I just found your channel on you tube.I think your videos are brilliant.I can show them to my children aged 10 and 14 and they understand.The way you explain things is excellent!!!Cheers!!
The explanation was crystal clear until 27:00 until the issue of the symmetry of the situation was introduced. In contrast to all the previous explanations which where very careful and rigorous, I felt this was more or less brushed off although it was probably the most crucial point and should have been explained more. Anyway, please consider this as my constructive feedback, I do appreciate giving us this video, it was a great introduction to the twin paradox although it failed to solve it for me.
@@AlericResident Well...although not explicitly stated before adding the acceleration factor, you understand the paradox by having in mind what's explained at 19:15 and then adding "1+1" when the travel is completely explained (up to 27:00). The paradox is that without acceleration (being picked up instantly, which is impossible) - both can say that the other one was the one moving. That's why there is no paradox in SR (because a travel like this without any acceleration is impossible)
@@tormodi5925 If that's your excuse, then clearly you fail to understand the actual claim of the Paradox. Which is an un-solvable paradox, meaning that the SR theory is wrong.
Why do you bother about the hooks if later you're gonna introduce acceleration? I thought you were gonna explain twin paradox without the acceleration effects.
The specific relativity part makes the returning twin just a few years younger. With the book and acceleration the returning twin is 59000 years younger. They seem not consistent.
You are right that if we could reach velocities near the speed of light which includes accelerating in order to get to those velocities, then we would travel through time much faster than those who are not subject to that acceleration. It doesn't mean we disappear from space. It simply means we move faster through space time. The disadvantage, however, is that there is no known way back.
There is no evidence that anything can move backwards in time. There is a convention when drawing Feynman diagrams for antimatter to be shown as travelling backwards in time. That is a convention. It does not mean that antimatter actually proceeds from today into yesterday.
Worse, the laws of physics have nothing at all that stops you from applying them backwards, giving rise to the almost philosophical question: why is there an arrow of time that goes the way it goes?!
However, because the earth is spinning, the guy on the top of the building is moving faster than the guy at the bottom. And since a=v^2/r, he is accelerating faster than the guy at the bottom. So does that cancel out the difference of the gravitational force?
Any clock has the same effect including a body clock. If you are travelling with the clock then it will appear to keep normal time. But an observer who is travelling at a velocity relative to you will perceive your clock to be running slow.
I think this is a poor video. It raises more questions than it answers. The graphs don't make sense. Why is he multiplying twin B's clock by 4/5 when he could just as easily move to twin B's reference frame and multiply twin A's clock by 4/5? Yes he said it was due to the acceleration, but to twin B, it looks like twin A was accelerating. Also, isn't the 1G of acceleration twin B experiences in the second example equivalent to the 1G of gravity twin A experiences on Earth? Why would twin B experience more time dilation? The video should address these questions.
planet42 The effect of time dilation doesn't only occur with acceleration, it also happens when frames are moving in uniform fashion relative to each. Devising an example completely without acceleration is trivial, so no the accelerations do not explain it. Your explanation and this video are both incorrect.
@Knowledge Rules There is an example totally without any accelerations used for example at the Fermilab channel where the elapsed time of the clocks is compared at the end without accelerations. Also, one can trivially construct a scenario with 2 travelers with symmetrical accelerations that therefore must cancel out.
If there is a relative velocity between two people, it is not possible for one to determine which of them is moving. But if one of them is accelerating and the other is not then it is very easy to determine experimentally which is the one that is accelerating.
Wont the person on earth also experience being in a acceleration of 9.8m/Sec2 , how is that different from the person who traveled in a space craft with accel of 9.8 , Both should have aged the same
Both twins were subject to g acceleration during the whole journey. The first one due to Earth gravitational attraction and the second one due to the space craft acceleration. So, both should be the same age when they meet again. Am I wrong?
I think the answer is that the twin that accelerated away from earth actually had 2g acceleration to start with, adding the acceleration of the rocket to the acceleration that the earth was exerting. So both twins are not subject to the same acceleration, and there would be different ageing, although the calculation is probably totally wrong because of the speed of light limit and also because g from earth would be decreasing with the distance. Why (on earth!) that point was not addressed by the video?
Same acceleration, same aging, that is what I was thinking as well. The equivalent principle. I haven't calculated the speed that would be accumulated by accelerating with 1G for 5y...
Thank you to your kind comments. Yes you are right. The whole point of relativity is that an observer in his own frame of reference sees no change in that frame of reference. Mass, length and time all remain the same. They only appear to change in another frame of reference.
The difference is that in one case you also travel at speed. Both special and general relativity contribute to time variations. There is also a changing acceleration for the spaceship. If you travel away from the Earth at g and then change the acceleration so that you slow down there is a significant shift in g at the point at which you change the direction of the acceleration.
You might want to look up "Time Dilation" in Wikipedia which has two useful sections: Relative velocity time dilation and Gravitational time dilation. Increasing velocity = slower time. Higher acceleration/ gravity = slower time. So an orbiting satellite has lower gravity than someone on earth (hence faster time) but is moving at speed (hence slower time). They do not cancel out. Velocity effect wins. Hence clock in satellite moves slower and astronaut is younger.
DrPhysicsA Just being picky, but you can get to New York from London in 5 minutes time because you wouldn't be traveling anywhere near the speed of light.
They are not moving at the speed of light relative to each other; what you are doing is to simply sum their speeds, but that's not how it works. From the perspective of one of the twins, the other one is traveling at 0.8c [here is the formula I used to calculate that: th-cam.com/video/IM630Z8lho8/w-d-xo.html].
When he said you can't travel from London to NY in 5 minutes I think he meant no you can't do it with today's technology. It requires at least 41k mph.
Thanks. In the (simplistic) example I have given all the acceleration is at the point where the direction changes. So the rocket goes away at 3/5c and then comes back at 3/5c and accelerates to change direction in an instant. A different approach is for the space craft to accelerate away from earth up to (say) 3/5, then decelerate to zero. Then accelerate to 3/5c in the other direction (ie towards earth) and finally to decelerate to zero, landing back on earth. But that's more difficult.
Many times during this lecture you jump between time "appearing" to slow down ... and actually slowing down. The two ideas are quite different. The appearance of time (observed via light) is just a matter of appearance .
What i don't understand is that both twins are moving with the same relative speed to each other but one is supposed to be older. but well i guess that's why it is called a paradox.
No, the twin on earth will be older because he is not traveling through space and his clock runs faster than a clock at the rocket. Accelerations and decelerations are only entered to avoid General Relativity problems. This is pure Special Relativity issue now and twin at the rocket is traveling through spacetime and the twin at the earth is traveling through time only.
The key point is that everyone perceives their own reference frame to be normal. Clocks run normally and body clocks run normally. But everyone else in a different frame appears to have all clocks including body clocks running slowly.
As I understand the problem, the stationary twin sees 10 years go by but the moving twin sees only 3 years go by. The traveling twin perceives himself to travel at just under c for 3 years. Neither of the twins can calculate that the speed of light has been exceeded in their own frame of reference.
Thilina Senevirathna Because as a massive object approaches the speed the of light, its mass begins to increase. The closer it gets to the speed of light the large it's mass becomes. Thus it requires and infinite amount of energy to transcend that last little bit of velocity.
A better way to think about it is that in the reference frame of the person on the rocket, the rocket CAN accelerate at 1g forever. That is, that person on the rocket can experience 1g acceleration forever, no problem. There is no reason why not... remember that the universe has no concept of any sort of absolute velocity. But in the reference frame of the person standing at the starting point, that person will note that the rocket can only approach C asymptotically. That is the application of a fixed rate of energy on the rocket to maintain the 1g from the point of view of the person on the rocket has a smaller and smaller effect on that person's velocity from the point of view of the person at the starting point. And from the reference frame of the person in the rocket, the relative velocity of the person standing at the starting point can also only approach C asymptotically. Even though we have an energy-mass equivalence, that equivalence is in the eye of the beholder... it depends on the frame of reference. The energy being applied to the rocket to maintain the 1g acceleration is in the ROCKET's frame of reference, not in the frame of reference of the person at the starting point. And, in fact, both frame of references will measure the same application of energy, just with different effects. The person in the rocket continues to feel 1g of acceleration and his velocity relative to where he was just a moment ago agrees, while the person at the start point notices that the application of energy to the rocket has less and less an effect on the rocket's velocity relative to the person at the start point. Hopefully I didn't make any mistakes there. Great video. -Matt
QUESTION: At 29:05 you talk about how the factor of "g" is caused by "Acceleration." This agrees with what you said earlier in the lesson about how "Acceleration" on a train causes you to "feel" the "Acceleration" associated with the increase in the "gravity" force of "g." However, you do not explain why the factor of "-g" that is caused by "Deceleration" also creates a form of gravity capable of slowing down time. I realize if the train suddenly decreases in speed, you "feel" the force of gravity in a "-g" sort of way similar to the way jamming on the brakes of a train would cause people to move forward into the seats of the people in front of them, just as quickly accelerating causes people on the train to move backward into their own seats. But your video seems to suggest the "Deceleration" is "gradual" over this ten year period of time. Similarly, the video seems to suggest the "Acceleration" takes place until the force of "g" is reached, and then from that point on the speed remains the same in a way that allows the "g" to remain constant for the ten year journey. In comparison, the "Deceleration" seems like it should happen quickly and then be over. This is a bit confusing - particularly in the area of "Deceleration" and whether the "Deceleration" is "gradual" for ten years, or some form of "Deceleration" that only happens at the end of the journey. Please explain this in further detail.
+shubham saxena No they wouldn't. Nothing with mass can cross the speed of light. You can only asymptotically approach it. The faster you go your relativistic mass increases requiring more and more energy to accelerate so eventually it would require infinite energy for something with mass to reach c. This is why nothing with mass travels at c.
I think he means 1g of force. 10m/s² is probably for Newtonian physics; assuming that, we would get to the speed of light at 3x10^7 Seconds [ Less than 1 year]. But a constant 1 force is possible, and after 10 years, u would be at close to the speed of light, but without surpassing it
Given enough fuel, You CAN accelerate locally at 1g for as many years as you want. If you have a spring mass system in your accelerating frame of reference, you will always see that the mass is pulling at mg. However, to external observer, you will not appear to be accelerating at 1g, but your mass will appear to be increasing. So, the local observer CAN accelerate at 1g for 40 years. If you disagree, then you are saying that the local observer can look at the spring-mass system and determines his acceleration is not 1g anymore and hence can calculate his speed with respect to C, which is absurd.
Hi Erica. I'll have a think. The problem is that there is no evidence of capacity to travel back in time - and there are lots of potential paradoxes if you could (eg killing your own grandfather before your father was born).
Acceleration is not part of the clock paradox. The clock paradox is about constant motion not acceleration. Acceleration is not even an issue here and thus has no consideration at all. Please stick to the experiment.
But without acceleration, the two frames of reference are each the same relative to each other. Each reference sees the other as moving, so all perceptions of time, plus aging, should be symmetrical. And how can each twin think the other one is younger than they themselves are once they reunite on earth? But acceleration does not explain it all either. For example, the first acceleration (that starts the journey) causes a slowdown of time that continues in the constant velocity section. How can it be that rapid acceleration causes a continuous slowdown of time between two frames of reference differing only by a constant velocity? Either we are smarter than the theoretical physicists, or the explanation in this video is incomplete.
Dan Austin The original thought experiment did not include the considerations of acceleration. The original thought experiment was about constant relativistic speeds. I believe the problem is is due to one of three issues. 1) We just do not have a complete description of the nature of light. 2) The mathematics to describe the relationship between the two frames of reference is incomplete. 3) The original data collected to lead to this problem is wrong or has been manipulated.
The experience of acceleration is the only difference in the two reference frames. It's critical to resolving the paradox. Otherwise either twin could claim to be at rest.
I agree. Acceleration has nothing to do with resolving the paradox. I think the paradox resolves itself if you also consider the viewpoint from the person in the spaceship. From that perspective, the earth goes whizzing off and then the spaceship goes whizzing off even faster to catch up with it. Because the spaceship is travelling faster than the earth, there is more time dilation on the spaceship than on the earth, which leads to the person on the spaceship being younger than the person on the earth. I haven't computed the numbers, but I'm pretty certain this resolves the paradox. The guy in the video failed to explain the paradox. This is a pretty poor video from that perspective.
How do you know the spaceship wasn't stationary and the solar system zooms off? This is a defining tenet of relativity. Anyone accelerating can claim to be in a gravitational field (inertial mass is the same as rest mass) and anyone not expediting an acceleration can claim to be at rest.
I think its best explained by saying that although there is a gravitational force acting on you on earth you aren't actually accelerating with g because the ground stops you. Whereas in space you actually accelerate at g and, in time, reach speeds which are close to c and that leads to time dilation.
Appreciate that response but I don't think it speaks to his question. I believe his question is more simply put by asking "aren't you trying to resolve this paradox by making the rocket twin your absolute frame of reference?"
There is a difference. The person on earth is subject to a constant acceleration of g. The other twin is subject initially to acceleration g but then will be subject to deceleration g, then the situation will be reversed again (if the twin is to get back to earth). Those changes in acceleration are key.
Really nice video! Very well explained. As with all such videos and as per many of the comments it leaves the question of who is moving to be a subjective one even better illustrated by the space ship which accelerates at exactly 1g. If we were able to realise our planets velocity through space and then have the accelerating twin accelerate himself to a standstill relative to background space and then re-accelerate himself to earth speed then would the mere realisation of this change his time dilation? The perception of your frame seems almost Quantum in the meaning it gives to the situation. Top marks for a great video with a beautifully short and elegant derivation of the Lorenz time dilation.
If A stays on Earth and B travels back and forth in a rocket, why does B's acceleration explain the proposition that the clocks are no longer in sync after B returns to Earth? Shouldn't time dilation work both ways? That is, if A see's B's clock running slow, doesn't B also see A's clock running slow because both reference frames are equal?
DrPhysicsA I'm sorry if this remark isn't necessarily relevant to the point you were making at about 30:30 into the video, but if one accelerates from rest at the rate of g ms-1 for 10 years, that would make you gain a velocity=a*t=9.81*10years*365days*.....=3.09*10^9 ms-1 which is bigger than the speed of light. however i dont think it changes what you were saying in any way, just thought its worth a mention.
The apparent velocity will not ever go beyond the speed of light, only approach it. But you can keep accelerating your own frame. Eventually, time will be so dilated that you can reach all of space in what YOU perceive as almost no time at all. Others will still see you moving slightly slower than light, though.
The calculation assumes that the speed of light can never be exceeded. As the spacecraft achieves relativistic speeds its speed increases asymptotically to the speed of light.
I was quoting the example given in the book "Why does E=mc2" by Profs Brian Cox and Jeff Forshaw (page 100). As you might expect the formula you have used needs to be modified to account for general relativity. And, of course, the spaceship will never exceed the speed of light but it will come very close.
That's the wierdness of special relativity. A moving clock is perceived to run slowly. So if you and I are moving apart at a relative speed which is large compared to c, then I see your clock running slower than mine and you see my clock running slower than yours.
As my videos on special relativity show - where 2 people are moving apart at a relative speed v then each will see the other's clock running slower. But they can never actually compare clocks in the same reference frame UNLESS one of them accelerates. When moving apart with velocity v there is no experiment which can determine which of the two is moving. But if one accelerates then that can be determined. The clock in the accelerating frame goes slower. So the accelerating twin will be younger.
It's instructive to draw an x-t diagram in the spaceship's frame(s). If you leave some extra vertical space below the diagram, you can extend Earth's worldline backward (i.e. downward) in the spaceship's returning frame to see that, as soon as the spaceship turns around, the beginning of the journey suddenly seems to the traveler like it happened a long time ago.
Yes, you could only accelerate until you were close to c. You couldn't exceed it. The big time differences arise because of the acceleration. And it is the person who is accelerating whose time slows down relative to the non-accelerating person's time. Once you get near c, there is virtually no acceleration. At that point an observer (at rest relative to you) would see your clock appear to stand still. But of course, you would also see that person's clock stand still.
I understand the effect of acceleration...the part I am not following is the use of the words "the one that is accelerating". Without an absolute frame of reference isn't it impossible to determine which is "the one that is accelerating"?
Any "clock" including your body clock behaves entirely normally as far as you (the local observer) is concerned. You will live your normal life span. But elsewhere time is moving at a different rate.
Well, Dr. Eagle, you have clearly explained a whole lot of complex things to a physics novice. I thank you! You have also, however, crushed my Star Trek fantasies, so I must also (with due shame) curse you, just a little. Thank you for taking the time to share your knowledge!
Yes, all clocks change including body clocks. So it is theoretically possible for someone to travel through space for say 40 years and return to the earth which will then be several thousand years into the future.
That is right. If two frameworks move apart at a relative constant velocity, there is no experiment which can establish if one is moving and one is stationary. But if one starts to accelerate it is easy to establish which of the two has accelerated.
Don't really understand the end where an acceleration of g will yield a slower time according to a person on earth, but since the person on earth is also accelerating at g (not in an inertial frame of reference) their time should slow as well. So since both twins are in a curved path through spacetime only one of them will have a slower time?
After looking around some more and thinking about what you said with acceleration actually causing time to slow down I think that the twin who went out in the space ship actually will be younger when they come back but it'd be nice if you could still confirm that for me :). Thanks and by the way, keep up the excellent work! You're the only physics person on youtube who puts the math (while still explaining concepts) into it like it should be done.
The observer inside the rocket sees the beam of light travel vertically upwards to the top of the rocket. The observer who sees the rocket traveling at v (tho he cannot know whether he or the rocket, or both, is actually moving) sees the light beam move upwards at an angle.
In special relativity terms there is no difference between the two observers if each is moving apart at a relative constant velocity. But if one is accelerating more than the other, that is not an invariant condition. In other words an experiment will demonstrate which of the two is subject to the greater acceleration.
I didn't Understand this term , ' your clock runs Slow if you are Accelerating ' so i wanna ask , Why It's said , if you some how Travel at the speed of light your time runs Extremely Slower then rest ...... My question is Speed of Light is Constant , 3x10^8 M/Sec so its fixed it is not Acceleration . So How ????
If the twin never comes back and they separate at a constant velocity then each will say that the other's clock is running slow (ie that the other person is ageing more slowly). The question of which of them gets older faster doesn't have any meaning since they are never in the same place at the same time to compare.
Aren't you breaking the math with that argument? Your argument suggests that time dilation doesn't occur in isolated systems. If that is the case then my mind is really I'm trouble. 😉
The mirror illustration is designed to show why time appears to differ. My special relativity video explains the transform without using mirrors. If the twins go off in opposite directions (same speed and acceleration) and arrive back on earth 40 years later they'd both be the same age but the earth would be 100 years (or whatever) in the future.
Combination of special and general relativity. And whether g changes. eg +g to speed up and -g to slow down. Massive change of accleration from +g to -g
you are defining time for the passenger in the spacecraft relative to someone outside as clicks of the clock. When it is moving the clock ticks slower for someone outside but ticks normally for the passenger. I'm a bit confused on why the time intervals would be defined as clicks on the clock but I'm pretty sure I got it and just need time sorting it all out.
If you could sit on a photon of light and travel with it, then the watch on your wrist would take away normally and time would pass exactly as you would expect. But the clocks in the rest of the universe would appear to stand still. Similarly anyone observing you from Earth would perceive that the watch on you all wrist had stopped.
DrPhysicsA, Thank you for this and every video. I finally, I think, understand how the paradox works and also how time dilation works, and not just in a nodding my head, the math adds up way, but in a fundamental way. And my understanding is slightly different from and simpler than, though not contradictory of, how I've always heard it explained as a question of time. Here's what I understand. Not only is light the speed *limit*, it is also *the* (only) speed at which every electron and sub-atomic particle moves and spins. When matter is "at rest" the particles are free to whizz away locally, and react locally, and so on, at light speed, and will "age away" at a standard rate as a result. Now, when moving at a high velocity, those local reactions -- the normal whizzing of the particles at light speed around and through each other -- must by necessity be slower in their local whizzing, because their required speed is now largely dedicated to this directional movement as well as the local movement. So much so that at high enough velocities the particles, still whirling at light speed, are not affecting and reacting with their local matter as quickly, and thus the matter is "aging" more slowly. But this is slightly different from what I've always thought I've heard. That is, it's less accurate to say that time itself moves slowly, and more accurate to say that all of the matter in motion is "aging" more slowly. Which, yes, is effectively the same thing, but to me it strikes me as making more intuitive sense than saying time itself is different. And it makes the resolution of the paradox more clear to me -- both bodies of matter are "moving", but it is the matter moving at the highest velocity in space that ages the slowest. So -- correct me if I'm wrong -- it isn't acceleration that's providing the effect but constant velocity. (Yes, acceleration is a function of velocity, so perhaps that's what you're referring to?). It also seems like it opens up another set of questions about relative velocity in space, but I understand there's no good way to address that without establishing a point/frame around which everything else is measured relatively. So the wildest notion I have: if you could theoretically measure some matter at a fixed point at rest -- it would be aging at the highest rate possible. Everything else in motion is simply aging and developing relatively slower as a function of its velocity. But time itself is always proceeding at some standard rate, as measured by the rate of such a fixed body. I think it's all in line with all of the math and logic, but it helps me understand better. Unless I'm off somehow.
Yojimbo That's not particularly helpful man. If you suggested a book because you think I'm making breakthroughs in my understanding, thank you. If you are insinuating my comment is off base somehow, it would be more helpful to point out where and a specific place I can look to to correct any misunderstanding. No offense, but I can't read tone and your replies to me and others here don't inspire me.
I think your understanding is considerably off base, and so much so that you might find reading up on this to be beneficial. Here is a website that I find useful for an explanation of special relativity. I'm sure there are others, but this one will be fine. galileo.phys.virginia.edu/classes/252/lecture1.htm I can't remember if they get into explaining the twin paradox which is the subject of our other discussion on this topic.
Your body is itself a clock. So the clock which is local to you will appear to be running normally. Your pulse will be normal. Half and hour will seem like half an hour etc. So if you happen to be in a spacecraft which is subject to huge acceleration such that the clock on the spacecraft moves forward only 25 years in the time in takes a clock on earth to move forward a few thousand years you would emerge on earth 25 years older into a time thousands of years in the future. But you cant go back!
A moving clock runs slower. Or more accurately, an observer observing a moving clock will see it running slowly. If the clock is moving at the speed of light it would appear to have stopped. But the person riding with the clock would perceive the clock to be ticking as normal. So from that point of view of a photon, time seems to pass by as normal. But anyone observing a clock on a photon would see it as if it had stopped.
Couldn't have explained it better, may I suggest though that you dont say "you're clock slows down" but rather "you're clock appears slower than earths clock" or visa versa. A point of confusion to a lot of people is that their clock changes but time always ticks at the same rate relative to you.
so how it is explained that even though from perspective of a travelling twin, the clocks on earth are slower, he is younger ?
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Langevin's twins are just a thought experiment, it's not a real experience, it's an *imaginary prediction,* you can make it say whatever you want: younger, older, same age. _To know more: __th-cam.com/video/vruCJAUxI0M_/w-d-xo.html_ (English subtitles available)_
This is a special relativity problem. Each of us would perceive the other traveling away at 3/5c. Each would observe time dilation and length contraction. That is, you would say that my clock was running slower than yours. And you would measure my distances as shorter than yours. Now, you are going to measure time by looking at my clock. But that won't change the fact that you will perceive length contraction in my frame of reference. So we won't measure the same distance.
The point I think I was making was that you might be able to leave earth now and through a combination of high speed and acceleration arrive back on earth 40 years later in your time but 59,000 yrs later in earth time. BUT you cant then go back to the earth as it was only 40 years after you took off.
FYI. Clocks in motion use the same amount of energy as stationary clocks. From E=mc, we can see that Energy equals Time so if both clocks used the same amount of Energy, then both clocks experienced the same amount of Time. Space and Time are separate frames of reference. Clocks are instruments that measure acceleration in Space. No paradox because there is no time-dilation.
Special relativity would kick in to prevent the rocket from exceeding the speed of light. It's mass would increase and therefore an increasing amount of force or thrust of its engines would be needed to accelerate an increasing mass. Also key here is the change in acceleration at the point where you stop accelerating outwards and start decelerating.
Great video, thanks, but I miss something fundamental. What is the actual significance of a physical clock moving slower during acceleration/high velocity on the biological processes in the human body that determine our 'age'? How are those biological processes 'aware' of a difference in time? Are they affected in the same way as a clock would be i.e. they slow down 'relative' to a stationary twin? I guess that is the whole point of relativity.. but would be great if you could clarify. Thanks
Old comment but still (theory of relativity is much older..hehe) - I think that there is no other way to explain it than to say that their time flows differently compared to each other, but each individual (the one moving and the one at rest) - will feel absolutely nothing special going on with their time. Our time down at earth runs slower compared to the time in the space station IIS (and the people living at the Maldives slower than the people in the Himalayas) - but none of them notice anything weird going on with there clocks or life.
28:42 - 10 years traveling with an acceleration of g (9.8 m/s^2) means that at the end of this time the rocket is going to have a speed of 3,094,761,600 m/s, roughly 10 times the speed of light so brian cox and jeff forshaw are speaking about the hyper-relativistic theory which is not yet discovered :) I love your series on youtube - it is a great chance for the young to understand with ease the base and medium in physics
That is one factor that also confused me. I did not think it was even possible for this scenario of "constantly" Acceleration for 10 years to be considered in the calculation. From that perspective, it must mean the Deceleration is also gradual over a 10 year period of time in the calculation.
@@rrrick2129 really sorry but you're completely wrong, the force applied is going to increase so that at some point the mass gets so big that an almost infinite force would be necessary to keep up the acceleration constant - sleep well
Both special & general relativity are at work. In the case of special relativity where there is no acceleration, each observer sees the other clock moving more slowly. For general relativity it is the reference frame which is accelerating whose clock will move slower. The combination of the two determines that when the two reference frames are once again the coincident, one clock will be slower than the other determined by the precise nature of the special and general relativity effects.
Because the accelerating spacecraft achieves high velocity so you get a general relativity and a special relativity effect. The person on the earth doesn't have the special relativity effect.
If I am standing still and you accelerate away in your car, it is true that both of us accelerate away from each other, but only one of us gets forced back in our seat. Of the twins in the paradox, one is subject to the acceleration g by standing on the earth. The other accelerates away at g but then is subject to a deceleration g in order to bring him back. It is in those changing decelerations that the difference lies.
That depends on the rotation of the Earth. If the building was located on the north pole or the south pole, or if the Earth had not been rotating, then they would travel through space at the same speed. So the question can be turned around and ask, how quickly would the Earth have to rotate in order for the two effects to cancel out? And would fall off the Earth, if it was rotating that fast?
If I have correctly understood this paradox the acceleration of the twin travelling on the spaceship is the only way to establish who is really moving with respect to the other twin, but the final difference of age is only caused by the velocity difference and the total period of the travel until the two twins can meet again.
So, being 40, and living constantly under 1g acceleration from earth during that time, am I actually 59,000 years old? I enjoy seeing these concepts written out mathematically. Thank you.
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You sir are a joy to listen to. Your articulating skills are unparalleled
Unparalleled in this universe anyway 😜
except on matters einstein he is wrong, because einstein is wrong.
I just found your channel on you tube.I think your videos are brilliant.I can show them to my children aged 10 and 14 and they understand.The way you explain things is excellent!!!Cheers!!
Absolutely brilliant! Thanks for the free education
Wrong, acceleration is not the answer.
@@jacekgodyka5542 And so the answer is .... ?
Remembering the letters is harder than the math.
This is the best video that incorporates the concept of the Lorentz factor that I've seen.
The explanation was crystal clear until 27:00 until the issue of the symmetry of the situation was introduced. In contrast to all the previous explanations which where very careful and rigorous, I felt this was more or less brushed off although it was probably the most crucial point and should have been explained more.
Anyway, please consider this as my constructive feedback, I do appreciate giving us this video, it was a great introduction to the twin paradox although it failed to solve it for me.
I just added a comment that was a lot less polite ;). Ihmo he failed completely in explaining the paradox.
@@AlericResident Well...although not explicitly stated before adding the acceleration factor, you understand the paradox by having in mind what's explained at 19:15 and then adding "1+1" when the travel is completely explained (up to 27:00). The paradox is that without acceleration (being picked up instantly, which is impossible) - both can say that the other one was the one moving. That's why there is no paradox in SR (because a travel like this without any acceleration is impossible)
It seems to me to relate to the Alice and Bob dilemma.
I believe he is an excellent teacher. It depends on what you are investigating and whether the lessons work for you.
@@tormodi5925 If that's your excuse, then clearly you fail to understand the actual claim of the Paradox. Which is an un-solvable paradox, meaning that the SR theory is wrong.
Man I wouldn't want to get hit by a hook going at 3/5 the speed of light. I'd be dead before I can even see the thing.
Milton Landrel. Ha Ha Ha.......love it ! Must have been " light " hook.
Yeah but a particle won't be dead!
Why do you bother about the hooks if later you're gonna introduce acceleration? I thought you were gonna explain twin paradox without the acceleration effects.
The specific relativity part makes the returning twin just a few years younger. With the book and acceleration the returning twin is 59000 years younger. They seem not consistent.
3133333 to 313131…
Sir I can't explain how you helped me to satisfy my hunger for knowledge of general relativity.thanks a lot.love from india
best channel on yt
You are right that if we could reach velocities near the speed of light which includes accelerating in order to get to those velocities, then we would travel through time much faster than those who are not subject to that acceleration. It doesn't mean we disappear from space. It simply means we move faster through space time. The disadvantage, however, is that there is no known way back.
There is no evidence that anything can move backwards in time. There is a convention when drawing Feynman diagrams for antimatter to be shown as travelling backwards in time. That is a convention. It does not mean that antimatter actually proceeds from today into yesterday.
Worse, the laws of physics have nothing at all that stops you from applying them backwards, giving rise to the almost philosophical question: why is there an arrow of time that goes the way it goes?!
However, because the earth is spinning, the guy on the top of the building is moving faster than the guy at the bottom. And since a=v^2/r, he is accelerating faster than the guy at the bottom. So does that cancel out the difference of the gravitational force?
Explained much better than other videos👍🌹
Any clock has the same effect including a body clock. If you are travelling with the clock then it will appear to keep normal time. But an observer who is travelling at a velocity relative to you will perceive your clock to be running slow.
I think this is a poor video. It raises more questions than it answers. The graphs don't make sense. Why is he multiplying twin B's clock by 4/5 when he could just as easily move to twin B's reference frame and multiply twin A's clock by 4/5? Yes he said it was due to the acceleration, but to twin B, it looks like twin A was accelerating. Also, isn't the 1G of acceleration twin B experiences in the second example equivalent to the 1G of gravity twin A experiences on Earth? Why would twin B experience more time dilation? The video should address these questions.
planet42 The effect of time dilation doesn't only occur with acceleration, it also happens when frames are moving in uniform fashion relative to each.
Devising an example completely without acceleration is trivial, so no the accelerations do not explain it. Your explanation and this video are both incorrect.
@Knowledge Rules There is an example totally without any accelerations used for example at the Fermilab channel where the elapsed time of the clocks is compared at the end without accelerations.
Also, one can trivially construct a scenario with 2 travelers with symmetrical accelerations that therefore must cancel out.
If there is a relative velocity between two people, it is not possible for one to determine which of them is moving. But if one of them is accelerating and the other is not then it is very easy to determine experimentally which is the one that is accelerating.
Wont the person on earth also experience being in a acceleration of 9.8m/Sec2 ,
how is that different from the person who traveled in a space craft with accel of 9.8 ,
Both should have aged the same
Shreenidhi Patil ones moving away
Thanks for totally disrupting my brain with that beautiful mathematics. Keep posting videos
Both twins were subject to g acceleration during the whole journey. The first one due to Earth gravitational attraction and the second one due to the space craft acceleration. So, both should be the same age when they meet again. Am I wrong?
I think the answer is that the twin that accelerated away from earth actually had 2g acceleration to start with, adding the acceleration of the rocket to the acceleration that the earth was exerting. So both twins are not subject to the same acceleration, and there would be different ageing, although the calculation is probably totally wrong because of the speed of light limit and also because g from earth would be decreasing with the distance. Why (on earth!) that point was not addressed by the video?
dmsamuel
Acceleration has nothing to do with it. It's just a poor video that doesn't explain the paradox.
Same acceleration, same aging, that is what I was thinking as well. The equivalent principle. I haven't calculated the speed that would be accumulated by accelerating with 1G for 5y...
Thank you to your kind comments. Yes you are right. The whole point of relativity is that an observer in his own frame of reference sees no change in that frame of reference. Mass, length and time all remain the same. They only appear to change in another frame of reference.
23:00 is when you go "oh my god"
The difference is that in one case you also travel at speed. Both special and general relativity contribute to time variations. There is also a changing acceleration for the spaceship. If you travel away from the Earth at g and then change the acceleration so that you slow down there is a significant shift in g at the point at which you change the direction of the acceleration.
my brain, is melting
You might want to look up "Time Dilation" in Wikipedia which has two useful sections: Relative velocity time dilation and Gravitational time dilation. Increasing velocity = slower time. Higher acceleration/ gravity = slower time. So an orbiting satellite has lower gravity than someone on earth (hence faster time) but is moving at speed (hence slower time). They do not cancel out. Velocity effect wins. Hence clock in satellite moves slower and astronaut is younger.
DrPhysicsA
Just being picky, but you can get to New York from London in 5 minutes time because you wouldn't be traveling anywhere near the speed of light.
Haha, how picky!
I just hope you assume no air friction
there will be a time tax. lol
They are not moving at the speed of light relative to each other; what you are doing is to simply sum their speeds, but that's not how it works. From the perspective of one of the twins, the other one is traveling at 0.8c [here is the formula I used to calculate that: th-cam.com/video/IM630Z8lho8/w-d-xo.html].
When he said you can't travel from London to NY in 5 minutes I think he meant no you can't do it with today's technology. It requires at least 41k mph.
Thanks. In the (simplistic) example I have given all the acceleration is at the point where the direction changes. So the rocket goes away at 3/5c and then comes back at 3/5c and accelerates to change direction in an instant. A different approach is for the space craft to accelerate away from earth up to (say) 3/5, then decelerate to zero. Then accelerate to 3/5c in the other direction (ie towards earth) and finally to decelerate to zero, landing back on earth. But that's more difficult.
R.I.P twin you were torn apart by a hook
one liek = 1 dab
Many times during this lecture you jump between time "appearing" to slow down ... and actually slowing down. The two ideas are quite different. The appearance of time (observed via light) is just a matter of appearance .
The twin on earth shouldn't also age 40 years since the absolute value of the accelerations is the same , according to equivalence principal ?
What i don't understand is that both twins are moving with the same relative speed to each other but one is supposed to be older. but well i guess that's why it is called a paradox.
I know right? Totally should be in the same age since colck on earth are slown by g...
No, the twin on earth will be older because he is not traveling through space and his clock runs faster than a clock at the rocket. Accelerations and decelerations are only entered to avoid General Relativity problems. This is pure Special Relativity issue now and twin at the rocket is traveling through spacetime and the twin at the earth is traveling through time only.
jacek0312 I get what you say only from the viewpoint of rocket the earth is moving trough spacetime. That is the point "paradox"...
But from both the viewpoint of Earth and the rocket, it is the rocket that accelerates. Acceleration isn't relative like velocity is.
The key point is that everyone perceives their own reference frame to be normal. Clocks run normally and body clocks run normally. But everyone else in a different frame appears to have all clocks including body clocks running slowly.
TIME DOES NOT PASS
Exactly, time does not pass and stays the same in every inertial reference frame. vixra.org/abs/1706.0498
As I understand the problem, the stationary twin sees 10 years go by but the moving twin sees only 3 years go by. The traveling twin perceives himself to travel at just under c for 3 years. Neither of the twins can calculate that the speed of light has been exceeded in their own frame of reference.
accelerating at 9.81m/s^2 for ten years would take you well beyond the speed of light.. no?
+Ali Syed No, it could take you to about 0.999999997c. You cannot get past the speed of the light.
Right, for some reason I was assuming the accerlation would be constant.. I'm guessing it decays pretty fast near c?
+Okaro X Why can't we get past speed of light if we could reach all the way up to the speed of light?
Thilina Senevirathna Because as a massive object approaches the speed the of light, its mass begins to increase. The closer it gets to the speed of light the large it's mass becomes. Thus it requires and infinite amount of energy to transcend that last little bit of velocity.
A better way to think about it is that in the reference frame of the person on the rocket, the rocket CAN accelerate at 1g forever. That is, that person on the rocket can experience 1g acceleration forever, no problem. There is no reason why not... remember that the universe has no concept of any sort of absolute velocity.
But in the reference frame of the person standing at the starting point, that person will note that the rocket can only approach C asymptotically. That is the application of a fixed rate of energy on the rocket to maintain the 1g from the point of view of the person on the rocket has a smaller and smaller effect on that person's velocity from the point of view of the person at the starting point.
And from the reference frame of the person in the rocket, the relative velocity of the person standing at the starting point can also only approach C asymptotically.
Even though we have an energy-mass equivalence, that equivalence is in the eye of the beholder... it depends on the frame of reference. The energy being applied to the rocket to maintain the 1g acceleration is in the ROCKET's frame of reference, not in the frame of reference of the person at the starting point. And, in fact, both frame of references will measure the same application of energy, just with different effects.
The person in the rocket continues to feel 1g of acceleration and his velocity relative to where he was just a moment ago agrees, while the person at the start point notices that the application of energy to the rocket has less and less an effect on the rocket's velocity relative to the person at the start point.
Hopefully I didn't make any mistakes there. Great video.
-Matt
QUESTION: At 29:05 you talk about how the factor of "g" is caused by "Acceleration." This agrees with what you said earlier in the lesson about how "Acceleration" on a train causes you to "feel" the "Acceleration" associated with the increase in the "gravity" force of "g." However, you do not explain why the factor of "-g" that is caused by "Deceleration" also creates a form of gravity capable of slowing down time.
I realize if the train suddenly decreases in speed, you "feel" the force of gravity in a "-g" sort of way similar to the way jamming on the brakes of a train would cause people to move forward into the seats of the people in front of them, just as quickly accelerating causes people on the train to move backward into their own seats.
But your video seems to suggest the "Deceleration" is "gradual" over this ten year period of time. Similarly, the video seems to suggest the "Acceleration" takes place until the force of "g" is reached, and then from that point on the speed remains the same in a way that allows the "g" to remain constant for the ten year journey. In comparison, the "Deceleration" seems like it should happen quickly and then be over.
This is a bit confusing - particularly in the area of "Deceleration" and whether the "Deceleration" is "gradual" for ten years, or some form of "Deceleration" that only happens at the end of the journey.
Please explain this in further detail.
one can't accelerate at g for ten years because they will cross c
+shubham saxena No they wouldn't. Nothing with mass can cross the speed of light. You can only asymptotically approach it. The faster you go your relativistic mass increases requiring more and more energy to accelerate so eventually it would require infinite energy for something with mass to reach c. This is why nothing with mass travels at c.
ok.......agreed
I think he means 1g of force.
10m/s² is probably for Newtonian physics; assuming that, we would get to the speed of light at 3x10^7 Seconds [ Less than 1 year].
But a constant 1 force is possible, and after 10 years, u would be at close to the speed of light, but without surpassing it
Given enough fuel, You CAN accelerate locally at 1g for as many years as you want. If you have a spring mass system in your accelerating frame of reference, you will always see that the mass is pulling at mg. However, to external observer, you will not appear to be accelerating at 1g, but your mass will appear to be increasing.
So, the local observer CAN accelerate at 1g for 40 years. If you disagree, then you are saying that the local observer can look at the spring-mass system and determines his acceleration is not 1g anymore and hence can calculate his speed with respect to C, which is absurd.
Hi Erica. I'll have a think. The problem is that there is no evidence of capacity to travel back in time - and there are lots of potential paradoxes if you could (eg killing your own grandfather before your father was born).
Acceleration is not part of the clock paradox. The clock paradox is about constant motion not acceleration. Acceleration is not even an issue here and thus has no consideration at all. Please stick to the experiment.
But without acceleration, the two frames of reference are each the same relative to each other. Each reference sees the other as moving, so all perceptions of time, plus aging, should be symmetrical. And how can each twin think the other one is younger than they themselves are once they reunite on earth?
But acceleration does not explain it all either. For example, the first acceleration (that starts the journey) causes a slowdown of time that continues in the constant velocity section. How can it be that rapid acceleration causes a continuous slowdown of time between two frames of reference differing only by a constant velocity?
Either we are smarter than the theoretical physicists, or the explanation in this video is incomplete.
Dan Austin
The original thought experiment did not include the considerations of acceleration. The original thought experiment was about constant relativistic speeds. I believe the problem is is due to one of three issues. 1) We just do not have a complete description of the nature of light. 2) The mathematics to describe the relationship between the two frames of reference is incomplete. 3) The original data collected to lead to this problem is wrong or has been manipulated.
The experience of acceleration is the only difference in the two reference frames. It's critical to resolving the paradox. Otherwise either twin could claim to be at rest.
I agree. Acceleration has nothing to do with resolving the paradox.
I think the paradox resolves itself if you also consider the viewpoint from the person in the spaceship. From that perspective, the earth goes whizzing off and then the spaceship goes whizzing off even faster to catch up with it. Because the spaceship is travelling faster than the earth, there is more time dilation on the spaceship than on the earth, which leads to the person on the spaceship being younger than the person on the earth.
I haven't computed the numbers, but I'm pretty certain this resolves the paradox.
The guy in the video failed to explain the paradox. This is a pretty poor video from that perspective.
How do you know the spaceship wasn't stationary and the solar system zooms off?
This is a defining tenet of relativity. Anyone accelerating can claim to be in a gravitational field (inertial mass is the same as rest mass) and anyone not expediting an acceleration can claim to be at rest.
I think its best explained by saying that although there is a gravitational force acting on you on earth you aren't actually accelerating with g because the ground stops you. Whereas in space you actually accelerate at g and, in time, reach speeds which are close to c and that leads to time dilation.
Appreciate that response but I don't think it speaks to his question. I believe his question is more simply put by asking "aren't you trying to resolve this paradox by making the rocket twin your absolute frame of reference?"
There is a difference. The person on earth is subject to a constant acceleration of g. The other twin is subject initially to acceleration g but then will be subject to deceleration g, then the situation will be reversed again (if the twin is to get back to earth). Those changes in acceleration are key.
Really nice video! Very well explained. As with all such videos and as per many of the comments it leaves the question of who is moving to be a subjective one even better illustrated by the space ship which accelerates at exactly 1g. If we were able to realise our planets velocity through space and then have the accelerating twin accelerate himself to a standstill relative to background space and then re-accelerate himself to earth speed then would the mere realisation of this change his time dilation? The perception of your frame seems almost Quantum in the meaning it gives to the situation. Top marks for a great video with a beautifully short and elegant derivation of the Lorenz time dilation.
Very well explained. I had never come across such a explanation of twin paradox before.
?a typical BS as in any textbook!
No. I am dividing both sides of the equation by c^2. Since the right-hand side is a product I only need to divide one part of the product by c^2.
If A stays on Earth and B travels back and forth in a rocket, why does B's acceleration explain the proposition that the clocks are no longer in sync after B returns to Earth?
Shouldn't time dilation work both ways? That is, if A see's B's clock running slow, doesn't B also see A's clock running slow because both reference frames are equal?
DrPhysicsA I'm sorry if this remark isn't necessarily relevant to the point you were making at about 30:30 into the video, but if one accelerates from rest at the rate of g ms-1 for 10 years, that would make you gain a velocity=a*t=9.81*10years*365days*.....=3.09*10^9 ms-1 which is bigger than the speed of light. however i dont think it changes what you were saying in any way, just thought its worth a mention.
The apparent velocity will not ever go beyond the speed of light, only approach it.
But you can keep accelerating your own frame. Eventually, time will be so dilated that you can reach all of space in what YOU perceive as almost no time at all. Others will still see you moving slightly slower than light, though.
***** The frame is not moving relative to itself, v is 0 in your rest frame.
The calculation assumes that the speed of light can never be exceeded. As the spacecraft achieves relativistic speeds its speed increases asymptotically to the speed of light.
I was quoting the example given in the book "Why does E=mc2" by Profs Brian Cox and Jeff Forshaw (page 100). As you might expect the formula you have used needs to be modified to account for general relativity. And, of course, the spaceship will never exceed the speed of light but it will come very close.
That's the wierdness of special relativity. A moving clock is perceived to run slowly. So if you and I are moving apart at a relative speed which is large compared to c, then I see your clock running slower than mine and you see my clock running slower than yours.
As my videos on special relativity show - where 2 people are moving apart at a relative speed v then each will see the other's clock running slower. But they can never actually compare clocks in the same reference frame UNLESS one of them accelerates. When moving apart with velocity v there is no experiment which can determine which of the two is moving. But if one accelerates then that can be determined. The clock in the accelerating frame goes slower. So the accelerating twin will be younger.
That acceleration can only be determined from a third reference frame...am I mistaken?
Fantastically well done! I gotta get me that book and see all the details!
It's instructive to draw an x-t diagram in the spaceship's frame(s). If you leave some extra vertical space below the diagram, you can extend Earth's worldline backward (i.e. downward) in the spaceship's returning frame to see that, as soon as the spaceship turns around, the beginning of the journey suddenly seems to the traveler like it happened a long time ago.
Yes, you could only accelerate until you were close to c. You couldn't exceed it. The big time differences arise because of the acceleration. And it is the person who is accelerating whose time slows down relative to the non-accelerating person's time. Once you get near c, there is virtually no acceleration.
At that point an observer (at rest relative to you) would see your clock appear to stand still. But of course, you would also see that person's clock stand still.
I understand the effect of acceleration...the part I am not following is the use of the words "the one that is accelerating". Without an absolute frame of reference isn't it impossible to determine which is "the one that is accelerating"?
Indeed you can. It is a clock. As a special treat, in one of my future videos I am going to show how it works.
Since both twins are subject to g (or minus g), how does time pass so differently ??"
Any "clock" including your body clock behaves entirely normally as far as you (the local observer) is concerned. You will live your normal life span. But elsewhere time is moving at a different rate.
Well, Dr. Eagle, you have clearly explained a whole lot of complex things to a physics novice. I thank you! You have also, however, crushed my Star Trek fantasies, so I must also (with due shame) curse you, just a little.
Thank you for taking the time to share your knowledge!
FAntastic fantastic fantastic!! You got me clapping,cheering and swearing during the whole video... what a great explanation!
Yes, all clocks change including body clocks. So it is theoretically possible for someone to travel through space for say 40 years and return to the earth which will then be several thousand years into the future.
I'm approaching the edge of my intellectual abilities here.
That is right. If two frameworks move apart at a relative constant velocity, there is no experiment which can establish if one is moving and one is stationary. But if one starts to accelerate it is easy to establish which of the two has accelerated.
Not if you are on earth. Both experience the same g.
You said "but if one starts to accelerate"...what absolute frame would you use to establish which is actually accelerating?
If one twin is travelling at 3/5c relative to the other twin there will be no acceleration. Each will see the other's clock showing time dilation.
Don't really understand the end where an acceleration of g will yield a slower time according to a person on earth, but since the person on earth is also accelerating at g (not in an inertial frame of reference) their time should slow as well. So since both twins are in a curved path through spacetime only one of them will have a slower time?
After looking around some more and thinking about what you said with acceleration actually causing time to slow down I think that the twin who went out in the space ship actually will be younger when they come back but it'd be nice if you could still confirm that for me :). Thanks and by the way, keep up the excellent work! You're the only physics person on youtube who puts the math (while still explaining concepts) into it like it should be done.
The observer inside the rocket sees the beam of light travel vertically upwards to the top of the rocket. The observer who sees the rocket traveling at v (tho he cannot know whether he or the rocket, or both, is actually moving) sees the light beam move upwards at an angle.
Direction doesn't matter. Moving clocks move slower. And clocks which are subject to greater acceleration move slower.
In special relativity terms there is no difference between the two observers if each is moving apart at a relative constant velocity. But if one is accelerating more than the other, that is not an invariant condition. In other words an experiment will demonstrate which of the two is subject to the greater acceleration.
I didn't Understand this term , ' your clock runs Slow if you are Accelerating ' so i wanna ask , Why It's said , if you some how Travel at the speed of light your time runs Extremely Slower then rest ...... My question is Speed of Light is Constant , 3x10^8 M/Sec so its fixed it is not Acceleration . So How ????
Yes. Information from one frame of reference is observed in another frame.
If the twin never comes back and they separate at a constant velocity then each will say that the other's clock is running slow (ie that the other person is ageing more slowly). The question of which of them gets older faster doesn't have any meaning since they are never in the same place at the same time to compare.
Aren't you breaking the math with that argument? Your argument suggests that time dilation doesn't occur in isolated systems. If that is the case then my mind is really I'm trouble. 😉
The mirror illustration is designed to show why time appears to differ. My special relativity video explains the transform without using mirrors. If the twins go off in opposite directions (same speed and acceleration) and arrive back on earth 40 years later they'd both be the same age but the earth would be 100 years (or whatever) in the future.
Combination of special and general relativity. And whether g changes. eg +g to speed up and -g to slow down. Massive change of accleration from +g to -g
hi, loved it...... please make a video briefly describing the terms, indices, etc.
you are defining time for the passenger in the spacecraft relative to someone outside as clicks of the clock. When it is moving the clock ticks slower for someone outside but ticks normally for the passenger. I'm a bit confused on why the time intervals would be defined as clicks on the clock but I'm pretty sure I got it and just need time sorting it all out.
If you could sit on a photon of light and travel with it, then the watch on your wrist would take away normally and time would pass exactly as you would expect. But the clocks in the rest of the universe would appear to stand still. Similarly anyone observing you from Earth would perceive that the watch on you all wrist had stopped.
DrPhysicsA, Thank you for this and every video. I finally, I think, understand how the paradox works and also how time dilation works, and not just in a nodding my head, the math adds up way, but in a fundamental way. And my understanding is slightly different from and simpler than, though not contradictory of, how I've always heard it explained as a question of time.
Here's what I understand. Not only is light the speed *limit*, it is also *the* (only) speed at which every electron and sub-atomic particle moves and spins. When matter is "at rest" the particles are free to whizz away locally, and react locally, and so on, at light speed, and will "age away" at a standard rate as a result.
Now, when moving at a high velocity, those local reactions -- the normal whizzing of the particles at light speed around and through each other -- must by necessity be slower in their local whizzing, because their required speed is now largely dedicated to this directional movement as well as the local movement. So much so that at high enough velocities the particles, still whirling at light speed, are not affecting and reacting with their local matter as quickly, and thus the matter is "aging" more slowly.
But this is slightly different from what I've always thought I've heard. That is, it's less accurate to say that time itself moves slowly, and more accurate to say that all of the matter in motion is "aging" more slowly. Which, yes, is effectively the same thing, but to me it strikes me as making more intuitive sense than saying time itself is different. And it makes the resolution of the paradox more clear to me -- both bodies of matter are "moving", but it is the matter moving at the highest velocity in space that ages the slowest.
So -- correct me if I'm wrong -- it isn't acceleration that's providing the effect but constant velocity. (Yes, acceleration is a function of velocity, so perhaps that's what you're referring to?). It also seems like it opens up another set of questions about relative velocity in space, but I understand there's no good way to address that without establishing a point/frame around which everything else is measured relatively.
So the wildest notion I have: if you could theoretically measure some matter at a fixed point at rest -- it would be aging at the highest rate possible. Everything else in motion is simply aging and developing relatively slower as a function of its velocity. But time itself is always proceeding at some standard rate, as measured by the rate of such a fixed body. I think it's all in line with all of the math and logic, but it helps me understand better. Unless I'm off somehow.
I think you should read a book on this.
Yojimbo I'm up for it. Do you have one to recommend?
Any basic university level physics textbook will do. And there's tons of stuff on the web on this.
Yojimbo That's not particularly helpful man. If you suggested a book because you think I'm making breakthroughs in my understanding, thank you. If you are insinuating my comment is off base somehow, it would be more helpful to point out where and a specific place I can look to to correct any misunderstanding. No offense, but I can't read tone and your replies to me and others here don't inspire me.
I think your understanding is considerably off base, and so much so that you might find reading up on this to be beneficial. Here is a website that I find useful for an explanation of special relativity. I'm sure there are others, but this one will be fine.
galileo.phys.virginia.edu/classes/252/lecture1.htm
I can't remember if they get into explaining the twin paradox which is the subject of our other discussion on this topic.
Hi, I'm just wondering, when did you get 1.25 on minute 22:39? Thanks!
Your body is itself a clock. So the clock which is local to you will appear to be running normally. Your pulse will be normal. Half and hour will seem like half an hour etc. So if you happen to be in a spacecraft which is subject to huge acceleration such that the clock on the spacecraft moves forward only 25 years in the time in takes a clock on earth to move forward a few thousand years you would emerge on earth 25 years older into a time thousands of years in the future. But you cant go back!
A moving clock runs slower. Or more accurately, an observer observing a moving clock will see it running slowly. If the clock is moving at the speed of light it would appear to have stopped. But the person riding with the clock would perceive the clock to be ticking as normal. So from that point of view of a photon, time seems to pass by as normal. But anyone observing a clock on a photon would see it as if it had stopped.
Thanks for the kind comments.
Couldn't have explained it better, may I suggest though that you dont say "you're clock slows down" but rather "you're clock appears slower than earths clock" or visa versa. A point of confusion to a lot of people is that their clock changes but time always ticks at the same rate relative to you.
so how it is explained that even though from perspective of a travelling twin, the clocks on earth are slower, he is younger ?
Langevin's twins are just a thought experiment, it's not a real experience, it's an *imaginary prediction,* you can make it say whatever you want: younger, older, same age.
_To know more: __th-cam.com/video/vruCJAUxI0M_/w-d-xo.html_ (English subtitles available)_
Sorry - missed this when it came in. I'm Dr Robert Eagle. Thanks for comment.
Have you seen my other videos in the Relativity playlist which can be found on the main TH-cam page for my channel.
This is a special relativity problem. Each of us would perceive the other traveling away at 3/5c. Each would observe time dilation and length contraction. That is, you would say that my clock was running slower than yours. And you would measure my distances as shorter than yours. Now, you are going to measure time by looking at my clock. But that won't change the fact that you will perceive length contraction in my frame of reference. So we won't measure the same distance.
That's comforting. You're a fount of gracious generosity. :)
The point I think I was making was that you might be able to leave earth now and through a combination of high speed and acceleration arrive back on earth 40 years later in your time but 59,000 yrs later in earth time. BUT you cant then go back to the earth as it was only 40 years after you took off.
You might want to look at Gravitational time dilation on Wikipedia which illustrates that you need a different approach.
FYI. Clocks in motion use the same amount of energy as stationary clocks. From E=mc, we can see that Energy equals Time so if both clocks used the same amount of Energy, then both clocks experienced the same amount of Time.
Space and Time are separate frames of reference.
Clocks are instruments that measure acceleration in Space.
No paradox because there is no time-dilation.
Special relativity would kick in to prevent the rocket from exceeding the speed of light. It's mass would increase and therefore an increasing amount of force or thrust of its engines would be needed to accelerate an increasing mass. Also key here is the change in acceleration at the point where you stop accelerating outwards and start decelerating.
Great video, thanks, but I miss something fundamental. What is the actual significance of a physical clock moving slower during acceleration/high velocity on the biological processes in the human body that determine our 'age'? How are those biological processes 'aware' of a difference in time? Are they affected in the same way as a clock would be i.e. they slow down 'relative' to a stationary twin? I guess that is the whole point of relativity.. but would be great if you could clarify. Thanks
Old comment but still (theory of relativity is much older..hehe) - I think that there is no other way to explain it than to say that their time flows differently compared to each other, but each individual (the one moving and the one at rest) - will feel absolutely nothing special going on with their time.
Our time down at earth runs slower compared to the time in the space station IIS (and the people living at the Maldives slower than the people in the Himalayas) - but none of them notice anything weird going on with there clocks or life.
Confuses: he excluded acceleration from the outset. Then comparing ages suddenly acc. is used as an argument.
Do Inertial Frames Resolve the Twin Paradox?
Let's find out!
why r u talking to yourself
28:42 - 10 years traveling with an acceleration of g (9.8 m/s^2) means that at the end of this time the rocket is going to have a speed of 3,094,761,600 m/s, roughly 10 times the speed of light so brian cox and jeff forshaw are speaking about the hyper-relativistic theory which is not yet discovered :)
I love your series on youtube - it is a great chance for the young to understand with ease the base and medium in physics
That is one factor that also confused me. I did not think it was even possible for this scenario of "constantly" Acceleration for 10 years to be considered in the calculation. From that perspective, it must mean the Deceleration is also gradual over a 10 year period of time in the calculation.
@@michaelzoran - it has to have a logic yet the actual process is not well explained
@@deuspax you have to have a torus.
@@rrrick2129 really sorry but you're completely wrong, the force applied is going to increase so that at some point the mass gets so big that an almost infinite force would be necessary to keep up the acceleration constant - sleep well
@@deuspax I work
Relativity kicks in. Spacecraft travels at very close to speed of light but never exceeds it.
Both special & general relativity are at work. In the case of special relativity where there is no acceleration, each observer sees the other clock moving more slowly. For general relativity it is the reference frame which is accelerating whose clock will move slower. The combination of the two determines that when the two reference frames are once again the coincident, one clock will be slower than the other determined by the precise nature of the special and general relativity effects.
Because the accelerating spacecraft achieves high velocity so you get a general relativity and a special relativity effect. The person on the earth doesn't have the special relativity effect.
If I am standing still and you accelerate away in your car, it is true that both of us accelerate away from each other, but only one of us gets forced back in our seat. Of the twins in the paradox, one is subject to the acceleration g by standing on the earth. The other accelerates away at g but then is subject to a deceleration g in order to bring him back. It is in those changing decelerations that the difference lies.
That helps... Thank you. Let me spend the rest of the night trying to figure that out...only one gets forced back in his seat...nice
Hi Doctor! I was wondering if you'd be able to represent why the time paradox is not actually paradoxical in terms of space time diagrams.
That depends on the rotation of the Earth. If the building was located on the north pole or the south pole, or if the Earth had not been rotating, then they would travel through space at the same speed.
So the question can be turned around and ask, how quickly would the Earth have to rotate in order for the two effects to cancel out? And would fall off the Earth, if it was rotating that fast?
If I have correctly understood this paradox the acceleration of the twin travelling on the spaceship is the only way to establish who is really moving with respect to the other twin, but the final difference of age is only caused by the velocity difference and the total period of the travel until the two twins can meet again.
So, being 40, and living constantly under 1g acceleration from earth during that time, am I actually 59,000 years old? I enjoy seeing these concepts written out mathematically. Thank you.