, DoS. Parth G etc & this episode is 1 of Arvin's best IMO but still I have to read a very good book: Engineering Mathematics by KA Stroud & only then will I really be able to start to learn quantum mechanics & I don't even know what to read to start to learn general relativity. Maybe eigenchris
Muck like the cosmological speed limit so to the grammatical one. You can not travel faster than, "favorite". Therefore there can be no , "most" favorite. Not even in Star Trek.
Firstly I would like to take refrence from an older video, Delayed choice quantam eraser. In the simplified experiment shown, I am taking at face value that the beam splitter were placed in such a way that there is absolutely 50% chance of the photon going downward or through the splitter(since I ain't a pro on the topic or the way actual experiment is conducted). Maybe there are some unknown variables/decoherence which is causing the photon to go through a certain path. And its just the past/present causing the future which is perceived vice versa. Now about this video, firstly I would like to say that the timeline graph example was one of the best to understand the concept as shown in graph form. But, assuming from this video if there is an AbsoluteTime which is represented in vertical axis, the whole process needs to be done from one reference point. ( I am talking about absolute time coz Earth is also moving, so is the solar system and the galaxy and so forth the universe is also expanding hence all are having some speed hence we may perceive time differently than the absolute time.) The reference point should not be changed mid process. It could be either the absolute time, Alice's time or BOB's time. So the graph shouldn't move after Bob receives message from Alice. If taken from absolute time reference, say Alice sent message at X seconds which is 4 seconds for Alice and 2 seconds for Bob, who even if replies instantaneously the absolute time would still be X which is 2 seconds for Bob and 4 seconds for Alice so how does the message travel back in time? I hope I am not mixed up in the term 'Special relativity'. But still assuming that somehow the message/information traveled back in time, doesn't it support the Idea of Determinism. The future effecting the past, A loop created which is caused by itself. I don't know if I am thinking in the right direction but still I loved the video cause it made me think all this. Thanks for the ignition of the thought process and would love to be enlightened/steered more. P.S. what I actually wrote is so different than what I thought I was going to write when I started. It is the thought process of writing which eventually made this so lengthy and the way it is. I don't know if that too is determinism. :-)
This is one of your best video, Arvin. Talking about breaking causality, what if: one entered a region of the universe where the entropy casually started decreasing time ago when there was already intelligent life formed, then has remained almost isolated from the rest of the universe, while entropy is casually still decreasing, its inhabitants are becoming younger and are losing memory from our perspective, like if time was reversed in that region. From their perspective instead WE are getting younger, since our past is their future, and this is perceived "until" ("since" from our perspective) the moment of entropy reversion. So how would it be communicating with each other? I know this is impossible because of statistics (second law of thermodynamics), but since spontaneous entropy reversion is at least theoretically possible, it must have sense and not evolve in paradoxes, and this would be interesting to analyze.
I noticed two flaws in this explanations. Arvin uses the explanation of light as he just freely chooses at his convenience to explain the speed of light and its limit. Once he says that light is an electromagnetic wave and once that it is photons. When Arvin explains the fields that are the root of forces he actually uses two electrons nagatively charged. And sure enough in such situation the two containing mass and charge objects will cause forces to act. Any mass object can be made to exert a force. But suddenly Arvin switches to light being an electomagnetic wave and then again to photons as he pleases. So, if Arvin chooses light to be photons and he further explains that photons are massless then, how can something massless exert any force? Photons have no charge either. So, again, how can photons create a force? But Arvin in a snide way swithes to waves. Arvin! Make up your mind! You can't have massless and chargeless photons exert a force. Another point is special relativity; there is one big difference in Alice and Bob situations. Bob flying away with the 0.87c experiences ACCELERATION whereas Alice does not. So, by twisting the time axle is wrong for Alice because she did not experience acceleration. I do not agree with Arvin; Alice or Bob can never receive a message before it was sent. It is just the manipulations with the graph which does work one way when Bob is accelerating but does not work for Alice because she is at stand still relative to Bob. Arvin uses a bit of sophistry here and a slight trick in mathematics. More on it here in a related correspondent: It is all fine and dandy. As it was stated here: light carries energy. Well, not so fast. If light is a wave then we can consider that as energy. But if we consider light as photons then we have a problem. Photons are massless. Therefore, how can a massless photon knock a free electron from a metal? I think this rather now has something to do with the observer who ''controls'' if he/she wants the light to be a particle or a wave. And therefore, also we come up to the paradox of Schrodinger's cat. Something massless cannot exert force and therfore cannot have energy and cannot do work. Massless photons cannot exert force either. How do they create photoelectric effect? That just does not make sense. Unless we have an active observer/experiment conducting person which leads us to the duality of light and the delayed choice experiment in Alain Aspect method. E=hf only pertains to frequency of light and the Einstein's KE for electrons is dealing with, well, electrons which have mass. So, the Einstein's equations using electrons with mass cannot be applied to kinetic energy of photons which have no mass. Something that has no mass cannot have kinetic nor potential energies.
Fantastic explanation, but now I'm more confused about time dilation. In your example about Bob and Alice and how each of them has a "normal" clock according to their own reference frame, and perceive time to be slower for the other - well then, how will their ages have changed when Bob finishes the trip? Isn't he supposed to age slower since he's traveling so fast?
Here is a "realistic" thought experiment along the lines of the moving laser dot on the moon. Since we should be seeing the summer triangle in the sky these days, I will use the star Vega, just about 26 light years distant, and easy to see with the naked eye. Now, watch it move across the sky. If you keep track of it all night, all the next day, and all the next evening until you see it in the same spot of the night sky, you realize something. With a radius of 26 light years, it has traveled a total distance (from within your reference frame) of pi x 52 light years or just about 163.3 light years in 24 hours, which works out to a speed of about 456 million miles per hour, which is just about 6.8 times the speed of light. Anyone can see Vega moving across the night sky and easily calculate this, so it's really not a thought experiment at all. So is Vega really moving that fast around the Earth ... ? lol
In your scenario Vega isn't moving (apart from its proper motion which we can ignore for the moment) its the earth that's rotating causing the observers field of view to move
so technically speaking if pistol star at 26090 light years moves within your frame of reference it would move 59.84 million times the speed of light. lmao. so its obvious, if the distance increase the speed of light in our reference frame also increases. but you should remember that since Vega is 26 light years away you are seeing the planet how it was 26 years ago and a day later you would see how the planet would look 26 years + 1 day ago so for subtract the first frame from the second frame, we are living 26 years ahead of when light left so (26 + 26 + 1) - (26 + 1) light years distance ( 1day later) - light years at present so 26 + 26 -26 + 1 - 1 = 26 light years paradox
Reminds me of the scene from an old Dr. Who is asked how the Tardis can be bigger on the inside than on the outside. He demonstrates how an object he's holding close to her looks bigger than one he put a ways away though they're the same size. The questioner said "well that's stupid" to which Dr. Who responded indignantly "that's the basis of all Timelord physics!"
The problem with the example of Bob and Alice is that in special relativity Bob's perception of time is irrelevant to the way Alice is aware of her relative time space
is it? if bob has instant communication by definition if he pinged Alice's ship he would see a live response the moment he communicated with Alice. So Alice pings Bob's ship at her second 4 and his second 2. It is precisely because Bob's perception of time has Alice moving at .87c and therefor form his perspective time is slowing down for her (and not him) then that means that if he sends an instantaneous communication back even after waiting two seconds it will go from his second 4 to her second 2 (because from his perception she is the one moving more slowly through time). Since Bob's response to Alice has arrived at her second 2, it has arrived before she sent her ping originally. If bob's frame of reference being that she is the one moving at relativistic speeds is precisely what causes the paradox in this example.
@@connorjensen9699 the problem is that it's only a perception of time, but if u look it like u are an observer in the middle of both alice and bob u will see both moving one to the left at half the speed and one to the right at half the speed, it's like the middle point from the 2 objects is moving away from alice at half the speed following bob at half the speed of bob, if bob go 1,00 the middle point is going at 0,5 in the same direction of bob, and alice is going at 0,5 in the opposite direction from our middle point of view, so there is no paradox, we can see the clock of alice and bob are still sincronized, if alice sent to bob an instant message at 4 on her clock bob will see the message at 4 second on him clock too, the crucial point is the speed of light change what my eyes can see of the information coming from alice, so bob see with him eyes alice clock tick slower but it's only an illusion caused by perspective, and the communication is istantanuosly, so bob should hear and reply to alice at 4 seconds for both, the only differences between the clocks ticks can occure cause time dilation due to mass, but if we assume they only moving in space at a certain speed with or without acceleration, there isn't a paradox cause they can only see delayed time apparently but their clock still sincronized so when alice is 4 and see bob is 2 it's cause light need to travel to alice eyes, but bob already traveled 4 seconds too in him point of view and when bob is 4 he can see alice is only 2, but it isn't only 2.... already happened for alice to reach 4 seconds and send the message but bob cannot see the information that is traveling through space meanwhile alice comunicate with him istantanously so the information of the communication start at the same time alice clock light start to travel to bob so bob can hear istantly alice but only see the light from the past of alice, so it's not a paradox but only an illusion caused by the light have maximum speed, the clocks do not tick differently it's only an illusion. would be different if bob travel near black holes and other things that make him clock really slowing down, but without mass time is the same everywhere independently by speed or acceleretion. alice can also ask bob how many seconds passed on him clock and bob should tell her that him clock is at 4 seconds, not 2... and viceversa bob can ask alice her time and she should tell him 4, but from him point of view cause light need to travel the information to him eyes alice is at 2 seconds..... so the paradox is not a paradox cause is only an illusion that clocks slow down for alice and bob in this scenario
@@connorjensen9699 It's just the image of her slowing down though she's not actually slowing down. It would look like her 2nd second but really it would be still received at her 4th second when he sent it. If light hits an object going .87c at 1.0., then light will leave at .13c. Meaning the light will reach you at a slower pace, which will look like slow-mo but its just an after image of the real thing. And yes light can travel at different speeds its been proven and the laws of conservation of energy also prove this.
Moving lazer across the moon from earth does not break the speed of light. It's like when you move a garden hose too fast and the stream breaks into chunks of streams and no longer stays a constant stream. Same is true for shadows.
Let say both Alice and Bob are moving away from Earth in opposite direction at the same speed, so that twin paradox is not a concern. Observing from Earth, at s=4 on Alice's clock, she sends a message using instant communication to Bob with a timestamp of s=4. Bob will not receive this message at s=2 on his clock but at s=4. He sees Alice's clock through a telescope that her clock is at s=2. There is a difference between observing Alice's clock through light speed or FLT, but no advance knowledge can be gained. Then it takes Bob's computer 1 second to send a message back to Alice, which makes it s=5 on his clock. This message will have a time stamp of s=5 and Alice receives this message at s=5 on her clock too. Now Alice looks at Bob's clock with her telescope and sees his clock is at s=2.5 and at that time he hasn't receive her message yet. But see, there is no break of causality, only that they observe an older history of the other side using light and they don't gain any info of the future from the FTL messages.
One must use the time frames of the person who may have a violation of causality. At 11:31, If that is Bob, he receives the instant signal at 8 seconds, and sends it back to Alice, thinking she will get it at 8:01; no violation of causality. If Alice is the one who will have the paradox, she sends the signal to bob at 4 seconds, whom she thinks gets it at 2 (but really gets it at 8), and Bob sends it back along Alice's timeline, for her to get it at 4:01. No paradox if you use the timelines of where you think the paradox will occur.
Maybe a god had something to do with it after all. He/she/they/it edited a line of code in an algorithm to make sure none of the values in the denominator were zero.
@@saifali7080 it's struggling to process so much information that it slows everything down, or might not responding right at the center. 1 planck length = 1 core/thread. That's how you get relativity (of cores)
This reality wouldn’t be consistent without a max resolution. C = max resolution of this virtual reality as a function of consciousness. Now you have an idea for what „god“ metaphorically stood for. PS there is no speed limit for thoughts. As we,ergo our players, are nonlocal consciousness ( part of the system formerly known metaphorically as „son of god“) there is no objective distance between us. And the distance to every point of of this virtual cosmos = 0 for consciousness. Thats why thoughts are transmitted instantaneously if two players are aware of their abilities. Thoughts don’t travel through virtual spacetime. Our players are not bounded to the physical ruleset of this VR. Just our virtual avatar is.
About the introduction thought experiment of (break the speed of light with laser), one channel(I think was a old Vsauce or Verittassium video) easy explained that in fact the lazer work like a photon water hose, where no matter how fast you change the angle of the hose the photon will still travel at speed of light and so you don't create a instant line that travel the circunference of the moon, but a speed of light shapped curve. Like how you wouldn't expect break the speed of light with a gattling gun just because you changed the angle and bullets ended far apart of each other. The only reason it seen instant to us is because in close distant is pretty much is instant.
11:17 isn't the paradox of causality here all due to time dilation. if u remove time dilation here when travelling fast, you remove the paradox. so a ship travelling at 0.87c that isn't experiencing time dilation, could theoretically travel ftl. so effectively any ftl system would have to be a form of bubble separating them from rest of the universe. so any ftl would effectively need to work like an instant transporter. and keep the time lines equal. so 4 sec to 4sec instant communication and travel would be possible ftl.
I love physics but I vote for compassion as more important - there's a meditation where you can love every1 eg start by feeling love for family & then see how other families are similar eg families in the holocaust & feel compassion for them & wish for them to be protected. Since this is a thing that the brain can do, 1 day brain scientists will be able to help everyone become much kinder & society will be happier & more harmonious & wealth will be distributed so no one suffers from eg lack of doctors or water & we'll maximise the global number of scientists & humanity will explore space & terraform other planets etc.. Therefore this knowledge is the GOAT
Its no that hard to process consider this: Mom: Cone here son, right now! In order for the son to appear right now, he would have to travel at the speed of light and yet faster, why? “Now” typically means current, “right now”typically pins a point in current time So in order for the son to appear at the end of the “right now!” He would have to appear instantly at the direct end of “now!” Which is hypothetically impossible because time is relative after she finishes the “w” in now her son must appear with no time in between, meaning no milliseconds, microseconds, or nanoseconds he must appear instantaneously at the end of the “w” if that were possible, it would still rise doubt as to when the mom finishes the word “now” in exact space time meaning the mother would have to instantly spew out the word “now” does this make sense?
@@PringleDinglesonThe3rd o snap, don't let the kids watch this one. Could you imagine? "but mum, it's theoretical impossible for me to be there right now, it will break causality"
If Alice on earth were considered moving away from Bob at the same rate Bob is moving from earth, there would be no dilation. Time dilation is due to Bob's velocity. Without a difference in velocity, there is no dilation. That is why an object in orbit experiences slower time even though it remains stationary over the same spot on earth. It is experiencing greater velocity. If there is no difference in velocity, then instantaneous communication does not break causality. If one is moving faster than the other, instantaneous communication still has not broken causality because both are still aging forward in time, but at different rates. If Alice communicates from earth's 4 to Bob's 2, it is because Bob has aged slower. It is NOT because Bob is further back in time. If he slows down in relation to Alice, then he shares the same frame as Alice, but he remains to have aged slower because he accelerated for a time. Instantaneous communication is EXACTLY THE SAME as if Bob stopped accelerating in that instant. So this explanation of instantaneous communication breaking causality is flat out wrong. If Bob is moving so that he is experiencing time at half the rate of Alice, then Alice's instantaneous communication will appear to be going 2x speed as he is talking to her, while Bob's instantaneous communication would appear to half the dilation as she observes it. They would be experiencing time dilation in real time from their vantage point. There would be no time travel, no breaking of causality, whatsoever. So the first diagram is correct because it represents Bob's acceleration, but the second diagram is wrong because it represents a backwards timeframe, which is NOT what is happening. the second diagram inverts their position while maintaining their time differences. This is an asymmetric inversion of the table, therefore it is wrong. This would represent if the first person were to move into the future though stationary, then they would certainly break causality if they communicated with the person backward in time. This is a time flow diagram, not an velocity difference diagram. Instantaneous communication is therefore possible without breaking causality because both people have moved forward to the same point in time, just with different experiences of time. Imagine if one person literally experienced time at half the rate as everyone else. It would not break causality for them to communicate with everyone around them. That person is not stuck in the past. He is just not as quick as everyone else. Is a fly 1 day in the future because it experiences time faster than us? Also, physicists always complicate the matter by directly relating two observers to each other. But it is not their relationship to each other that causes dilation, but it is due only to their differences in velocity, that is, their frame of reference in time. Slowing back down to their starting speed, or returning to their staring position, does not erase the loss of time because time only ever moves forward, NEVER backward. But slowing down below the speed of those in the starting frame does allow their frame to return to be potentially equal with the one who left. For instance, if a person orbits the sun in the earth's path of transit at twice the velocity that the earth orbits the sun, then all that person needs to do is remain in place at the original starting point and move in a circle equal to the diameter of the earth's surface as at the same latitude for a year, or at least move out and back to match the same distance and velocity until the earth reaches that point again to undo the time variance. But this requires no moving backward in time. It is merely the tortoise and the hare switching places.
Keep in mind there is no time dilation. Alice nor bob experience time dilation. That’s relativity. At no time is “time” Moving faster or slower for either party and this is the trick of relativity. We are moving at bear half the speed of light right now here on earth…do you experience time dilation? No.
@@aaronesaxton Excuse me for that comment. I just started studying the issue then. I understand it better now. It is due to velocity, not acceleration, nor distance. But the diagram issues remain. I have now replaced "acceleration" in the comment with velocity terms. As to dilation, dilation is real and it does affect clocks for someone that has achieved a higher velocity. It is not an illusion. Dilation is not something that is experienced, but rather is observed. They have to adjust the clocks of satelites because even if, and because of being in a higher orbit, and therefore faster velocity, their clocks run slower than on earth, so a calculation is done to distinguish the difference. When astronauts leave earth and come back, their clocks are always offset from when they left. If time dilation was not an observable phenomenon, that would not happen. Time dilation is very real, but can only be measured, not experienced.
I saw cool world's YT video on this topic and it was a great video, but I had hard time understanding the causality breakage. Your video almost made me understood the issue of causality breakage. Right on man, keep it up.
The reality is a little different, although not much different from the thought model. As Florian advises you, buy a garden hose and water the lawn. If you move sharply to the right or left, the water flow will bend, which in a small way also happens with a laser beam, so you have to include this laser pig, which is delayed, in addition to the speed of the light sent and reflected by the reflection.
The universe does have lag though. Since information, even forces including gravity, have to travel at c. If you were to summon a second moon from nothingness, it would take at least a second for it to "realise" that it's actually supposed to be attracted by the Earth and change its trajectory and start orbiting/falling. Just like these cartoons, if you see what I mean.
The sub space analogy had the famous "who's on first" come to mind. For those younger look it up by, Abbot and Costello. Back to the subject. It's interesting but the interaction between the two units are not within one time frame, but a third party observation. There are actually three separate frames of time. Causality intact.
There are always an infinite number of reference frames that could be observing this experiment. Adding more does not change the fact that the person on earth can send a message to the ship and get the answer before they sent the question.
I think the issue here is an incomplete view of reference frames. Einstein stated that all reference frames are equal but that’s likely not the case. In your example it’s likely that it will ultimately be shown that only the rocket is actually moving through space at the velocity stated and that even though the rocket could send a message instantly to the earth the message would always be received at the relative time on earth (i.e. if sent 2 seconds for the rocket, received 4 seconds for the earth) and that any message from earth sent would be in this example received at half the time on the rocket. Although time is moving slower on the rocket, there can be no time travel.
I've now seen this example a dozen times. There seems to be a flaw in the initial condition. For that STL ship to get up to speed it has to accelerate and that acceleration forces the clock to be slower so the paradox does seem to go away. It's the same as the twin paradox when they come back together one is definitely younger. In fact the only way this example works is if the spaceship popped into existence at that speed which cannot happen.
From Alice's perspective Bob's time ticks slower (9:56). From Bob's perspective Alice's clock is running slower (10:32). That means both are right and wrong at the same time. And from the instant message's point of view, both Alice and Bob are moving at the same speed in opposite directions thus keeping the same timestamp. (something like pause at 13:09)
This is exactly my thought!! I still can't understand how causality breaks. This contradicts all explanations about Alice having aged at a faster rate than Bob. So, what is it? I feel that I am either missing something or the explanations are not complete. Even looking at your point of view (where they are both moving in opposite directions to each other at half the speed mentioned) where both of their clocks tick at the same time, there's still the problem with how much Alice has aged compared to Bob. It is a fact that Bob is traveling faster than Alice since they both have a reference frame (Alice) and also, by the time bob gets the message and replies back, for Alice some time has passed but Bob is not getting the message in the past, it looks like that because his time has dilatated. When he receives the first message at his 2 seconds mark, it's still 4 seconds for Alice, then, when he replies back at his 4 seconds mark, the time dilatation has made Alice's time to be 8. Because if we stop the rocket right there, Bob is 4 seconds younger than Alice, hence, the message is not traveling back in time, just from a different frame of time. I see it almost as sending messages back and forth between 2 people at different time zones. Anyways, I've seen and read at least 5 articles and videos about FTL communication and how it breaks causality and still can't understand how it happens to break it, because it contradicts all the other explanations about how you can basically age at a slower rate inside a ship that moves at relativistic speeds.
@@nosmirck Nice one with the Time Zones analogy. Usually the causality break statement is based on the established rules of the frame of reference. It's easier to dismiss the idea based on resulting paradoxes than to make it viable by adding extra unknown variables to the frame. Maybe one day quantum computing and AI will give us another perspective on this topic.
No. Bob and Alice are not moving at the same speed from each other. Bob is accelerating. Alice is not. I noticed two flaws in this explanations. Arvin uses the explanation of light as he just freely chooses at his convenience to explain the speed of light and its limit. Once he says that light is an electromagnetic wave and once that it is photons. When Arvin explains the fields that are the root of forces he actually uses two electrons nagatively charged. And sure enough in such situation the two containing mass and charge objects will cause forces to act. Any mass object can be made to exert a force. But suddenly Arvin switches to light being an electomagnetic wave and then again to photons as he pleases. So, if Arvin chooses light to be photons and he further explains that photons are massless then, how can something massless exert any force? Photons have no charge either. So, again, how can photons create a force? But Arvin in a snide way swithes to waves. Arvin! Make up your mind! You can't have massless and chargeless photons exert a force. Another point is special relativity; there is one big difference in Alice and Bob situations. Bob flying away with the 0.87c experiences ACCELERATION whereas Alice does not. So, by twisting the time axle is wrong for Alice because she did not experience acceleration. I do not agree with Arvin; Alice or Bob can never receive a message before it was sent. It is just the manipulations with the graph which does work one way when Bob is accelerating but does not work for Alice because she is at stand still relative to Bob. Arvin uses a bit of sophistry here and a slight trick in mathematics. More on it here in a related correspondent: It is all fine and dandy. As it was stated here: light carries energy. Well, not so fast. If light is a wave then we can consider that as energy. But if we consider light as photons then we have a problem. Photons are massless. Therefore, how can a massless photon knock a free electron from a metal? I think this rather now has something to do with the observer who ''controls'' if he/she wants the light to be a particle or a wave. And therefore, also we come up to the paradox of Schrodinger's cat. Something massless cannot exert force and therfore cannot have energy and cannot do work. Massless photons cannot exert force either. How do they create photoelectric effect? That just does not make sense. Unless we have an active observer/experiment conducting person which leads us to the duality of light and the delayed choice experiment in Alain Aspect method. E=hf only pertains to frequency of light and the Einstein's KE for electrons is dealing with, well, electrons which have mass. So, the Einstein's equations using electrons with mass cannot be applied to kinetic energy of photons which have no mass. Something that has no mass cannot have kinetic nor potential energies.
@@ericephemetherson3964 Yes, I know that. It was another point of view. Let's say Bob uses a warp drive engine, meaning that he isn't really accelerating, but the space around him warps. Although both of them are relatively stationary the distance between them grows.
This is the clearest explanation of how FTL communication can cause causality problems that I have ever seen. No arbitrary line is drawn that the narrator just states is FTL. Every line of the "subspace" message is explained. Good job, sir! Thank you, sir. BTW, in Star Trek TOS episode "The Enterprise Incident," Tal says, "The subspace message will take three weeks to reach Starfleet." I don't know where the instantaneous bit comes from. Maybe from one of the later series. No matter. Your explanation is superb! Thanks again. -- Recall that what's instantaneous for Alice is not, in general, instantaneous for Bob, and vice versa. And that's part of the answer.
I must not be understanding Something though… because even how he described it it doesn’t seem like a paradox, only an illusion based off of each others reference..
@@stonesymmetry How is receiving a reply to a message you haven't sent yet, after which you can decide to not send the initial message, just an illusion?
its the difference between group velocity and phase velocity. A wave approaching the shore has a group velocity less than c. But the a point of constant phase on a breaking wave, parallel to the shore, can move at speed greater than c, as it moves from left to right.
Sorry but there is no paradox: If you send a question with instant communication from 4 it will arrive at 4. The time runs the same for both. The only difference is that visually it looks from Alice perspective that Bob's timer is at 2 (if she was able to see his timer) while her's is at 4 but in fact both timers are at 4 in the same moment. The same way Alice timer would look like it is at 2 for Bob when actually both are at 4. It is just a visual thing what the eyes see because the light travels at c.
If Bob turned around part of the way during his trip and returned at the same speed, then according to Alice he should have experienced half the time she did. But according to Bob, Alice should have experienced half the time he did. Isn't that also a paradox? Or is the direction of motion change the speed of time?
I think as Bob slows down, Alice's time will 'catch up' with him, and then overtake him, so Bob will see Alice age quickly during deceleration. It's like Bob is riding in front of a wave of light that struggles to catch up with him, so it looks like Alice is aging slowly. Then when Bob slows down, the light catches up normally and Alice looks like she is aging normally. Then when Bob turns around and comes back, he smashes into the light at nearly twice the rate compared to if he was stationary, so Alice appears to age twice as fast (or is it 4x as fast, as time for Bob also moves twice as slow?). In any event, the paradox solves itself and, when back on earth, both Alice and Bob agree that it was Bob who has aged slowly. That's probably not quite the right explanation though.
So there is one thing by I’ve always been confused about when it comes to relativity. If the astronaut leaves earth according to earths reference frame he’d age slower to those on earth, but on his reference frame earth would be the thing moving and therefore be experiencing the opposite effect and people on earth would age slower then the astronaut, this seems like a contradiction, the only solution I can see is that if they met again their clocks would sink(excluding gravitational time dilation) but any signals exchanged before then are just temporally shifted. Am I correct in this analysis of relativistic time dilation, or am I missing a critical understanding of some part of it?
I like how you referenced the light dot on moon to give an example of faster than light speed. First of all, it takes some time for light to travel from Earth to the moon. The longer the distance, the more time it would take for light to propagate. Secondly, try firing water from a hose pipe at your car in a circular direction. You'll find that as you increase the distance, there is a time lag between the expected and observed propagation. This is because the circumference of the circle you're moving your hand in is much smaller than the one you're plotting on your car using the water jet. The water jet in between would resemble a helical shape if it is continuous. So yes, the expectation that light would travel instantly to moon as and when the laser rotates on Earth clearly isn't true, even if we do not consider photons. Thank you for pointing that out.
Why do people say "if speed of light is infinite, then you don't get to use special relativity anymore" and then turn around and say "infinite speed means that due to relativity e=mc2, no mass can exist, therefore speed can't be infinite"? Trying to use that equation means you haven't thrown out relativity.
Question- some galaxies are so far away from us that even at the speed of light we could never get to them. If the speed of light is actually the speed limit of causality in the Universe rather than just a speed limit, then how could galaxies this far away possibly have any gravitational effect on us since we are moving apart faster that the speed of causality could reach us?
@@stanleysteamer3212 wrong, gravity have a delay from different point on the universe, gravitational wave travel at the speed of light, so the real answer is, if there is some galaxy that is travelling faster than speed of light from our perspective thant their gravitational effect would be none to us but also we wouldn't be able to see them for the same speed limit
I love how they always tell me how a guy flying in a rocket at super-speed will age slower than us on earth... but in the next sentence they tell me how everything is relative and to the guy in the rocket, we are travelling at super-speed, thus we age slower than him.
How can time for the rocket to be flowing at half the rate of the earth, yet when you swap perspective it's the opposite. How can they both be experiencing time faster than the other?
This is because the light from both, the ship to earth, and from earth to the ship will be outdated by years respectively, but appear instantaneous from those perspectives. Likewise, for very large distances, we are looking at the past of other galaxies because the light from their present hasn't reached us yet, and vice versa
The relative part here means the guy in the ship will not feel like he is aging slower, he won't look at his clock and see it tick slower. Only an observer outside the ship will see him age slower.
Just to be picky: Where we see Alice moving from Bob's perspective we see that his engines are firing. This is probably a mistake because it would mean that he is accelerating and hence not in an inertial frame of reference 🙂
You are absolutely correct. I noticed two flaws in this explanations. Arvin uses the explanation of light as he just freely chooses at his convenience to explain the speed of light and its limit. Once he says that light is an electromagnetic wave and once that it is photons. When Arvin explains the fields that are the root of forces he actually uses two electrons nagatively charged. And sure enough in such situation the two containing mass and charge objects will cause forces to act. Any mass object can be made to exert a force. But suddenly Arvin switches to light being an electomagnetic wave and then again to photons as he pleases. So, if Arvin chooses light to be photons and he further explains that photons are massless then, how can something massless exert any force? Photons have no charge either. So, again, how can photons create a force? But Arvin in a snide way swithes to waves. Arvin! Make up your mind! You can't have massless and chargeless photons exert a force. Another point is special relativity; there is one big difference in Alice and Bob situations. Bob flying away with the 0.87c experiences ACCELERATION whereas Alice does not. So, by twisting the time axle is wrong for Alice because she did not experience acceleration. I do not agree with Arvin; Alice or Bob can never receive a message before it was sent. It is just the manipulations with the graph which does work one way when Bob is accelerating but does not work for Alice because she is at stand still relative to Bob. Arvin uses a bit of sophistry here and a slight trick in mathematics. More on it here in a related correspondent: It is all fine and dandy. As it was stated here: light carries energy. Well, not so fast. If light is a wave then we can consider that as energy. But if we consider light as photons then we have a problem. Photons are massless. Therefore, how can a massless photon knock a free electron from a metal? I think this rather now has something to do with the observer who ''controls'' if he/she wants the light to be a particle or a wave. And therefore, also we come up to the paradox of Schrodinger's cat. Something massless cannot exert force and therfore cannot have energy and cannot do work. Massless photons cannot exert force either. How do they create photoelectric effect? That just does not make sense. Unless we have an active observer/experiment conducting person which leads us to the duality of light and the delayed choice experiment in Alain Aspect method. E=hf only pertains to frequency of light and the Einstein's KE for electrons is dealing with, well, electrons which have mass. So, the Einstein's equations using electrons with mass cannot be applied to kinetic energy of photons which have no mass. Something that has no mass cannot have kinetic nor potential energies.
@ Arvin Ash Hi, I watch this video and there are 2 big mistakes. 1. We don't know what the speed of light really is. We can measure only with reflection from mirror. The speed of the light can be 2C in one direction and 1/2C after reflection and we have no idea how to measure it. 2. Bob and Alice communicating with infinite transmission speed when Bob is moving with 0.87C. Your mistake is with their clocks. You cannot change one part of equation only. If you postulate that the communication speed between them is infinite, then you have to change their clocks that are ticking with the same speed One last info for you. The matter can't reach even 0.87C, because it will be ripped apart if C is the real limit. Why? Because electrons are moving in the atoms and can't reach that speed, they must slow down. It means, they will start to fly freely, they breaks away from the atom's core. Take a look at this: th-cam.com/video/pTn6Ewhb27k/w-d-xo.html
Imagine a load on a wire that is already 300km away, when the shadow is cast on the arc interrupting the flow of electricity does the load drop to 0 300km away? No. The shadow is delayed by all of the photons already in transmission; it’s not instantaneous.
That’s the only thing about Star Trek that gets to me, the communication stuff, and a few other things like travel speed sometimes seems to be inconsistent. This is episode wise.
@@garethhanby The only relevant motion in this scenario is the motion of light from the star being observed from (x) light years distance. You are seeing it as it existed however many light years in the past when these photons started their journey.
Your explanation with just the 2 reference points is the easiest I have seen so far. I have seen a few very confusing diagrams that didn't make much sense too me because i forgot relativity and how time moves differently
Amazing video! I'm sure you already made a video about quantum entanglement but I think it would be an excellent topic for next time as it seems to break everything that you spoke about in this video.
Yeah I agree. He says it doesn't because the entangled particles form 1 single object. IMO it doesn't matter if they do form 1 object, it's still FTL cause & effect
what if when he doesn't send the message, he creat a parallel universe or even creates a new consciousness within himself? we did not test this theory whatsoever
Hmm? Einstein...said all of this over a hundred years ago......Nothing new here then!? “Consciousness” still one of the greatest Unknowns.....Nothing new there either.... It is repeat...get paid...repeat get paid....Nothing new there then!? Next repeat..get paid!
DEBUNKING??? Claims of speed of light. It is your active tool of masurements that can't violate the speed of light. But objects can violate that speed of light. (e.g.Two objects approaching each other at 0.6 the speed of light. How fast do they actually pass one another? ) Both objects pass each other faster than light. Attempts to measure this is trapped/limited by the tool which you are using to measure it. Your problem is: 1) You are using an inferior ruler. that is subject to relativity itself. 2) The two objects perception of space distance are no longer equal. 3) You don't realize that difference in the size of space bcs it only manifests at speeds we are not use to. Those that use the cone of time example and paradox of information fail to address one important thing. They fail to explain why/how point A exist both in the future and in the past at the same time. They fail to address that their tool of measurement and its limits are creating the paradox.
I like how you at the end say you hope someone comes along to prove it wrong or incomplete. In the past we constantly say things are impossible until we end up doing it. It's entirely possible that it's only our limited understanding of the universe is why we think faster than light travel is impossible. I think we need to be very careful when we say something is impossible.
@@ДаниилРабинович-б9п Not thar I am physicist, but if I remember it right, isn't it so that they are connected and can be measured no matter the distance between, but as the result is random and you can't affect it, you don't transfer information
@@ДаниилРабинович-б9п Like the spin can't be affected, but if you take the Quantum Eraser experiment, I still don't get it. Like it basically knows if it will be erased or observed even if it gets erased after the entangled particle hits the screen. Like how... To make it even more complicated, what happens if you put in a mechanism which recognises the interference pattern so if it predicts that it will be erased, the mechanism observe it. But then you get a new version of the grandfather paradox.
Just as there are different experiences of time due to relativity, what if there are also different causal chains? What if these temporal paradoxes are possible due to different timeline threads or time forks that occur? What I'm saying is what if time travel produces new timelines such that you can actually go back in time and kill your grandfather but that wouldn't result in your death but the death of another you in the original time thread; because the act of going back in time put you in a new and separate time thread independent of the one from which you came.
8:38 "There is no absolute "now". The "now" depends on a reference frame." Kinda sounds to me like quantum theory. The current state of a particle is fully dependent upon the circumstances that led to its being observed. The perspective of the observer is still the catalyst, in both Relativity and Quantum Theory. Interesting 🤔
Indeed Quantum mechanics via the Feynman path integral can actually be formulated in context of a relative observer with an entanglement cone that propagates outwards faster than light(but is unable to transmit information) One fascinating prospect within Wolfram's physics project is that the constraints of a Turing machine operating on some network in the limit as the network becomes extremely large can be shown to reproduce Einstein's Field Equations where distance is derived from the rate of information propagation but if allowing the network to update in all possible unique configurations in super position it automatically reproduces the Feynman path integral of quantum field theory within different possible unique states branch out from each other not unlike the many worlds hypothesis except where these are only reference frames and not truly separate i.e. they will due to branchial metric curvature (the branchial component of the metric tensor turns out to be the wave guide of Broglie-Bohm theory the rest of this interpretation doesn't really hold though) gravitate towards each other resulting in different branches converging or collapsing resulting in an apparent probability of outcomes. Naturally this is far from confirmed and any extraordinary claims require extraordinary evidence but this model places some strong constraints on quantum computers and has an interesting alternative possibility for inflation based off of the emergent nature of space allowing the number of spatial dimensions to change via a phase transition. But if this is true then rather than molding gravity into quantum mechanics quantum mechanics will fold into gravity with the microstructure below normal space being very very weird based on causal hypergraphs and geometric edges. It gets really weird with further conclusions being hard if not impossible to draw without the development of entirely new areas of mathematics.
Imagine Alice and Bob are the same age. Alice stays on Earth and Bob flies away at relativistic speeds and then flies back to Earth. In this scenario, Bob would age more slowly than Alice, since time is passing more slowly for him. Bob would then be younger than Alice. At the same time, from Bob's perspective, he could say that Alice is flying away from him at relativistic speeds. This would mean that Alice should age more slowly. Thus when they reconnect, Alice would be younger than Bob. Both of these things cannot be true. So, something is wrong with stating that their relative motions are equivalent. The explanation used in this video seems to have the same problem. Also, spacetime is an actual thing. It is an object, not just an abstraction. Alice's speed, relative to spacetime, would be different than Bob's speed, relative to spacetime. Spacetime is not absolute frame of reference, since it also moves, twists, expands, etc., but still, Bob's motion and Alice's motion would not be equivalent because their speeds, relative to spacetime, would be different. I know folks will say that this is wrong, but I do not (yet) see the flaw.
While Bob is moving, from his perspective Alice is aging more slowly. However, if he comes back to earth, and stops moving so that both Alice and Bob are not stationary relative to each other, then they will see that Bob has aged more slowly.
@@ArvinAsh I very much appreciate your taking the time to respond! If Bob aged more slowly, then Bob flying away from Alice is not functionally the same as Alice flying away from Bob. It seems like that would take away the justification for saying that we can treat their two perspectives as equivalent, as the video does at 10:30. When we say that Bob would perceive Alice's time as moving at 1/2 pace, we have to ask what we mean by perceive. Is he perceiving via light speed communication? Because the light speed delays would throw off his perception. If we are not allowing for instant communication, then Bob would have no means of perceiving Alice's flow of time. The only way they could truly perceive each other's flow of time would be retrospectively, when they reconnect. When this happens, we see that their motions away from each other were not equivalent, because Bob actually aged slower.
I found an explanation that explains that the twin paradox is explained by acceleration and general relativity: th-cam.com/video/ERgwVm9qWKA/w-d-xo.html. However, it does not really provide an explanation for why those things make the difference. I will keep reading on this.
@@ArvinAsh I still cannot get past the notion that it seems like we are mixing up two different things. So, Bob really is experiencing 1/2 the flow of time. He might perceive Alice as moving at 1/2 speed, but that is only a misperception brought about because of a signal delay from the light speed limit and because his own flow of time is slower. In terms of instantaneous communication, Bob's instant communication at his 4 seconds would not reach Alice at her 2 seconds, it would reach Alice at her 8 seconds. The realigned chart at 10:30 does not represent what is actually happening, just what Bob would perceive due to communication delays and changes in his flow rate.
If you know time dilatation, you know about existence of time contraction. It is every day event around us, from our perspective. Light is not the fastest limit in science.
Thank you! This explains the concept of "no universal now" way better than static diagrams. Question: If Bob were to stop moving relative to Alice at 8 seconds, would their "now" become simultaneous again, such that if they could communicate instantaneously from that point onward, there would be no backward propagation of the signal through time?
If Bob stops he gets the same notion of "now" as Alice. So it's t=16 for Alice and t=8 for Bob, these two events are simultaneous now for both of them, and in a second it's t=17 for Alice and t=9 for Bob etc. Bob is younger, he lived 8 seconds less.
@@thedeemon Thank you. I suppose we know it's Bob who's youngest, because he's the one who had to decelerate to get back in sync with Alice's time line. Still, a difficult concept to wrap your head around --- that Bob's deceleration at a distance from Alice re-synchs their time. Now THAT is "spooky action at a distance!"
@@alansewell7810 Maybe. Don Lincoln on Fermilab's channel did a video a few years ago on the "twin's paradox" (i.e. why is it the "moving" twin that ages slower) where he explains why there really isn't a paradox even if there is no acceleration / deceleration involved. Actually he did two videos: one with the math and one without. Basically Bob and Alice still don't have a consistent "now" even if not moving relative to each other, because Bob has been in two different reference frames. So you can't even say one is younger than the other "now" because there is no "now".
@@rancidbeef582 I watched Don's videos on that, and will watch them again, now that you've jogged my memory. Don knows his Relativity but is less lucid than Arvin Ash. One thing I have picked up from the discussions on several TH-cam channels is that when objects begin relative motion starting at a time "X," the "X" becomes a different time for each of them, because the Lorentz transformation modifies the time when an event begins as well as dilates it. For example, if the sun goes supernova at 12:00 and a spaceship leaves earth a half the speed of light, then the wave front must overtake the moving spaceship and stationary Earth at 12:08, since the velocity of light is the same for both of them. However, the moving spaceship, which is overtaken at twice the distance of Earth, perceives the supernova originating at 11:52. If instantaneous communication existed between the Sun, Earth, and Spaceship (assuming no vaporization by the supernova) then at the moment the supernova wave front reaches Earth, someone on Earth could relay a message to the spaceship, the spaceship relays it to the Sun, and the Sun bounces it back to Earth 8 minutes before it was sent. My understanding of what would happen if signals could travel instantaneously.
If warp drives and hyperdrives are physically impossible, what about a ‘jump drive’? This science fiction idea postulates that a craft can create an alternate pocket universe around itself and remove itself from normal space - thus circumventing real space. There is also Dune’s notion of 'folding space’. Technologically, this is pie in the sky of course, but is it hypothetically possible?
Star Trek's warp drive involves a warp bubble that collapses space in the direction of travel. The ships don't travel faster than light in the usual sense.
Quantum eraser fundamentally doesn't involve any backwards causation. Some articles/videos are unfortunately confused or misleading on this point. Its just something that superficially appears retrocausal based on snuck-in classical thinking. Quantum mechanics is perfectly consistent with special relativity, as formulated in i.e. the Standard Model (relativistic quantum field theory).
@@cloffff Pls explain more for me. If the which-way detection is after the wave/particle goes through the slit why does it collapse the probability function?
@@jtcorey7681 It doesn't work because the detector sends a message back in time to tell the photon which way to go; it works because you can't know which detector got set off until after the photon chose its path and had time to reach both detectors. It's an oddity because we can't tell the difference between the two: they both look the same.
Okay the part I’m still struggling with is, why would his reply go to her in the past? So if she sent a message at her 4 which is his 2, wouldn’t his reply at 4 be her 8? Because of Bob’s time dilation even though the clocks were synced they would go out of sync over his journey. So the more they spoke the quicker time for Alice would move.
Agreed. This is the part I do not understand either. When he flips the graph to Bob's perspective it looks almost as if Alice's time is going 1/2 f Bob's.... Shouldnt this still be double?? How did Bob's 2 seconds all of a sudden become Alice's 1 second?
@@michaelhearn3043 Bob's 2 seconds became Alice's 1 second since by BOB's point of view, HE IS the one who is standing STILL while ALICE is moving. That is from HIS point of view (which is equally valid as Alice's point of view, this is why it is called theory of relativity. There is not absolute truth of who is moving and who is stationary. It is all relative and based of perspective).
@@giannisms1861 i know this is like 5 months old and i'm probably pretty off base here but isn't there a difference in energy regardless of perceptive, bob is moving through space time with enough force to create time dilation while Alice is not, shouldn't that be the end all be all idk how perspective matters other wise time dilation makes zero sense.
@@porky5567 well i get what you mean but i am pretty sure the theory of relativity does not recognize WHO is moving, it only recognizes that two people are moving relative to each other. This creates paradoxes, maybe there is a way to tell which one is legit moving thorugh space and who is not, but as of now i beleive we cant tell.
Instant communication should follow the world lines so it is the same from any frame of reference. Because when you bend space you also bend the world line. So in essence ftl can not be put in these diagrams as easily as other movements.
"My heart hopes that a future Einstein...proves these equations to be incomplete and finds a loophole that allows us to keep hope alive. Zephraim Chochrane, I hope you're out there." You and I both, brother. You and I both. TOS rules!
1:18 If the speed of the laser dot can be any and it is possible to code information according speed or direction of such dot why not to use two coder-decoder (on moon and on earth) devices to communicate faster than ligth? I think this example with moving laser dot needs more detailed explanation ;)
At first that’s what I thought too but when you think about it more carefully, it isn’t instantaneous Because the movement of the dot shining on the moon will be delayed compared to the movement of the light from earth. So if you moved the dot from the left side to the right side of the moon very fast, it will still take whatever amount of seconds for it to actually move. The dot will move at the exact same rate as you are, but by a few seconds behind. So in the end, it’s the same as just communicating normally through radios or whatever.
@@ArvinAsh The article Bells Theorem for Temporal Order published in Nature Communication, August 2019 seems to be saying that cause and effect do not necessarily follow a classical model in superposition state in a gravity field. I could be misinterpreting what they are saying but it seems that quantum gravity may have not have classical cause and effect in order to integrate relatively with quantum mechanics.
If there is a time delay between switching on the laser and seeing the dot appear on the moon, then this is like holding a garden hose and pointing it at a dry wall. Even if you flick your wrist real fast, the wet streak on the wall will lag behind. Also, the dot is not an object. It’s merely an indication of where the laser’s photons are reflecting from at any given moment.
Quick question: For your example at ~ 11:00, is this not simply a perception issue? shouldn't Annie and Bob both see each other at a time sooner than they are actually at, but be able to agree that those times are earlier than present for each of them with ftl communication?
I don't quite understand you but i dont think they would be seeing each other earlier than they are actually at because the moving observer will just experience time slower than the inertial one but to the moving observer time will just tick normally. So they won't be looking at each other's past selves but rather present selves, just that one feels time to pass slower than the other
No, because there is no absolute time. Time is relative, as he explained in the video. It is hard to wrap our head around the idea, but it's true, it's been proven many times.
I have a question, though. I've always thought that if you left on a spaceship going a high fraction of the speed of light for a while, that you would return to Earth and all your friends and family would be long dead. But according to Bob and Alice around 11:48, 4 seconds for her is 8 seconds for him, meaning that if he returned at that moment, HE would be older than SHE would be? I thought I understood the time dilation effect until your video. I get that from her perspective, he would barely age and she would age normally. However, even from his perspective, he would age normally, but she would age like REALLY fast, right? What am I missing?
I see this way: time is actually absolute. Alice SEES bob slow because he is going away from her at near light speed, as bob also sees her slow. If bob were to do a 180° turn, each would SEE the other fast. When they encounter, both would be the same as if they were always together, with clock times alike. In truth, they were always at the same time, they were only PERCEIVED by each other differently.
This would only happen if one side were the decelerate back into the same velocity frame. If both sides decelerated with the same force, the effect won’t happen
In my engineering degree we did an experiment to show that phase and group velocity are different ... I am sure any scientist could explain that this can make something appear to be going faster than light. A simple explanation is a sea wave hits a sea wall at an angle ... the wave is approaching the wall at a particular speed ... however when it hits the sea wall it will APPEAR to travel along the sea wall faster than it is travelling towards it. If the sea wall is nearly 90° to the group velocity (direction) of the wave front then the phase velocity of the wave front along the wall may be many times faster. Transform this to a wave front of light hitting something at not quite 90° and we can observe a phenomena appearing to travel many times faster than light ... but it is an illusion! One may say that the sea wave is also an illusion as the water molecules in an advancing sinusoidal wave actually travel in a circular motion and do not advance in the direction of the wave! NB this does not apply to breaking waves ... although the impingement of the breaking wave along the wall will be faster but just an ugly step function and not a nice wave. Can I also point out that the light used to observe this water wave phenomena is 'relatively' instantaneous which is not so with light waves ... hence every time you appear to have won and transferred information faster than light you haven''t as the observation of that information can only travel at the speed of light in your reference frame.
Arvin, there is an objection: Alcubierre metrics allows FTL (Faster Than Light) travel of mass and information. And this is compatible with relativity, since it only applies within the same frame of reference aka spacetime. Whenever a piece of spacetime is cut out from another (including all quantum field layers), there is no longer a speed limit, since relativity vanishes and there is no longer any interaction between these two pieces of spacetime. In other words: When Bob went with 10c to Alpha Centauri during the time of travel, he could not exchange information with Alice. Upon arrival, his clock showed the same time as that of Alice. When he returns with the same vehicle, the same happende agains - and nothing has changed. This has far more implications, one might think, since, we have already such a situation, where the visible edge of the universe expands faster than the speed of light. Therefore less and less objects will be visible in the future. Therefore stretching spacetime with FTL-speed seems to break down relativity too.
@@saucevc8353 There is no such thing like «instant communication». Information can only travel faster than light, when either i) you bend spacetime - as you can pull a long stiff wire without any delay - but gravity itself travels with c. This is a very important property of spacetime, otherwise two celestial bodies, that orbit each other would experience a torque due to lag of gravitation field update. ii) Whenever Alcubierre metric is engaged. In this case, there is no longer any connection between the spacetime of the universe and the cut-out-portion of spacetime, used for FTL-travel. This is an important prerequisite to avoid all sorts of relativistic paradoxons. Nobody can violate physical laws, but circumvent it by suitable technology.
@@debrainwasher But you can just launch an alcubierre drive from the time dilated non ftl spaceship containing a message. Boom, ftl communication. Which creates a paradox. Is the time it takes for the drive to arrive at it's destination determined by the viewpoint of the launcher, or the viewpoint of the recipient?
@@sylviarogier1 I can't fault your judgement there. :) I'm not usually a fan of horror, but I make an exception for those two. The atmosphere and tension in both are just brilliant.
This whole point is moot, since we do not know the 1 way speed of light. That’s right, we have no clue how fast light travels from the source to our observation. And with current tech, we can not know this. We only know the reflected speed of light or 2 way, from source to reflected point to our observation. Einstein acknowledged this in his papers on the subject. He postulates that from to the source to our observation could be instantaneous or 186k mps or anywhere in between. He chose the constant speed 186k mps round trip. This does not make it so and if it is different from the constant it does not break physics, it changes the answers. Suppose light from the source to the observer was instantaneous and then from the reflected point to our observation would mean light speed would be half of what we think it is. Also if it is instantaneous from the source, we would be seeing the universe in real time. The one way speed is unknown. The best we can do is say that the speed of light is always 186k mps and that is and assumption. Proof is when you look at a prism light not only bends but changes speed and or wave length and we can see this in the different colors coming out of the prism. Truth is we have no way of knowing what effects interstellar space will have one light or any type of transmitted data. We don’t even really know how far away anything is past 400 light years. Best guess after that.
I think the main flaw in the Alice and Bob example is the time slowing on the traveler perspective...that would mean that the light is quite instant even from sun to us, and so on so a future Einstein will find something based on this... Also in this example the speed of Alice isn't 0 in absolute terms, cause the earth is moving around Sun, and after this the whole solar system moves in the galaxy and so on so the proportion would not be 1/2...
Yeah, objects in rest shouldn't be moving regardless of the observer. Moving takes energy. If I move and see earth and universe moving away from me, that doesn't mean that I exert energy to every object in the universe so that they can move away from me.
hold on i thought time dilation was theorised to reconcile the finite speed of information with the lack of an absolute reference frame. surely then, faster than light communication would disallow time dilation. eg, if bob and allice have clocks that run at the same speed and are synchronised when bob leaves, if alice then sends bob a "subspace" communication when her clock reaches 4s bob would receive it when his clock 4s, because the subspace comms would not experience time dilation, because it isnt restrained by a finite speed. if viewed through subspace comms rather than electromagnetic comms, the clocks would appear to stay synchronized. The subspace would appear to be to light as light is to sound. if you had a train moving some large fraction the speed of sound and you communicated the ticking of clocks with small explosives the stationary observer would hear the trains pulses to be slower and the train would hear the stationary observers pulses to be slower, however they would both see the pulses much closer to being in sync due to how much faster the propagation of light is. Therefore if there were a method of moving data faster than the speed of light the effects of time dilation would diminish proportionally to how much faster this method is trending to 0 as the speed of the method trends towards infinite.
There was another thing he didn't say and that is that our current understanding of physics is extremely closely in agreement with experiment. Where current theory fails is in extreme situations such as near black holes and the Planck length. The gaps where theory is weak have gotten smaller over time. For example, QFT has theory matching experiment to 10 decimal places. That doesn't leave much wiggle room for unknown unknowns.
I could easily imagine a scenario where Alice only recieves messages from the future in the cases where she *somehow* decides to actually sends the message, while any instant where she decides not to are irrelevant cause she never recieved a message from the future in those cases.
Robert Forward wrote a book, Timemaster in which he speculated that you could travel into the past but still could not change the past. This was because the past occurred with your future self in it. If you went into the past with the goal of changing it then the universe would make random chaotic events more likely in order to thwart you. So if you shot yourself for instance you actually shot a clone or some other unlikely occurrence would occur to stop you. It would have to as changing the past would be impossible. It was a wild book.
He didnt take into account that the universe can create local event bubbles that can contain information and stuff. This bubble can collapse and cease to exist when some kind of paradox happen. This requires a lot of energy to create, but hey FTL speed require infinite energy to do so in the first place. This actually happen every time in quantum scale since the energy required is suf
This problem occurs because we over believe that space is relative, not absolute. Because space is believed to be relative, we get time to be relative as well. It is just like assuming a is true -> (a+C) is true as well. Eventually when an object is moving, only that object sees the resistance of space. As we all know, space is not empty and rather object keep coming in and going out of existence in something that we call emptiness. We cannot move faster than light as the friction of space does not allow us to. If the friction which electromagnetic waves see (the epsilon and muo values) change, the speed of light will change as well. Simple Newtonian physics. Why do objects move slower in time when they move. This is because of atomic forces. When proton sends electric waves to electron who is revolving around the proton, the wave has to travel to the electron before any cause-effect thing happens. In a moving atom, that wave will take longer as the path which the causality wave has to travel will be longer due to the atom moving at a speed. This will make one second for a relativistic object appear longer. As on an absolute time scale, it took the wave more time to reach the electron v/s it would have taken in a steady object. Since these waves have no mass, they don't have momentum and hence unlike other moving objects, they don't follow newton's first law of motion (an object at motion will remain at motion) which applies only to objects with mass or momentum.
I used instant communication to ask you a question but you gave me the answer before I asked so no need to ask. Thanks Arvin!
You still have to ask otherwise he never knew lol
@@spotifyvibes319 😂😂😂
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If the glove doesn't fit then this is all just water under the bridge.
but its not instant
The matter of why FTL breaks casuality in special relativity was never explained better than in this video! Kudos!
I noticed two flaws in this explanations. Arvin uses the explanation of light as he just freely chooses at his convenience to explain the speed of light and its limit. Once he says that light is an electromagnetic wave and once that it is photons. When Arvin explains the fields that are the root of forces he actually uses two electrons nagatively charged. And sure enough in such situation the two containing mass and charge objects will cause forces to act. Any mass object can be made to exert a force. But suddenly Arvin switches to light being an electomagnetic wave and then again to photons as he pleases. So, if Arvin chooses light to be photons and he further explains that photons are massless then, how can something massless exert any force? Photons have no charge either. So, again, how can photons create a force? But Arvin in a snide way swithes to waves. Arvin! Make up your mind! You can't have massless and chargeless photons exert a force.
Another point is special relativity; there is one big difference in Alice and Bob situations. Bob flying away with the 0.87c experiences ACCELERATION whereas Alice does not. So, by twisting the time axle is wrong for Alice because she did not experience acceleration. I do not agree with Arvin; Alice or Bob can never receive a message before it was sent. It is just the manipulations with the graph which does work one way when Bob is accelerating but does not work for Alice because she is at stand still relative to Bob. Arvin uses a bit of sophistry here and a slight trick in mathematics. More on it here in a related correspondent:
It is all fine and dandy. As it was stated here: light carries energy. Well, not so fast. If light is a wave then we can consider that as energy. But if we consider light as photons then we have a problem. Photons are massless. Therefore, how can a massless photon knock a free electron from a metal? I think this rather now has something to do with the observer who ''controls'' if he/she wants the light to be a particle or a wave. And therefore, also we come up to the paradox of Schrodinger's cat. Something massless cannot exert force and therfore cannot have energy and cannot do work. Massless photons cannot exert force either. How do they create photoelectric effect? That just does not make sense. Unless we have an active observer/experiment conducting person which leads us to the duality of light and the delayed choice experiment in Alain Aspect method. E=hf only pertains to frequency of light and the Einstein's KE for electrons is dealing with, well, electrons which have mass. So, the Einstein's equations using electrons with mass cannot be applied to kinetic energy of photons which have no mass. Something that has no mass cannot have kinetic nor potential energies.
Thats the most unscientist explaination i can imagine. And more then totaly wrong. That are FAKENEWS!!!! And also stupid bullshit. Each photon of that LASER still moves with the speed of light. Light is not a beam. Dont spread such lies. Dont speak when you do not know what you tell. Liar who put there lies in public should get punished for making mankind into idiots who do not even care to give criminals control about nuklearweapons. Sry, but dont thank someone who just fools you.
@@ericephemetherson3964
What's about electromagnetic energy?
@@boudewijnj.m.kegels5198 What is about electromagnetic energy? This energy is very peculiar. Please, state your question more precisely.
@@ericephemetherson3964
Why should photon not carry pure energy without being a mass-body?
I'd say a foton IS energy and nothing else, else it wouldn't travel with the speed of light. E=mc² says in which relation mass can be converted into energy and the other way round. So lots of energy are needed to create mass: 2,176 x 10^59 times.
Man, I would say the typical "I wish I had teachers like Arvin when I was in school", but nah - I'm glad Arvin's precisely where he is right now. He reaches way more people this way, and his miraculous ability to put complex topics into plain English as well as these mad useful graphics couldn't exist so completely in any other medium. The fact that anyone can access it, for free, at any time, from any place, and pause/fast-forward/rewind or watch at 2x speed or 1/2 speed - this - everything right here - it's just perfect.
If you're reading this, thank you Arvin (:
Thanks for the kind words. Much appreciated my friend.
The problem here lies in time dilation. If the message is sent at 2 seconds and gets to the target at 2 seconds, that's true instantaneous.Same if the response is sent at 3 seconds and gets back at 3 seconds. Causality and paradoxes occur because what's described here is a message traveling back in time itself in the first place. But that's an issue of perspective with time dilation taken into account. If the message travels instantaneously between two points that are not experiencing time dilation, then there is no paradox.
So the answer to this paradox is that even though the transmission to bob was instantaneous and bob appears to have received it in the past according to his clock (but only his clock) the response still comes back instantaneously at 4 seconds and is received at 4 seconds by Alice.
Remember, we're not talking about actual time travel, just perceived time. The universe itself isn't changing, only the way the people involved experience it.
Yeah, Alice is sending the message when the ship has traveled 4 sec at 0.87C. Thus Bob cannot receive the message before the ship he is on has actually traveled the distance from Alice perspective.
Bobs perspective of Alice might say one thing, but for actual physics? Bob is on a ship that accelerated to 0.87C.
No idea if this view on it actually holds water or not, but the explanation in the video is lacking.
in short, from the perspective of the information they are both at 4s.
@@alichi101 I also don't understand why would this be a message sent back in time. We (but also Bob and Alice) know who accelerated and whose time is slower. Why is it a problem for the universe to know this? :)
What if Bob slows down to normal speed for a microsecond (after he travelled 2 sec his time), will he be able to correctly observe that his time was the slower one? Will he see that 2 seconds passed for him and 4 for Alice? I assume yes and in this case I don't see the paradox.
@@csmcs2 My understanding is that theres is no such thing as knowing which person "really" accelerated. As mentioned, the laws of physics are the same everywhere in the universe, and both of the two of them would perceive their physical reality to be the true one.
You keep using the word instantaneous as though two events could be instantaneous in different reference frames. The reality is that instantaneity itself is relative even after accounting for time dilation.
Two events that occur at different points in space cannot be said to have happened at the same time for all references frames. Instead, different observers traveling at different velocities will disagree as to how much time has elapsed between the two events. And if there is at least one reference frame in which both events happened at the same time then different observers will disagree as to which event happened first.
Your understanding of the problem wrongly assumes that you can simply give a timestamp as “2 seconds” without specifying the reference frame.
11:40 I don't understand why the case isn't: Alice sends a signal at 2 seconds, from Bob's perspective it arrives at 2 seconds but it _appears_ as though Alice doesn't send it until 4 seconds relative to Bob, and when Bob sends a reply at 4 seconds, from Alice's perspective it arrives at 4 seconds but it _appears_ as though Bob doesn't send it until 8 seconds relative to Alice.
I never intuitively understood what light was or how it propagated until I saw this. I've known logically for ages, but never really 'got' it. For some reason this description really clicked with me, thank you so much for this video.
I’ve heard of the back in time paradox before, but not this well explained, thanks!
Yeah I think Dr Ash explained that time-backwards signal travel exeptionally well!
lol don't tell this dude that you can send text to different time zones... like so what if it back in time it is all relative... it could still work..
like who cares the time stamp on the message.
@@patrickgronemeyer3375 th-cam.com/video/KGBMf-U-uD0/w-d-xo.html - Will We Ever Build Warp Drives? (feat Dr. Miguel Alcubierre!)
@@whatitis4872 th-cam.com/video/KGBMf-U-uD0/w-d-xo.html - Will We Ever Build Warp Drives? (feat Dr. Miguel Alcubierre!)
Einstein's idea of space and time not being absolute, but the speed of light being absolute is nothing short of groundbreaking.
This simple yet complex idea has opened so many doors to the answers to the nature of the universe.
There is another factor to consider here... a ship within an Alcubierre style field exists in normal spacetime, and is in fact relatively stationary. There would be no time dilation between the inside of the bubble and the outside observer. The contraction and expansion of space is what moves the field at relativistic speeds, but information in and out of that field would still be constrained by C.
If person A sends a message to person B who is moving at high warp speeds, then person B will either outrun the message or intercept it head on, but no matter the relative direction of travel, the message would always be received after person A sends it, because there is no time dilation involved.
You could even send a message in one direction, warp past it, and then receive it yourself. This still doesn't break causality, because you cannot reply to the message before you send it, because, again, at no point is time dilated.
Any travel faster than light will break causality. Take the example given of Alice and Bob. If an Alcubierre style travel method exists, then a third person C could travel between Alice and Bob faster than light and send messages between them. You then run into the same grandfather paradox situation
@@nirmalasokan1687 Even if you travel instantly, you still cannot intercept a message before it is sent. You can tell Bob that Alice sent a message, even its contents, and you can tell Alice of Bob's reply before he receives Alice's message, but this is no different than comparing physical mail to email. You can't instantly email someone to tell them not to send a letter after they've already posted it.
@@DanGovier Any faster than light travel will appear to go back in time from a different inertial observer's point of view
This video explains it better than I can
th-cam.com/video/an0M-wcHw5A/w-d-xo.html
Time dilation is caused by Light moving the same speeds in all reference frames. If there's no dilation, wouldn't light then move far beyond C within the Alcubierre field? And In the video He talks about the issues with infinite speed of light
This channel is probably one of my most favorite. A gem in a sea of garbage. Never stop making these.
A sea of garbage is actually a thing. Plastic in the Pacific.
i agree there are many garbage explanations & Arvin Ash is a gem among with a few others such as ScienceClic, DrPhysicsA, DrBecky, DoS. Parth G etc
& this episode is 1 of Arvin's best IMO
but still I have to read a very good book: Engineering Mathematics by KA Stroud & only then will I really be able to start to learn quantum mechanics
& I don't even know what to read to start to learn general relativity. Maybe eigenchris
Muck like the cosmological speed limit so to the grammatical one. You can not travel faster than, "favorite".
Therefore there can be no , "most" favorite. Not even in Star Trek.
lol
Amusing statement considering that this video is wrong.
Firstly I would like to take refrence from an older video, Delayed choice quantam eraser. In the simplified experiment shown, I am taking at face value that the beam splitter were placed in such a way that there is absolutely 50% chance of the photon going downward or through the splitter(since I ain't a pro on the topic or the way actual experiment is conducted). Maybe there are some unknown variables/decoherence which is causing the photon to go through a certain path. And its just the past/present causing the future which is perceived vice versa.
Now about this video, firstly I would like to say that the timeline graph example was one of the best to understand the concept as shown in graph form. But, assuming from this video if there is an AbsoluteTime which is represented in vertical axis, the whole process needs to be done from one reference point. ( I am talking about absolute time coz Earth is also moving, so is the solar system and the galaxy and so forth the universe is also expanding hence all are having some speed hence we may perceive time differently than the absolute time.) The reference point should not be changed mid process. It could be either the absolute time, Alice's time or BOB's time. So the graph shouldn't move after Bob receives message from Alice. If taken from absolute time reference, say Alice sent message at X seconds which is 4 seconds for Alice and 2 seconds for Bob, who even if replies instantaneously the absolute time would still be X which is 2 seconds for Bob and 4 seconds for Alice so how does the message travel back in time? I hope I am not mixed up in the term 'Special relativity'.
But still assuming that somehow the message/information traveled back in time, doesn't it support the Idea of Determinism. The future effecting the past, A loop created which is caused by itself.
I don't know if I am thinking in the right direction but still I loved the video cause it made me think all this. Thanks for the ignition of the thought process and would love to be enlightened/steered more.
P.S. what I actually wrote is so different than what I thought I was going to write when I started. It is the thought process of writing which eventually made this so lengthy and the way it is. I don't know if that too is determinism. :-)
This is one of your best video, Arvin.
Talking about breaking causality, what if: one entered a region of the universe where the entropy casually started decreasing time ago when there was already intelligent life formed, then has remained almost isolated from the rest of the universe, while entropy is casually still decreasing, its inhabitants are becoming younger and are losing memory from our perspective, like if time was reversed in that region. From their perspective instead WE are getting younger, since our past is their future, and this is perceived "until" ("since" from our perspective) the moment of entropy reversion. So how would it be communicating with each other?
I know this is impossible because of statistics (second law of thermodynamics), but since spontaneous entropy reversion is at least theoretically possible, it must have sense and not evolve in paradoxes, and this would be interesting to analyze.
You would dig Dan Winter's work on Negentropy, if you haven't already. ✌🌐
I noticed two flaws in this explanations. Arvin uses the explanation of light as he just freely chooses at his convenience to explain the speed of light and its limit. Once he says that light is an electromagnetic wave and once that it is photons. When Arvin explains the fields that are the root of forces he actually uses two electrons nagatively charged. And sure enough in such situation the two containing mass and charge objects will cause forces to act. Any mass object can be made to exert a force. But suddenly Arvin switches to light being an electomagnetic wave and then again to photons as he pleases. So, if Arvin chooses light to be photons and he further explains that photons are massless then, how can something massless exert any force? Photons have no charge either. So, again, how can photons create a force? But Arvin in a snide way swithes to waves. Arvin! Make up your mind! You can't have massless and chargeless photons exert a force.
Another point is special relativity; there is one big difference in Alice and Bob situations. Bob flying away with the 0.87c experiences ACCELERATION whereas Alice does not. So, by twisting the time axle is wrong for Alice because she did not experience acceleration. I do not agree with Arvin; Alice or Bob can never receive a message before it was sent. It is just the manipulations with the graph which does work one way when Bob is accelerating but does not work for Alice because she is at stand still relative to Bob. Arvin uses a bit of sophistry here and a slight trick in mathematics. More on it here in a related correspondent:
It is all fine and dandy. As it was stated here: light carries energy. Well, not so fast. If light is a wave then we can consider that as energy. But if we consider light as photons then we have a problem. Photons are massless. Therefore, how can a massless photon knock a free electron from a metal? I think this rather now has something to do with the observer who ''controls'' if he/she wants the light to be a particle or a wave. And therefore, also we come up to the paradox of Schrodinger's cat. Something massless cannot exert force and therfore cannot have energy and cannot do work. Massless photons cannot exert force either. How do they create photoelectric effect? That just does not make sense. Unless we have an active observer/experiment conducting person which leads us to the duality of light and the delayed choice experiment in Alain Aspect method. E=hf only pertains to frequency of light and the Einstein's KE for electrons is dealing with, well, electrons which have mass. So, the Einstein's equations using electrons with mass cannot be applied to kinetic energy of photons which have no mass. Something that has no mass cannot have kinetic nor potential energies.
Fantastic explanation, but now I'm more confused about time dilation. In your example about Bob and Alice and how each of them has a "normal" clock according to their own reference frame, and perceive time to be slower for the other - well then, how will their ages have changed when Bob finishes the trip? Isn't he supposed to age slower since he's traveling so fast?
Just what I've got really confused about from this video :(
Here is a "realistic" thought experiment along the lines of the moving laser dot on the moon.
Since we should be seeing the summer triangle in the sky these days, I will use the star Vega, just about 26 light years distant, and easy to see with the naked eye. Now, watch it move across the sky. If you keep track of it all night, all the next day, and all the next evening until you see it in the same spot of the night sky, you realize something. With a radius of 26 light years, it has traveled a total distance (from within your reference frame) of pi x 52 light years or just about 163.3 light years in 24 hours, which works out to a speed of about 456 million miles per hour, which is just about 6.8 times the speed of light. Anyone can see Vega moving across the night sky and easily calculate this, so it's really not a thought experiment at all. So is Vega really moving that fast around the Earth ... ?
lol
Thinking Vega is moving that distance in 24 hrs and it's not the earth spinning that makes it look that way is definitely a interesting take.
In your scenario Vega isn't moving (apart from its proper motion which we can ignore for the moment) its the earth that's rotating causing the observers field of view to move
@@paulhunter7002 th-cam.com/video/5q_z8BjiYng/w-d-xo.html - Starship Congress 2017: Dr. Miguel Alcubierre, "Faster Than The Speed Of Light"
so technically speaking if pistol star at 26090 light years moves within your frame of reference it would move 59.84 million times the speed of light. lmao. so its obvious, if the distance increase the speed of light in our reference frame also increases. but you should remember that since Vega is 26 light years away you are seeing the planet how it was 26 years ago and a day later you would see how the planet would look 26 years + 1 day ago so for subtract the first frame from the second frame, we are living 26 years ahead of when light left so
(26 + 26 + 1) - (26 + 1)
light years distance ( 1day later) - light years at present
so 26 + 26 -26 + 1 - 1 = 26 light years
paradox
Reminds me of the scene from an old Dr. Who is asked how the Tardis can be bigger on the inside than on the outside. He demonstrates how an object he's holding close to her looks bigger than one he put a ways away though they're the same size. The questioner said "well that's stupid" to which Dr. Who responded indignantly "that's the basis of all Timelord physics!"
The problem with the example of Bob and Alice is that in special relativity Bob's perception of time is irrelevant to the way Alice is aware of her relative time space
is it? if bob has instant communication by definition if he pinged Alice's ship he would see a live response the moment he communicated with Alice. So Alice pings Bob's ship at her second 4 and his second 2. It is precisely because Bob's perception of time has Alice moving at .87c and therefor form his perspective time is slowing down for her (and not him) then that means that if he sends an instantaneous communication back even after waiting two seconds it will go from his second 4 to her second 2 (because from his perception she is the one moving more slowly through time). Since Bob's response to Alice has arrived at her second 2, it has arrived before she sent her ping originally. If bob's frame of reference being that she is the one moving at relativistic speeds is precisely what causes the paradox in this example.
@@connorjensen9699 the problem is that it's only a perception of time, but if u look it like u are an observer in the middle of both alice and bob u will see both moving one to the left at half the speed and one to the right at half the speed, it's like the middle point from the 2 objects is moving away from alice at half the speed following bob at half the speed of bob, if bob go 1,00 the middle point is going at 0,5 in the same direction of bob, and alice is going at 0,5 in the opposite direction from our middle point of view, so there is no paradox, we can see the clock of alice and bob are still sincronized, if alice sent to bob an instant message at 4 on her clock bob will see the message at 4 second on him clock too, the crucial point is the speed of light change what my eyes can see of the information coming from alice, so bob see with him eyes alice clock tick slower but it's only an illusion caused by perspective, and the communication is istantanuosly, so bob should hear and reply to alice at 4 seconds for both, the only differences between the clocks ticks can occure cause time dilation due to mass, but if we assume they only moving in space at a certain speed with or without acceleration, there isn't a paradox cause they can only see delayed time apparently but their clock still sincronized so when alice is 4 and see bob is 2 it's cause light need to travel to alice eyes, but bob already traveled 4 seconds too in him point of view and when bob is 4 he can see alice is only 2, but it isn't only 2.... already happened for alice to reach 4 seconds and send the message but bob cannot see the information that is traveling through space meanwhile alice comunicate with him istantanously so the information of the communication start at the same time alice clock light start to travel to bob so bob can hear istantly alice but only see the light from the past of alice, so it's not a paradox but only an illusion caused by the light have maximum speed, the clocks do not tick differently it's only an illusion. would be different if bob travel near black holes and other things that make him clock really slowing down, but without mass time is the same everywhere independently by speed or acceleretion. alice can also ask bob how many seconds passed on him clock and bob should tell her that him clock is at 4 seconds, not 2... and viceversa bob can ask alice her time and she should tell him 4, but from him point of view cause light need to travel the information to him eyes alice is at 2 seconds..... so the paradox is not a paradox cause is only an illusion that clocks slow down for alice and bob in this scenario
@@connorjensen9699 It's just the image of her slowing down though she's not actually slowing down. It would look like her 2nd second but really it would be still received at her 4th second when he sent it. If light hits an object going .87c at 1.0., then light will leave at .13c. Meaning the light will reach you at a slower pace, which will look like slow-mo but its just an after image of the real thing. And yes light can travel at different speeds its been proven and the laws of conservation of energy also prove this.
Moving lazer across the moon from earth does not break the speed of light. It's like when you move a garden hose too fast and the stream breaks into chunks of streams and no longer stays a constant stream. Same is true for shadows.
Arvin Ash is the most underrated youtube channel in existence. Keep up the beautifully simple explanations to the equally beautiful complex ideas
The moment you say "coming up right now", is the moment to destroy the like button, in every video.
I was about to say a similar thing. I dunno why but the way he says that gets me really excited. 😂
It's just a ripoff of the introduction to PBS nova which does the same thing dude. I thought it was super corny myself.
@@NR-fg2qc me too
@@medexamtoolscom Except PBS Nova is boring as fuck. This guy knows how to present.
But if it's destroyed then how do you use it again?
Let say both Alice and Bob are moving away from Earth in opposite direction at the same speed, so that twin paradox is not a concern. Observing from Earth, at s=4 on Alice's clock, she sends a message using instant communication to Bob with a timestamp of s=4. Bob will not receive this message at s=2 on his clock but at s=4. He sees Alice's clock through a telescope that her clock is at s=2. There is a difference between observing Alice's clock through light speed or FLT, but no advance knowledge can be gained.
Then it takes Bob's computer 1 second to send a message back to Alice, which makes it s=5 on his clock. This message will have a time stamp of s=5 and Alice receives this message at s=5 on her clock too. Now Alice looks at Bob's clock with her telescope and sees his clock is at s=2.5 and at that time he hasn't receive her message yet. But see, there is no break of causality, only that they observe an older history of the other side using light and they don't gain any info of the future from the FTL messages.
One must use the time frames of the person who may have a violation of causality. At 11:31, If that is Bob, he receives the instant signal at 8 seconds, and sends it back to Alice, thinking she will get it at 8:01; no violation of causality. If Alice is the one who will have the paradox, she sends the signal to bob at 4 seconds, whom she thinks gets it at 2 (but really gets it at 8), and Bob sends it back along Alice's timeline, for her to get it at 4:01. No paradox if you use the timelines of where you think the paradox will occur.
Thumbs up for the Zefram Cochrane reference.
In 42 years, we’ll see what happens…
🖖🖖🖖
And God said "let there be a speed limit to information transmission, that there may be light" and there WAS light.
Maybe a god had something to do with it after all. He/she/they/it edited a line of code in an algorithm to make sure none of the values in the denominator were zero.
@@ModMINI what about black hole?
@@saifali7080 it's struggling to process so much information that it slows everything down, or might not responding right at the center. 1 planck length = 1 core/thread. That's how you get relativity (of cores)
@@varthshenon so we livin in matrix right?
This reality wouldn’t be consistent without a max resolution. C = max resolution of this virtual reality as a function of consciousness. Now you have an idea for what „god“ metaphorically stood for. PS there is no speed limit for thoughts. As we,ergo our players, are nonlocal consciousness ( part of the system formerly known metaphorically as „son of god“) there is no objective distance between us. And the distance to every point of of this virtual cosmos = 0 for consciousness. Thats why thoughts are transmitted instantaneously if two players are aware of their abilities. Thoughts don’t travel through virtual spacetime. Our players are not bounded to the physical ruleset of this VR. Just our virtual avatar is.
This was really well done and explained with great clarity. I had not found this channel before but im sure to keep watching
About the introduction thought experiment of (break the speed of light with laser), one channel(I think was a old Vsauce or Verittassium video) easy explained that in fact the lazer work like a photon water hose, where no matter how fast you change the angle of the hose the photon will still travel at speed of light and so you don't create a instant line that travel the circunference of the moon, but a speed of light shapped curve. Like how you wouldn't expect break the speed of light with a gattling gun just because you changed the angle and bullets ended far apart of each other. The only reason it seen instant to us is because in close distant is pretty much is instant.
11:17 isn't the paradox of causality here all due to time dilation. if u remove time dilation here when travelling fast, you remove the paradox. so a ship travelling at 0.87c that isn't experiencing time dilation, could theoretically travel ftl.
so effectively any ftl system would have to be a form of bubble separating them from rest of the universe. so any ftl would effectively need to work like an instant transporter. and keep the time lines equal. so 4 sec to 4sec instant communication and travel would be possible ftl.
Physics is the most incredible, beautiful and important knowledge ever created.
Discovered not created, we just observe and record the grand scheme of universe
@@atharvsharma7648 hmm,knowledge cant be created
Nope. Mathamatics is.
Agree
I love physics but I vote for compassion as more important - there's a meditation where you can love every1 eg start by feeling love for family & then see how other families are similar eg families in the holocaust & feel compassion for them & wish for them to be protected. Since this is a thing that the brain can do, 1 day brain scientists will be able to help everyone become much kinder & society will be happier & more harmonious & wealth will be distributed so no one suffers from eg lack of doctors or water & we'll maximise the global number of scientists & humanity will explore space & terraform other planets etc.. Therefore this knowledge is the GOAT
I feel a great disturbance in the Force as though thousands of Trekkie heads exploded at once.
@John Smith Much facts. Many logic.
@John Smith You have authentic research paper linked?
@John Smith The video, troll.
They exploded when you mixed Star Wars and Star Trek together. Like having Hobbits and a Zombie Apocalypse in the same movie.
@@theophrastus3.056 Can Orcs digest zombies, or vice versa?
The fact that there's no absolute "now" is mindblowing and hard to process.
How does this factor in with wormholes?
It proves that wormholes can't exist.
Its no that hard to process consider this:
Mom: Cone here son, right now!
In order for the son to appear right now, he would have to travel at the speed of light and yet faster, why?
“Now” typically means current, “right now”typically pins a point in current time
So in order for the son to appear at the end of the “right now!” He would have to appear instantly at the direct end of “now!” Which is hypothetically impossible because time is relative after she finishes the “w” in now her son must appear with no time in between, meaning no milliseconds, microseconds, or nanoseconds he must appear instantaneously at the end of the “w” if that were possible, it would still rise doubt as to when the mom finishes the word “now” in exact space time meaning the mother would have to instantly spew out the word “now” does this make sense?
@@thuggeegaming659 th-cam.com/video/Vk5bxHetL4s/w-d-xo.html - The NEW Warp Drive Possibilities
@@PringleDinglesonThe3rd th-cam.com/video/5q_z8BjiYng/w-d-xo.html - Starship Congress 2017: Miguel Alcubierre, "Faster Than The Speed Of Light"
@@PringleDinglesonThe3rd o snap, don't let the kids watch this one. Could you imagine? "but mum, it's theoretical impossible for me to be there right now, it will break causality"
If Alice on earth were considered moving away from Bob at the same rate Bob is moving from earth, there would be no dilation. Time dilation is due to Bob's velocity. Without a difference in velocity, there is no dilation. That is why an object in orbit experiences slower time even though it remains stationary over the same spot on earth. It is experiencing greater velocity.
If there is no difference in velocity, then instantaneous communication does not break causality. If one is moving faster than the other, instantaneous communication still has not broken causality because both are still aging forward in time, but at different rates.
If Alice communicates from earth's 4 to Bob's 2, it is because Bob has aged slower. It is NOT because Bob is further back in time. If he slows down in relation to Alice, then he shares the same frame as Alice, but he remains to have aged slower because he accelerated for a time.
Instantaneous communication is EXACTLY THE SAME as if Bob stopped accelerating in that instant. So this explanation of instantaneous communication breaking causality is flat out wrong.
If Bob is moving so that he is experiencing time at half the rate of Alice, then Alice's instantaneous communication will appear to be going 2x speed as he is talking to her, while Bob's instantaneous communication would appear to half the dilation as she observes it. They would be experiencing time dilation in real time from their vantage point. There would be no time travel, no breaking of causality, whatsoever.
So the first diagram is correct because it represents Bob's acceleration, but the second diagram is wrong because it represents a backwards timeframe, which is NOT what is happening.
the second diagram inverts their position while maintaining their time differences. This is an asymmetric inversion of the table, therefore it is wrong. This would represent if the first person were to move into the future though stationary, then they would certainly break causality if they communicated with the person backward in time. This is a time flow diagram, not an velocity difference diagram.
Instantaneous communication is therefore possible without breaking causality because both people have moved forward to the same point in time, just with different experiences of time. Imagine if one person literally experienced time at half the rate as everyone else. It would not break causality for them to communicate with everyone around them. That person is not stuck in the past. He is just not as quick as everyone else. Is a fly 1 day in the future because it experiences time faster than us?
Also, physicists always complicate the matter by directly relating two observers to each other. But it is not their relationship to each other that causes dilation, but it is due only to their differences in velocity, that is, their frame of reference in time. Slowing back down to their starting speed, or returning to their staring position, does not erase the loss of time because time only ever moves forward, NEVER backward.
But slowing down below the speed of those in the starting frame does allow their frame to return to be potentially equal with the one who left. For instance, if a person orbits the sun in the earth's path of transit at twice the velocity that the earth orbits the sun, then all that person needs to do is remain in place at the original starting point and move in a circle equal to the diameter of the earth's surface as at the same latitude for a year, or at least move out and back to match the same distance and velocity until the earth reaches that point again to undo the time variance. But this requires no moving backward in time. It is merely the tortoise and the hare switching places.
I'd add that the 'easy' way to see that the images are flat-out wrong, is that they cause a paradox when Bob comes back to earth.
Keep in mind there is no time dilation. Alice nor bob experience time dilation. That’s relativity. At no time is “time”
Moving faster or slower for either party and this is the trick of relativity. We are moving at bear half the speed of light right now here on earth…do you experience time dilation? No.
@@aaronesaxton Excuse me for that comment. I just started studying the issue then. I understand it better now. It is due to velocity, not acceleration, nor distance. But the diagram issues remain. I have now replaced "acceleration" in the comment with velocity terms.
As to dilation, dilation is real and it does affect clocks for someone that has achieved a higher velocity. It is not an illusion. Dilation is not something that is experienced, but rather is observed. They have to adjust the clocks of satelites because even if, and because of being in a higher orbit, and therefore faster velocity, their clocks run slower than on earth, so a calculation is done to distinguish the difference. When astronauts leave earth and come back, their clocks are always offset from when they left. If time dilation was not an observable phenomenon, that would not happen. Time dilation is very real, but can only be measured, not experienced.
I saw cool world's YT video on this topic and it was a great video, but I had hard time understanding the causality breakage. Your video almost made me understood the issue of causality breakage.
Right on man, keep it up.
I would like to see the 2 second delay between me moving the laser pointer and me seeing, how the dot moved. A true real-life lag.
If you like these kind of things, try with a garden hose.
‘
it's 200 ms or a 5th of a second.
The reality is a little different, although not much different from the thought model. As Florian advises you, buy a garden hose and water the lawn. If you move sharply to the right or left, the water flow will bend, which in a small way also happens with a laser beam, so you have to include this laser pig, which is delayed, in addition to the speed of the light sent and reflected by the reflection.
The universe does have lag though. Since information, even forces including gravity, have to travel at c. If you were to summon a second moon from nothingness, it would take at least a second for it to "realise" that it's actually supposed to be attracted by the Earth and change its trajectory and start orbiting/falling. Just like these cartoons, if you see what I mean.
The sub space analogy had the famous "who's on first" come to mind. For those younger look it up by, Abbot and Costello. Back to the subject. It's interesting but the interaction between the two units are not within one time frame, but a third party observation. There are actually three separate frames of time. Causality intact.
Here's a classic from Spaceballs: th-cam.com/video/nRGCZh5A8T4/w-d-xo.html
There are always an infinite number of reference frames that could be observing this experiment. Adding more does not change the fact that the person on earth can send a message to the ship and get the answer before they sent the question.
Do you know it is a meditation to watch your videos. They just make no thoughts situation in the mind Sir. all the best and keep it up Sir. ......
I think the issue here is an incomplete view of reference frames. Einstein stated that all reference frames are equal but that’s likely not the case. In your example it’s likely that it will ultimately be shown that only the rocket is actually moving through space at the velocity stated and that even though the rocket could send a message instantly to the earth the message would always be received at the relative time on earth (i.e. if sent 2 seconds for the rocket, received 4 seconds for the earth) and that any message from earth sent would be in this example received at half the time on the rocket. Although time is moving slower on the rocket, there can be no time travel.
I've now seen this example a dozen times. There seems to be a flaw in the initial condition. For that STL ship to get up to speed it has to accelerate and that acceleration forces the clock to be slower so the paradox does seem to go away. It's the same as the twin paradox when they come back together one is definitely younger.
In fact the only way this example works is if the spaceship popped into existence at that speed which cannot happen.
Yes. That was what I was thinking. Wouldn’t bob see earth’s time moving faster. Unless I am missing something.
@@Need_better_handle Both perspectives see the other moving away from each other, so the outcome is always the same.
Doesn’t work like that at all. Simply accelerating does not stop the paradoxes
From Alice's perspective Bob's time ticks slower (9:56). From Bob's perspective Alice's clock is running slower (10:32). That means both are right and wrong at the same time. And from the instant message's point of view, both Alice and Bob are moving at the same speed in opposite directions thus keeping the same timestamp. (something like pause at 13:09)
This is exactly my thought!! I still can't understand how causality breaks. This contradicts all explanations about Alice having aged at a faster rate than Bob. So, what is it? I feel that I am either missing something or the explanations are not complete.
Even looking at your point of view (where they are both moving in opposite directions to each other at half the speed mentioned) where both of their clocks tick at the same time, there's still the problem with how much Alice has aged compared to Bob.
It is a fact that Bob is traveling faster than Alice since they both have a reference frame (Alice) and also, by the time bob gets the message and replies back, for Alice some time has passed but Bob is not getting the message in the past, it looks like that because his time has dilatated. When he receives the first message at his 2 seconds mark, it's still 4 seconds for Alice, then, when he replies back at his 4 seconds mark, the time dilatation has made Alice's time to be 8. Because if we stop the rocket right there, Bob is 4 seconds younger than Alice, hence, the message is not traveling back in time, just from a different frame of time. I see it almost as sending messages back and forth between 2 people at different time zones.
Anyways, I've seen and read at least 5 articles and videos about FTL communication and how it breaks causality and still can't understand how it happens to break it, because it contradicts all the other explanations about how you can basically age at a slower rate inside a ship that moves at relativistic speeds.
@@nosmirck Nice one with the Time Zones analogy. Usually the causality break statement is based on the established rules of the frame of reference. It's easier to dismiss the idea based on resulting paradoxes than to make it viable by adding extra unknown variables to the frame. Maybe one day quantum computing and AI will give us another perspective on this topic.
No. Bob and Alice are not moving at the same speed from each other. Bob is accelerating. Alice is not.
I noticed two flaws in this explanations. Arvin uses the explanation of light as he just freely chooses at his convenience to explain the speed of light and its limit. Once he says that light is an electromagnetic wave and once that it is photons. When Arvin explains the fields that are the root of forces he actually uses two electrons nagatively charged. And sure enough in such situation the two containing mass and charge objects will cause forces to act. Any mass object can be made to exert a force. But suddenly Arvin switches to light being an electomagnetic wave and then again to photons as he pleases. So, if Arvin chooses light to be photons and he further explains that photons are massless then, how can something massless exert any force? Photons have no charge either. So, again, how can photons create a force? But Arvin in a snide way swithes to waves. Arvin! Make up your mind! You can't have massless and chargeless photons exert a force.
Another point is special relativity; there is one big difference in Alice and Bob situations. Bob flying away with the 0.87c experiences ACCELERATION whereas Alice does not. So, by twisting the time axle is wrong for Alice because she did not experience acceleration. I do not agree with Arvin; Alice or Bob can never receive a message before it was sent. It is just the manipulations with the graph which does work one way when Bob is accelerating but does not work for Alice because she is at stand still relative to Bob. Arvin uses a bit of sophistry here and a slight trick in mathematics. More on it here in a related correspondent:
It is all fine and dandy. As it was stated here: light carries energy. Well, not so fast. If light is a wave then we can consider that as energy. But if we consider light as photons then we have a problem. Photons are massless. Therefore, how can a massless photon knock a free electron from a metal? I think this rather now has something to do with the observer who ''controls'' if he/she wants the light to be a particle or a wave. And therefore, also we come up to the paradox of Schrodinger's cat. Something massless cannot exert force and therfore cannot have energy and cannot do work. Massless photons cannot exert force either. How do they create photoelectric effect? That just does not make sense. Unless we have an active observer/experiment conducting person which leads us to the duality of light and the delayed choice experiment in Alain Aspect method. E=hf only pertains to frequency of light and the Einstein's KE for electrons is dealing with, well, electrons which have mass. So, the Einstein's equations using electrons with mass cannot be applied to kinetic energy of photons which have no mass. Something that has no mass cannot have kinetic nor potential energies.
@@ericephemetherson3964 Yes, I know that. It was another point of view. Let's say Bob uses a warp drive engine, meaning that he isn't really accelerating, but the space around him warps. Although both of them are relatively stationary the distance between them grows.
@@yinfest I would agree with you explanation, too.
This is the clearest explanation of causality paradoxes I've yet seen. Great job!
This is the clearest explanation of how FTL communication can cause causality problems that I have ever seen. No arbitrary line is drawn that the narrator just states is FTL. Every line of the "subspace" message is explained. Good job, sir! Thank you, sir. BTW, in Star Trek TOS episode "The Enterprise Incident," Tal says, "The subspace message will take three weeks to reach Starfleet." I don't know where the instantaneous bit comes from. Maybe from one of the later series. No matter. Your explanation is superb! Thanks again. -- Recall that what's instantaneous for Alice is not, in general, instantaneous for Bob, and vice versa. And that's part of the answer.
I must not be understanding Something though… because even how he described it it doesn’t seem like a paradox, only an illusion based off of each others reference..
@@stonesymmetry How is receiving a reply to a message you haven't sent yet, after which you can decide to not send the initial message, just an illusion?
its the difference between group velocity and phase velocity. A wave approaching the shore has a group velocity less than c. But the a point of constant phase on a breaking wave, parallel to the shore, can move at speed greater than c, as it moves from left to right.
Sorry but there is no paradox: If you send a question with instant communication from 4 it will arrive at 4. The time runs the same for both. The only difference is that visually it looks from Alice perspective that Bob's timer is at 2 (if she was able to see his timer) while her's is at 4 but in fact both timers are at 4 in the same moment. The same way Alice timer would look like it is at 2 for Bob when actually both are at 4. It is just a visual thing what the eyes see because the light travels at c.
You are excellent at delving into very deep topics and explaining them in ways that people can understand. Always great content as usual
E=mc2 has stood correct for over a century. He’s just using big words like “delicatessen”
If Bob turned around part of the way during his trip and returned at the same speed, then according to Alice he should have experienced half the time she did. But according to Bob, Alice should have experienced half the time he did. Isn't that also a paradox? Or is the direction of motion change the speed of time?
I think as Bob slows down, Alice's time will 'catch up' with him, and then overtake him, so Bob will see Alice age quickly during deceleration. It's like Bob is riding in front of a wave of light that struggles to catch up with him, so it looks like Alice is aging slowly. Then when Bob slows down, the light catches up normally and Alice looks like she is aging normally. Then when Bob turns around and comes back, he smashes into the light at nearly twice the rate compared to if he was stationary, so Alice appears to age twice as fast (or is it 4x as fast, as time for Bob also moves twice as slow?). In any event, the paradox solves itself and, when back on earth, both Alice and Bob agree that it was Bob who has aged slowly. That's probably not quite the right explanation though.
yea, basically on return the oppisite happens. But this was a simple explanation of it because it doesnt take into account time dilatation.
So there is one thing by I’ve always been confused about when it comes to relativity. If the astronaut leaves earth according to earths reference frame he’d age slower to those on earth, but on his reference frame earth would be the thing moving and therefore be experiencing the opposite effect and people on earth would age slower then the astronaut, this seems like a contradiction, the only solution I can see is that if they met again their clocks would sink(excluding gravitational time dilation) but any signals exchanged before then are just temporally shifted. Am I correct in this analysis of relativistic time dilation, or am I missing a critical understanding of some part of it?
I like how you referenced the light dot on moon to give an example of faster than light speed.
First of all, it takes some time for light to travel from Earth to the moon. The longer the distance, the more time it would take for light to propagate.
Secondly, try firing water from a hose pipe at your car in a circular direction. You'll find that as you increase the distance, there is a time lag between the expected and observed propagation. This is because the circumference of the circle you're moving your hand in is much smaller than the one you're plotting on your car using the water jet. The water jet in between would resemble a helical shape if it is continuous.
So yes, the expectation that light would travel instantly to moon as and when the laser rotates on Earth clearly isn't true, even if we do not consider photons. Thank you for pointing that out.
Why do people say "if speed of light is infinite, then you don't get to use special relativity anymore" and then turn around and say "infinite speed means that due to relativity e=mc2, no mass can exist, therefore speed can't be infinite"?
Trying to use that equation means you haven't thrown out relativity.
Science is amazing
Wow! I finally understand why we can't break the speed on light!
Amazing! Thank you!
I’ve just ordered a laser pen thanks Arvin. I’ll see if I can speed up light with radiation
Explanations like this can shed years of ignorance in minutes. Now here’s a channel turning back time on falsehoods. Wonderful; thank you.
Your channel is blessing for those of us who are curious souls!
Question- some galaxies are so far away from us that even at the speed of light we could never get to them. If the speed of light is actually the speed limit of causality in the Universe rather than just a speed limit, then how could galaxies this far away possibly have any gravitational effect on us since we are moving apart faster that the speed of causality could reach us?
Space has no speed limit..gravity is spacetime
@@stanleysteamer3212 wrong, gravity have a delay from different point on the universe, gravitational wave travel at the speed of light, so the real answer is, if there is some galaxy that is travelling faster than speed of light from our perspective thant their gravitational effect would be none to us but also we wouldn't be able to see them for the same speed limit
I love how they always tell me how a guy flying in a rocket at super-speed will age slower than us on earth... but in the next sentence they tell me how everything is relative and to the guy in the rocket, we are travelling at super-speed, thus we age slower than him.
How can time for the rocket to be flowing at half the rate of the earth, yet when you swap perspective it's the opposite. How can they both be experiencing time faster than the other?
@@mityace "Uh... shut up."
This is because the light from both, the ship to earth, and from earth to the ship will be outdated by years respectively, but appear instantaneous from those perspectives. Likewise, for very large distances, we are looking at the past of other galaxies because the light from their present hasn't reached us yet, and vice versa
@@mityaceEasy. We are both seeing each other’s past.
The relative part here means the guy in the ship will not feel like he is aging slower, he won't look at his clock and see it tick slower. Only an observer outside the ship will see him age slower.
Just to be picky: Where we see Alice moving from Bob's perspective we see that his engines are firing. This is probably a mistake because it would mean that he is accelerating and hence not in an inertial frame of reference 🙂
You are absolutely correct.
I noticed two flaws in this explanations. Arvin uses the explanation of light as he just freely chooses at his convenience to explain the speed of light and its limit. Once he says that light is an electromagnetic wave and once that it is photons. When Arvin explains the fields that are the root of forces he actually uses two electrons nagatively charged. And sure enough in such situation the two containing mass and charge objects will cause forces to act. Any mass object can be made to exert a force. But suddenly Arvin switches to light being an electomagnetic wave and then again to photons as he pleases. So, if Arvin chooses light to be photons and he further explains that photons are massless then, how can something massless exert any force? Photons have no charge either. So, again, how can photons create a force? But Arvin in a snide way swithes to waves. Arvin! Make up your mind! You can't have massless and chargeless photons exert a force.
Another point is special relativity; there is one big difference in Alice and Bob situations. Bob flying away with the 0.87c experiences ACCELERATION whereas Alice does not. So, by twisting the time axle is wrong for Alice because she did not experience acceleration. I do not agree with Arvin; Alice or Bob can never receive a message before it was sent. It is just the manipulations with the graph which does work one way when Bob is accelerating but does not work for Alice because she is at stand still relative to Bob. Arvin uses a bit of sophistry here and a slight trick in mathematics. More on it here in a related correspondent:
It is all fine and dandy. As it was stated here: light carries energy. Well, not so fast. If light is a wave then we can consider that as energy. But if we consider light as photons then we have a problem. Photons are massless. Therefore, how can a massless photon knock a free electron from a metal? I think this rather now has something to do with the observer who ''controls'' if he/she wants the light to be a particle or a wave. And therefore, also we come up to the paradox of Schrodinger's cat. Something massless cannot exert force and therfore cannot have energy and cannot do work. Massless photons cannot exert force either. How do they create photoelectric effect? That just does not make sense. Unless we have an active observer/experiment conducting person which leads us to the duality of light and the delayed choice experiment in Alain Aspect method. E=hf only pertains to frequency of light and the Einstein's KE for electrons is dealing with, well, electrons which have mass. So, the Einstein's equations using electrons with mass cannot be applied to kinetic energy of photons which have no mass. Something that has no mass cannot have kinetic nor potential energies.
@@ericephemetherson3964Massless photons can exert a force, they carry momentum.
@
Arvin Ash
Hi, I watch this video and there are 2 big mistakes.
1. We don't know what the speed of light really is. We can measure only with reflection from mirror. The speed of the light can be 2C in one direction and 1/2C after reflection and we have no idea how to measure it.
2. Bob and Alice communicating with infinite transmission speed when Bob is moving with 0.87C. Your mistake is with their clocks. You cannot change one part of equation only. If you postulate that the communication speed between them is infinite, then you have to change their clocks that are ticking with the same speed
One last info for you. The matter can't reach even 0.87C, because it will be ripped apart if C is the real limit. Why? Because electrons are moving in the atoms and can't reach that speed, they must slow down. It means, they will start to fly freely, they breaks away from the atom's core.
Take a look at this: th-cam.com/video/pTn6Ewhb27k/w-d-xo.html
Imagine a load on a wire that is already 300km away, when the shadow is cast on the arc interrupting the flow of electricity does the load drop to 0 300km away? No.
The shadow is delayed by all of the photons already in transmission; it’s not instantaneous.
9:40 it really explains space time, time dialation, special relativity in a simple way good job!
Even Zefram Cochrane did not know he invented an impossible machine.
That’s the only thing about Star Trek that gets to me, the communication stuff, and a few other things like travel speed sometimes seems to be inconsistent. This is episode wise.
When we look from one star to another our pupils travel light year's :)
@Kelvin It's a joke lol.
@@SyenPie A bad one. It is the point you are looking at that has travelled.
@@garethhanby The only relevant motion in this scenario is the motion of light from the star being observed from (x) light years distance. You are seeing it as it existed however many light years in the past when these photons started their journey.
@@MarinCipollina doesn't that contradict the whole point of casualty this video just discussed...?
Your explanation with just the 2 reference points is the easiest I have seen so far. I have seen a few very confusing diagrams that didn't make much sense too me because i forgot relativity and how time moves differently
Tilting the graph has been the clearest explanation of this I have seen thus far. Well done.
13:56 thanks for the special mention😂
😛
Indians will be Indians
@@somyajain593 I'm not.
Amazing video! I'm sure you already made a video about quantum entanglement but I think it would be an excellent topic for next time as it seems to break everything that you spoke about in this video.
Yeah I agree. He says it doesn't because the entangled particles form 1 single object. IMO it doesn't matter if they do form 1 object, it's still FTL cause & effect
what if when he doesn't send the message, he creat a parallel universe or even creates a new consciousness within himself? we did not test this theory whatsoever
Hmm? Einstein...said all of this over a hundred years ago......Nothing new here then!? “Consciousness” still one of the greatest Unknowns.....Nothing new there either.... It is repeat...get paid...repeat get paid....Nothing new there then!? Next repeat..get paid!
@@1SpudderR this is a bot? What do u mean by this words?? I dont get it
DEBUNKING???
Claims of speed of light.
It is your active tool of masurements that can't violate the speed of light. But objects can violate that speed of light.
(e.g.Two objects approaching each other at 0.6 the speed of light. How fast do they actually pass one another? )
Both objects pass each other faster than light. Attempts to measure this is trapped/limited by the tool which you are using to measure it.
Your problem is:
1) You are using an inferior ruler. that is subject to relativity itself.
2) The two objects perception of space distance are no longer equal.
3) You don't realize that difference in the size of space bcs it only manifests at speeds we are not use to.
Those that use the cone of time example and paradox of information fail to address one important thing. They fail to explain why/how point A exist both in the future and in the past at the same time. They fail to address that their tool of measurement and its limits are creating the paradox.
I like how you at the end say you hope someone comes along to prove it wrong or incomplete. In the past we constantly say things are impossible until we end up doing it. It's entirely possible that it's only our limited understanding of the universe is why we think faster than light travel is impossible. I think we need to be very careful when we say something is impossible.
In comes Quantum mechanics, with Quantum Eraser experiment. Maybe close to nothing is good enough, who knows.
the Quantum Eraser doesn't allow to transmit information faster than light. In fact no entanglement based thing does.
@@ДаниилРабинович-б9п Not thar I am physicist, but if I remember it right, isn't it so that they are connected and can be measured no matter the distance between, but as the result is random and you can't affect it, you don't transfer information
@@andersolsson148 exactly
@@ДаниилРабинович-б9п Like the spin can't be affected, but if you take the Quantum Eraser experiment, I still don't get it. Like it basically knows if it will be erased or observed even if it gets erased after the entangled particle hits the screen. Like how... To make it even more complicated, what happens if you put in a mechanism which recognises the interference pattern so if it predicts that it will be erased, the mechanism observe it. But then you get a new version of the grandfather paradox.
@@andersolsson148 th-cam.com/video/8ReFpvPGFeg/w-d-xo.html - Warp drive
Just as there are different experiences of time due to relativity, what if there are also different causal chains? What if these temporal paradoxes are possible due to different timeline threads or time forks that occur? What I'm saying is what if time travel produces new timelines such that you can actually go back in time and kill your grandfather but that wouldn't result in your death but the death of another you in the original time thread; because the act of going back in time put you in a new and separate time thread independent of the one from which you came.
8:38 "There is no absolute "now". The "now" depends on a reference frame."
Kinda sounds to me like quantum theory. The current state of a particle is fully dependent upon the circumstances that led to its being observed.
The perspective of the observer is still the catalyst, in both Relativity and Quantum Theory.
Interesting 🤔
Actually in quantum mechanics time is linear, what is a problem.
Indeed Quantum mechanics via the Feynman path integral can actually be formulated in context of a relative observer with an entanglement cone that propagates outwards faster than light(but is unable to transmit information)
One fascinating prospect within Wolfram's physics project is that the constraints of a Turing machine operating on some network in the limit as the network becomes extremely large can be shown to reproduce Einstein's Field Equations where distance is derived from the rate of information propagation but if allowing the network to update in all possible unique configurations in super position it automatically reproduces the Feynman path integral of quantum field theory within different possible unique states branch out from each other not unlike the many worlds hypothesis except where these are only reference frames and not truly separate i.e. they will due to branchial metric curvature (the branchial component of the metric tensor turns out to be the wave guide of Broglie-Bohm theory the rest of this interpretation doesn't really hold though) gravitate towards each other resulting in different branches converging or collapsing resulting in an apparent probability of outcomes.
Naturally this is far from confirmed and any extraordinary claims require extraordinary evidence but this model places some strong constraints on quantum computers and has an interesting alternative possibility for inflation based off of the emergent nature of space allowing the number of spatial dimensions to change via a phase transition. But if this is true then rather than molding gravity into quantum mechanics quantum mechanics will fold into gravity with the microstructure below normal space being very very weird based on causal hypergraphs and geometric edges. It gets really weird with further conclusions being hard if not impossible to draw without the development of entirely new areas of mathematics.
@@Dragrath1 Have you ever heard of punctuation? You run-on sentences are unbearable.
@@Dragrath1 - Never seen it that way and also the fundamental theorem of QM, Shcrödinger's equation, requires of linear time.
@@FobbitMike - He only missed one stop sign, after "Indeed" and before capitalized "Quantum". Obviously a typo.
Imagine Alice and Bob are the same age. Alice stays on Earth and Bob flies away at relativistic speeds and then flies back to Earth. In this scenario, Bob would age more slowly than Alice, since time is passing more slowly for him. Bob would then be younger than Alice.
At the same time, from Bob's perspective, he could say that Alice is flying away from him at relativistic speeds. This would mean that Alice should age more slowly. Thus when they reconnect, Alice would be younger than Bob.
Both of these things cannot be true. So, something is wrong with stating that their relative motions are equivalent. The explanation used in this video seems to have the same problem.
Also, spacetime is an actual thing. It is an object, not just an abstraction. Alice's speed, relative to spacetime, would be different than Bob's speed, relative to spacetime. Spacetime is not absolute frame of reference, since it also moves, twists, expands, etc., but still, Bob's motion and Alice's motion would not be equivalent because their speeds, relative to spacetime, would be different.
I know folks will say that this is wrong, but I do not (yet) see the flaw.
While Bob is moving, from his perspective Alice is aging more slowly. However, if he comes back to earth, and stops moving so that both Alice and Bob are not stationary relative to each other, then they will see that Bob has aged more slowly.
@@ArvinAsh I very much appreciate your taking the time to respond!
If Bob aged more slowly, then Bob flying away from Alice is not functionally the same as Alice flying away from Bob. It seems like that would take away the justification for saying that we can treat their two perspectives as equivalent, as the video does at 10:30.
When we say that Bob would perceive Alice's time as moving at 1/2 pace, we have to ask what we mean by perceive. Is he perceiving via light speed communication? Because the light speed delays would throw off his perception. If we are not allowing for instant communication, then Bob would have no means of perceiving Alice's flow of time.
The only way they could truly perceive each other's flow of time would be retrospectively, when they reconnect. When this happens, we see that their motions away from each other were not equivalent, because Bob actually aged slower.
I found an explanation that explains that the twin paradox is explained by acceleration and general relativity: th-cam.com/video/ERgwVm9qWKA/w-d-xo.html.
However, it does not really provide an explanation for why those things make the difference. I will keep reading on this.
@@ArvinAsh I still cannot get past the notion that it seems like we are mixing up two different things. So, Bob really is experiencing 1/2 the flow of time. He might perceive Alice as moving at 1/2 speed, but that is only a misperception brought about because of a signal delay from the light speed limit and because his own flow of time is slower.
In terms of instantaneous communication, Bob's instant communication at his 4 seconds would not reach Alice at her 2 seconds, it would reach Alice at her 8 seconds. The realigned chart at 10:30 does not represent what is actually happening, just what Bob would perceive due to communication delays and changes in his flow rate.
If you know time dilatation, you know about existence of time contraction. It is every day event around us, from our perspective. Light is not the fastest limit in science.
Thank you! This explains the concept of "no universal now" way better than static diagrams. Question: If Bob were to stop moving relative to Alice at 8 seconds, would their "now" become simultaneous again, such that if they could communicate instantaneously from that point onward, there would be no backward propagation of the signal through time?
If Bob stops he gets the same notion of "now" as Alice. So it's t=16 for Alice and t=8 for Bob, these two events are simultaneous now for both of them, and in a second it's t=17 for Alice and t=9 for Bob etc. Bob is younger, he lived 8 seconds less.
@@thedeemon Thank you. I suppose we know it's Bob who's youngest, because he's the one who had to decelerate to get back in sync with Alice's time line. Still, a difficult concept to wrap your head around --- that Bob's deceleration at a distance from Alice re-synchs their time. Now THAT is "spooky action at a distance!"
@@alansewell7810 Maybe. Don Lincoln on Fermilab's channel did a video a few years ago on the "twin's paradox" (i.e. why is it the "moving" twin that ages slower) where he explains why there really isn't a paradox even if there is no acceleration / deceleration involved. Actually he did two videos: one with the math and one without. Basically Bob and Alice still don't have a consistent "now" even if not moving relative to each other, because Bob has been in two different reference frames. So you can't even say one is younger than the other "now" because there is no "now".
@@rancidbeef582 I watched Don's videos on that, and will watch them again, now that you've jogged my memory. Don knows his Relativity but is less lucid than Arvin Ash. One thing I have picked up from the discussions on several TH-cam channels is that when objects begin relative motion starting at a time "X," the "X" becomes a different time for each of them, because the Lorentz transformation modifies the time when an event begins as well as dilates it. For example, if the sun goes supernova at 12:00 and a spaceship leaves earth a half the speed of light, then the wave front must overtake the moving spaceship and stationary Earth at 12:08, since the velocity of light is the same for both of them. However, the moving spaceship, which is overtaken at twice the distance of Earth, perceives the supernova originating at 11:52. If instantaneous communication existed between the Sun, Earth, and Spaceship (assuming no vaporization by the supernova) then at the moment the supernova wave front reaches Earth, someone on Earth could relay a message to the spaceship, the spaceship relays it to the Sun, and the Sun bounces it back to Earth 8 minutes before it was sent. My understanding of what would happen if signals could travel instantaneously.
@@alansewell7810 Ok, that scenario just broke my brain. I suspect you're right. I'm going to have to think about it for a while.
My brain hurts. Thank you for amazing content👍👍
If warp drives and hyperdrives are physically impossible, what about a ‘jump drive’? This science fiction idea postulates that a craft can create an alternate pocket universe around itself and remove itself from normal space - thus circumventing real space. There is also Dune’s notion of 'folding space’. Technologically, this is pie in the sky of course, but is it hypothetically possible?
I'll have more on this in the next video. Stay tuned.
Isaac Asimov postulated hyper space where whole universe was a single point
Star Trek's warp drive involves a warp bubble that collapses space in the direction of travel. The ships don't travel faster than light in the usual sense.
If alice time goes slower from bob's perspective but bob's time goes slower from alice perspective. If bob returned to earth who would be older?
Best explanation about why FTL breaks causality I have ever seen. Much better than the explanation that chatgpt gave to me for example.
So how does this impact the results of the quantum eraser experiment?
Quantum eraser fundamentally doesn't involve any backwards causation. Some articles/videos are unfortunately confused or misleading on this point. Its just something that superficially appears retrocausal based on snuck-in classical thinking. Quantum mechanics is perfectly consistent with special relativity, as formulated in i.e. the Standard Model (relativistic quantum field theory).
@@cloffff Pls explain more for me. If the which-way detection is after the wave/particle goes through the slit why does it collapse the probability function?
@@jtcorey7681 It doesn't work because the detector sends a message back in time to tell the photon which way to go; it works because you can't know which detector got set off until after the photon chose its path and had time to reach both detectors. It's an oddity because we can't tell the difference between the two: they both look the same.
Okay the part I’m still struggling with is, why would his reply go to her in the past?
So if she sent a message at her 4 which is his 2, wouldn’t his reply at 4 be her 8?
Because of Bob’s time dilation even though the clocks were synced they would go out of sync over his journey. So the more they spoke the quicker time for Alice would move.
Agreed. This is the part I do not understand either. When he flips the graph to Bob's perspective it looks almost as if Alice's time is going 1/2 f Bob's.... Shouldnt this still be double?? How did Bob's 2 seconds all of a sudden become Alice's 1 second?
Since no one replied it has to do with frame of reference. Remember that the signal is going faster than light
@@michaelhearn3043 Bob's 2 seconds became Alice's 1 second since by BOB's point of view, HE IS the one who is standing STILL while ALICE is moving. That is from HIS point of view (which is equally valid as Alice's point of view, this is why it is called theory of relativity. There is not absolute truth of who is moving and who is stationary. It is all relative and based of perspective).
@@giannisms1861 i know this is like 5 months old and i'm probably pretty off base here but isn't there a difference in energy regardless of perceptive, bob is moving through space time with enough force to create time dilation while Alice is not, shouldn't that be the end all be all idk how perspective matters other wise time dilation makes zero sense.
@@porky5567 well i get what you mean but i am pretty sure the theory of relativity does not recognize WHO is moving, it only recognizes that two people are moving relative to each other. This creates paradoxes, maybe there is a way to tell which one is legit moving thorugh space and who is not, but as of now i beleive we cant tell.
A wonderful invitation to all the Zefrins out there!
Zefram
This is probably my favorite youtube channel. I love how Arvin explains things
Instant communication should follow the world lines so it is the same from any frame of reference. Because when you bend space you also bend the world line. So in essence ftl can not be put in these diagrams as easily as other movements.
"My heart hopes that a future Einstein...proves these equations to be incomplete and finds a loophole that allows us to keep hope alive. Zephraim Chochrane, I hope you're out there."
You and I both, brother. You and I both. TOS rules!
1:18 If the speed of the laser dot can be any and it is possible to code information according speed or direction of such dot why not to use two coder-decoder (on moon and on earth) devices to communicate faster than ligth? I think this example with moving laser dot needs more detailed explanation ;)
At first that’s what I thought too but when you think about it more carefully, it isn’t instantaneous
Because the movement of the dot shining on the moon will be delayed compared to the movement of the light from earth.
So if you moved the dot from the left side to the right side of the moon very fast, it will still take whatever amount of seconds for it to actually move. The dot will move at the exact same rate as you are, but by a few seconds behind. So in the end, it’s the same as just communicating normally through radios or whatever.
So what happens if Alice and Bob are entangled? Wouldn't this break causality?
No communication would be able to take place if entangled.
@@ArvinAsh The article Bells Theorem for Temporal Order published in Nature Communication, August 2019 seems to be saying that cause and effect do not necessarily follow a classical model in superposition state in a gravity field. I could be misinterpreting what they are saying but it seems that quantum gravity may have not have classical cause and effect in order to integrate relatively with quantum mechanics.
This is the greatest explanation I have ever watched about relativity, thanks
If there is a time delay between switching on the laser and seeing the dot appear on the moon, then this is like holding a garden hose and pointing it at a dry wall. Even if you flick your wrist real fast, the wet streak on the wall will lag behind.
Also, the dot is not an object. It’s merely an indication of where the laser’s photons are reflecting from at any given moment.
Quick question: For your example at ~ 11:00, is this not simply a perception issue? shouldn't Annie and Bob both see each other at a time sooner than they are actually at, but be able to agree that those times are earlier than present for each of them with ftl communication?
I don't quite understand you but i dont think they would be seeing each other earlier than they are actually at because the moving observer will just experience time slower than the inertial one but to the moving observer time will just tick normally. So they won't be looking at each other's past selves but rather present selves, just that one feels time to pass slower than the other
No, because there is no absolute time. Time is relative, as he explained in the video. It is hard to wrap our head around the idea, but it's true, it's been proven many times.
This episode is a Classic (with a capital c.)
I have a question, though. I've always thought that if you left on a spaceship going a high fraction of the speed of light for a while, that you would return to Earth and all your friends and family would be long dead. But according to Bob and Alice around 11:48, 4 seconds for her is 8 seconds for him, meaning that if he returned at that moment, HE would be older than SHE would be? I thought I understood the time dilation effect until your video. I get that from her perspective, he would barely age and she would age normally. However, even from his perspective, he would age normally, but she would age like REALLY fast, right? What am I missing?
I see this way: time is actually absolute. Alice SEES bob slow because he is going away from her at near light speed, as bob also sees her slow. If bob were to do a 180° turn, each would SEE the other fast. When they encounter, both would be the same as if they were always together, with clock times alike. In truth, they were always at the same time, they were only PERCEIVED by each other differently.
This would only happen if one side were the decelerate back into the same velocity frame. If both sides decelerated with the same force, the effect won’t happen
The best explanation I heard about causality in the context of special relativity.
In my engineering degree we did an experiment to show that phase and group velocity are different ... I am sure any scientist could explain that this can make something appear to be going faster than light.
A simple explanation is a sea wave hits a sea wall at an angle ... the wave is approaching the wall at a particular speed ... however when it hits the sea wall it will APPEAR to travel along the sea wall faster than it is travelling towards it. If the sea wall is nearly 90° to the group velocity (direction) of the wave front then the phase velocity of the wave front along the wall may be many times faster.
Transform this to a wave front of light hitting something at not quite 90° and we can observe a phenomena appearing to travel many times faster than light ... but it is an illusion!
One may say that the sea wave is also an illusion as the water molecules in an advancing sinusoidal wave actually travel in a circular motion and do not advance in the direction of the wave!
NB this does not apply to breaking waves ... although the impingement of the breaking wave along the wall will be faster but just an ugly step function and not a nice wave.
Can I also point out that the light used to observe this water wave phenomena is 'relatively' instantaneous which is not so with light waves ... hence every time you appear to have won and transferred information faster than light you haven''t as the observation of that information can only travel at the speed of light in your reference frame.
Arvin, there is an objection: Alcubierre metrics allows FTL (Faster Than Light) travel of mass and information. And this is compatible with relativity, since it only applies within the same frame of reference aka spacetime. Whenever a piece of spacetime is cut out from another (including all quantum field layers), there is no longer a speed limit, since relativity vanishes and there is no longer any interaction between these two pieces of spacetime. In other words: When Bob went with 10c to Alpha Centauri during the time of travel, he could not exchange information with Alice. Upon arrival, his clock showed the same time as that of Alice. When he returns with the same vehicle, the same happende agains - and nothing has changed. This has far more implications, one might think, since, we have already such a situation, where the visible edge of the universe expands faster than the speed of light. Therefore less and less objects will be visible in the future. Therefore stretching spacetime with FTL-speed seems to break down relativity too.
This still doesn't answer what happens with the instant communication scenario.
@@saucevc8353 There is no such thing like «instant communication». Information can only travel faster than light, when either i) you bend spacetime - as you can pull a long stiff wire without any delay - but gravity itself travels with c. This is a very important property of spacetime, otherwise two celestial bodies, that orbit each other would experience a torque due to lag of gravitation field update. ii) Whenever Alcubierre metric is engaged. In this case, there is no longer any connection between the spacetime of the universe and the cut-out-portion of spacetime, used for FTL-travel. This is an important prerequisite to avoid all sorts of relativistic paradoxons. Nobody can violate physical laws, but circumvent it by suitable technology.
@@debrainwasher But you can just launch an alcubierre drive from the time dilated non ftl spaceship containing a message. Boom, ftl communication. Which creates a paradox. Is the time it takes for the drive to arrive at it's destination determined by the viewpoint of the launcher, or the viewpoint of the recipient?
The Albuquerque drive! Albuquerque -- the city that's probably spelled wrong. --Dave Barry.
I don't understand this Albuquerque drive. You still violate causality with it, so I don't see how it can be comptiable with SR.
I hit the 👍 faster than the speed of light.
In Event Horizon, a sci-fi horror movie, they bend space in order to travel faster than light.
That's just shortening the distance
between two points in space aka a wormhole not increasing speed past that of massless particles.
Yeah, and how did that work out for them? :P
Seriously, I love that movie.
@@MrDJAK777 Quite right.
@@TheSwiftFalcon My second favorite sci-fi horror movie after Alien.
@@sylviarogier1 I can't fault your judgement there. :) I'm not usually a fan of horror, but I make an exception for those two. The atmosphere and tension in both are just brilliant.
This whole point is moot, since we do not know the 1 way speed of light. That’s right, we have no clue how fast light travels from the source to our observation. And with current tech, we can not know this. We only know the reflected speed of light or 2 way, from source to reflected point to our observation. Einstein acknowledged this in his papers on the subject. He postulates that from to the source to our observation could be instantaneous or 186k mps or anywhere in between. He chose the constant speed 186k mps round trip. This does not make it so and if it is different from the constant it does not break physics, it changes the answers.
Suppose light from the source to the observer was instantaneous and then from the reflected point to our observation would mean light speed would be half of what we think it is. Also if it is instantaneous from the source, we would be seeing the universe in real time. The one way speed is unknown. The best we can do is say that the speed of light is always 186k mps and that is and assumption. Proof is when you look at a prism light not only bends but changes speed and or wave length and we can see this in the different colors coming out of the prism. Truth is we have no way of knowing what effects interstellar space will have one light or any type of transmitted data. We don’t even really know how far away anything is past 400 light years. Best guess after that.
I think the main flaw in the Alice and Bob example is the time slowing on the traveler perspective...that would mean that the light is quite instant even from sun to us, and so on so a future Einstein will find something based on this... Also in this example the speed of Alice isn't 0 in absolute terms, cause the earth is moving around Sun, and after this the whole solar system moves in the galaxy and so on so the proportion would not be 1/2...
Yeah, objects in rest shouldn't be moving regardless of the observer. Moving takes energy.
If I move and see earth and universe moving away from me, that doesn't mean that I exert energy to every object in the universe so that they can move away from me.
hold on i thought time dilation was theorised to reconcile the finite speed of information with the lack of an absolute reference frame. surely then, faster than light communication would disallow time dilation.
eg, if bob and allice have clocks that run at the same speed and are synchronised when bob leaves, if alice then sends bob a "subspace" communication when her clock reaches 4s bob would receive it when his clock 4s, because the subspace comms would not experience time dilation, because it isnt restrained by a finite speed. if viewed through subspace comms rather than electromagnetic comms, the clocks would appear to stay synchronized.
The subspace would appear to be to light as light is to sound. if you had a train moving some large fraction the speed of sound and you communicated the ticking of clocks with small explosives the stationary observer would hear the trains pulses to be slower and the train would hear the stationary observers pulses to be slower, however they would both see the pulses much closer to being in sync due to how much faster the propagation of light is.
Therefore if there were a method of moving data faster than the speed of light the effects of time dilation would diminish proportionally to how much faster this method is trending to 0 as the speed of the method trends towards infinite.
Key phrase: “according to physics as we understand it.”
There was another thing he didn't say and that is that our current understanding of physics is extremely closely in agreement with experiment. Where current theory fails is in extreme situations such as near black holes and the Planck length. The gaps where theory is weak have gotten smaller over time. For example, QFT has theory matching experiment to 10 decimal places. That doesn't leave much wiggle room for unknown unknowns.
@@lupus7194 I mean Newtonian mechanics was extremely close to experiment.
I could easily imagine a scenario where Alice only recieves messages from the future in the cases where she *somehow* decides to actually sends the message, while any instant where she decides not to are irrelevant cause she never recieved a message from the future in those cases.
Robert Forward wrote a book, Timemaster in which he speculated that you could travel into the past but still could not change the past. This was because the past occurred with your future self in it.
If you went into the past with the goal of changing it then the universe would make random chaotic events more likely in order to thwart you. So if you shot yourself for instance you actually shot a clone or some other unlikely occurrence would occur to stop you. It would have to as changing the past would be impossible.
It was a wild book.
He didnt take into account that the universe can create local event bubbles that can contain information and stuff. This bubble can collapse and cease to exist when some kind of paradox happen. This requires a lot of energy to create, but hey FTL speed require infinite energy to do so in the first place. This actually happen every time in quantum scale since the energy required is suf
A true teacher is one who can take something that's complex and break it down so that even a 3rd grader can understand it..
This problem occurs because we over believe that space is relative, not absolute. Because space is believed to be relative, we get time to be relative as well.
It is just like assuming a is true -> (a+C) is true as well.
Eventually when an object is moving, only that object sees the resistance of space. As we all know, space is not empty and rather object keep coming in and going out of existence in something that we call emptiness.
We cannot move faster than light as the friction of space does not allow us to. If the friction which electromagnetic waves see (the epsilon and muo values) change, the speed of light will change as well. Simple Newtonian physics.
Why do objects move slower in time when they move. This is because of atomic forces.
When proton sends electric waves to electron who is revolving around the proton, the wave has to travel to the electron before any cause-effect thing happens.
In a moving atom, that wave will take longer as the path which the causality wave has to travel will be longer due to the atom moving at a speed.
This will make one second for a relativistic object appear longer. As on an absolute time scale, it took the wave more time to reach the electron v/s it would have taken in a steady object.
Since these waves have no mass, they don't have momentum and hence unlike other moving objects, they don't follow newton's first law of motion (an object at motion will remain at motion) which applies only to objects with mass or momentum.