To clarify: I understand the "detection" at the screen is a different kind than one that might happen at the slits or any other detectors. It was just meant as a segue/transition to the main conclusion of the video: that quantum mechanics is just a model. The way quantum mechanics works doesn't necessarily say _anything_ about the nature of matter/light. If we input some initial conditions and a few interactions, it will give us a result that matches an experiment really well... but only after we do the experiment with a _bunch_ of particles over time. That doesn't mean we can make any judgements _at all_ about what individual particles are doing at any given moment.
I disagree. The detection at the screen is in fact exactly like the detection at the eraser. The only difference is that it is "softer". When photons hit the screen, their position is consistent with both patterns, but each position has different probability for each pattern. Hitting the peak of the "detected" pattern, makes the second photon guaranteed to hit the detector. Hitting the troth of the "detected" pattern, makes the second photon guaranteed to hit the eraser. When the photon hits the screen somewhere in between, the second photon gets some probability of landing either in detector or eraser. The probability is proportional to the "consistency" of the first photon's position on the screen with given outcome.
@@adb012 It doesn't matter. The overall pattern on the screen is the same. It is the way it is, because the second photon leaks information into the wild. Whether you actually happen to utilize that leaked information to figure out something about the first photon is irrelevant.
I actually thought that was the most important insight in the video. It is something most other Q.E. videos don't even mention. There absolutely is an interaction at the screen, and the pattern on the screen, taken alone, is the same either way. So the inference is that the detection at the screen can select for the result at the eraser without having to deal with anything going back in time. It's the simpler explanation of the two. Result at screen selects for the result at the eraser rather than the result at the eraser going back in time and selecting for the result on the screen. I was thinking about the Q.E. experiment all wrong until I watched this video (and I've gone through at least a dozen videos and two papers trying to understand it). I'm really happy you did a video on the quantum eraser. You have a way of explaining things that allows one to home in on the parts that are most important. -Matt
Another excellent physics video. It seems to me and a few others that the only non-hocus-pocus model is the de Broglie/Bohm pilot wave model. Its really too bad de Broglie let his contemporaries talk him out of further study of his insightful realistic model.
I love the “QM is not magic” mantra! Also particles being tiny waves makes a lot more sense than “they’re particle-wave magic hand-wavy particles!”. I’m so tired of being bamboozled by QM and double slits stealing my lunch money. The final part was a bit rushed for me though, I’d love to have the controversy around what’s going on addressed more meticulously maybe in another video.
@@jurusco Nope. Still not magic. Our inability to understand the universe with current models does not mean the universe is inherently not understandable. We just need better models. Of course its possible that there is no model that can be completely understood by humans due to our inability to measure things beyond the limits of our own existence (multiple universes or dimensions, for example.) But that still doesn't make it magic in the sense of "there are no rules".. it just means we can't necessarily know all the rules.
@@altrag depends how you define magic. There are soft (no rules) magic and hard (with rules, like conservation of energy) magic systems in myths, legends and video games. You can also define magic as things that you don't understand. So, until we understand it better, one could argue that QM is magic (a system that has rules which we don't know about). The same way fire and thunder were regarded as gods until we devised tools not only explain, but understand at fundamental levels; enough to reproduce them (We still don't know a lot about Space, so that's why many people are into Zoroastrianism). Naming QM (or anything) magic should not prevent us from investigating it further though.
@@JamesSarantidis I'm defining magic not as "things we don't understand," or even as "things we can't understand due to the limitations of the human mind." I'm defining magic as "things that are fundamentally not understandable _even by God_ " (that is, by a being that has absolutely no limitations on their intellect.) So kind of along the line of your "soft" definition, but in the most strict formulation of "no rules."
Nope. He's full of crap. Photon. Now explain it with a protein molecule. He's also ignoring the wave collapse experiments that were done purely on thought. He's clinging grimly to his imagined materialist pseudo reality. lol
@@RAF71chingachgook Having looked at your channel I think I can safely ignore your opinion without losing any sleep over what I might have missed out knowing but thanks for throwing your 2 cents worth in anyway.
If someone says physics is ... - boring - for nerds - difficult to understand - badly explained - needs advanced math knowledge - or any other misconception ... Just send them to the Science Asylum and they 'll see the 'light' in all its beauty and clarity !
many people who try to explain the double slit experiment always do so in a very vague manner , its like they don't fully understand it .... but your explanation was very elaborate nick, you did the best way ... better than all other explanations.
I think the often quoted alleged Feynman statements are by far too pessimistic. I don't know whether he still believed that there must still be something "thingy" about elementary particles like the electron ("wherever it is, it is a point charge") not regarding this as a mere artefact of it being elementary and thus just interacting as a whole.
@@jensphiliphohmann1876 , How much Feynman have you read? He was never pessimistic in this way. Also, the idea that there is some "point charge" is meaningless to him and all advanced physicists. QED. Feynman diagram.
@@photonboy999 I read some lectures and the QED book where Feynman is not pessimistic at all. My statement refers to some alleged Feynman quotes like "shut up and calculate" or so, and the alleged Feynman statement that no one understands quantum mechanics. Statements that don't really fit to Feynman's books.
Feynman also said very clearly in his NZ QED lectures that light is a particle... though I think I prefer Nicks view that the detection of a single photon is indeterminate about its particle or waviness properties.
This is a very useful and important video. The mystification of quantum mechanics is pedagogically horrific. We should always assume that everything can be understood, even if we don't fully understand it at this time.
Observing collapses the wave function...I want to try this test in water or some other medium to record the track to the interference pattern rather than detector at the slits...hmmm...I’m going to solve this...
Quite bold to assume that the human mind can understand everything. That’s like saying we are the supreme intelligent beings in the universe. There might be things in the universe that we can never comprehend.
@@aaambrosio3622 If that were true, then by contrapoint you have said there is a finite amount of knowledge we can understand. Which is as implausible as saying there are things we can never understand.
Quantum mechanics might not be magic, but you have a magical gift for explaining things. You just demystified in mere minutes all the other videos and books I have read about the double slit excitement. I do believe Albert Einstein would clap for you.
The 6 dislikes are the ancient keepers of the light. They’re displeased that you know their deepest secrets. We love your insight Nick and your ability to share it!
@@xyz.ijk. Maybe religious absolutists insisting God did it or tin foil hat types who think all science is nefarious shadow governments trying to control us for reasons or people who don't like his voice or the colour of his shirt. People are weird like that. Hahaha
I think for these topics video length around 10-ish minutes is more appropriate - otherwise (ie if shorter) explanations could be lacking (and thus understanding)
Nick just totally slam dunked the mysticism around the double slit experiment with extreme agression. This was VITAL, because as someone who have watched those vids, and been mystified, I needed this snap back to reality. Thanks!
Here's a mystery. If a photon is the smallest unit of light, how do you end up with two entangled half-a-photons? How do you detect half-a-photon? I think the last part of your video requires watching a few times.
@@crouchingtigerhiddenadam1352 It is the smalles packet of electromagnetic energy of a given frequency. That means the frequency is halved when the photon pair is created (it can also be any other proportion as long as it sums up to the original frequency) because energy is proportional to frequency. Such a process is also known as down conversion if you want to read up on it. The crystal needs some special properties that generate a so called nonlinear optical response. Most everyday materials only have a linear response, which merely alters the path of the light (reflection and refraction).
@@halbeard2996 there is a common definition given in physics which goes something like this: _the photon cannot be split. It has zero mass. It cannot decay. It can interact with another particle lose part of its energy and change wavelength._ ... and then there is QED and the *vertex function.* Questions like, _if light is an electromagnetic wave, what is the relationship between a photon and an electron?_ or _how many times can you split a photon?_ _(why is the answer 3?)_ On TH-cam you would think Quantum starts and stops with the wave function. Yet the meaty stuff is in vectors.
"I think I can safely say that nobody understands quantum mechanics." -Richard Feynman, The Character of Physical Law (MIT Press: Cambridge, Massachusetts, 1995), 129.
CJ the baka, There are various levels of understanding. However, NOBODY is confident that they understand what's going on here completely. How do the particles change what they are doing based on a seemingly future detection? Nobody knows.
It doesn't _have_ to be polarizing film. It could be anything that reveals the slit information. I just chose polarizing film as an example of something that could do that.
Nick, this is by far the best explanation of the double slit experiment on the internet. I have watched most of them without helping me understand the deeper details. Thanks!
Great job Nick, after watching so many videos about this trying to figure out what kind of magic was happening here, it seems you are the only one that deserves the Nobel prize that has been put at stake! great explanation & a really conclusive model
The interference pattern was observed not only for photons but also for large atomic and molecular classical particles. All your explanation is valid only for photons which are not actually classical particles.
is anything a "classical particle"? it's all made of energy and waves... with larger amount of elements involved, the harder it gets not to collapse the waveform because chances of interactions are getting bigger. thats why we don't fall thru the floor.
1) The double-slit experiment can be done with all sorts of particles. The same rules/results apply to all of them. I just used photons in my video because it was a good transition from my previous video and because they're the first particle we did this experiment with. 2) Larger particles only behave "classically" because they're constantly interacting with things making them localize. If you could get a dust particle to be isolated for long enough, it... _should_ behave like a quantum particle too.
This was a fantastic explanation. I've been trying to wrap my head around the particle/wave duality thing for ages, not to mention entanglement. This video makes things so much clearer now. Keep up the great work!
I think I've seen or read dozens of explanations of the topic along the years. This is the best one yet. You kicked the most famous QM experiment right in the mystification tender spot.
Lot to soak in with this, but I think the biggest thing that meant the most to me personally was understanding that tiny particles like photons are still waves. Just really tiny ones. I admit I still thought of them as, like... a very tiny ball. Of course it makes much more sense now that something so small wouldn't be represented that way. It's still kind of crazy to think of these photon waves as having probability fields and such. But you make another good point that it's just hard to accurately measure how such small things behave when we're so incomprehensibly massive compared to them. I find quantum mechanics (and trying to understand them rationally) an engaging subject, maybe because I want to understand how the universe and physics work at its most basic, fundamental level. Also, kind of trippy to think everything that exists is fundamentally made of very, very tiny waves colliding and interacting with each other.
On wave/particle duality. Sub-atomic particles very well may be point charges moving about in a wave-like pattern. The PROBABILITY is based on interacting with particles and finding the likelihood of them being in a certain point. It's not a provable "thing" in the sense of physicality. It's probably best to think of them as PARTICLES which locally vibrate in a wave-like pattern whether standing still or propagating through space.
I saw a theory today about this which was the particle is ' riding ' the wave much like a surfer . The wave splits into 2 and the particle goes one way and the two waves create the interferance, this I think would account for the interference but not the collapse when which way is detected
Hi Nick. My 12 year old son wandered across your videos started asking me a lot of questions. He loves your videos and I must say that I am also highly impressed. One part of your explanation, and that of other videos on this topic, show that when an observer is present (a measurement process that must in some way interact with the particle moving through the slits) the act of observing is "collapsing the wavefunction". The interference pattern changes quite significantly from the far-field pattern of a normal two slit case to two stripes. Given that the far-field pattern is itself due to the size of the slits, their spacing and that sufficient distance has been allowed to propagate the wave/particle to the screen to show a particular diffraction pattern, wouldn't the two individual stripes be overlapping, representing the overlap of the single-slit far-field diffraction patterns; one from each slit. However, if this is the near-field (to the slits) pattern of the "single-slit case" wouldn't the interference pattern of the "two-slit case" look more like the fresnel diffraction pattern with a higher-spatial frequency interference pattern than you present? When you take into account that the BBO crystal is causing a degree of decoherence between successive photons entering the system, the interference pattern should wash out into a incoherent pattern. You may want to review the comments I made and links I provided to Sabine Hossenfelder's video: th-cam.com/video/RQv5CVELG3U/w-d-xo.html This is a difficult experiment to use as a basis of explanation of the Delayed Choice Quantum Eraser but variations on it investigated by Alain Aspect offer less room for misinterpretation and highlight the weirdness of such quantum experiments. Using an experimental setup which does not use a BBO crystal to achieve entanglement limits the complexity of the analysis. For those interested to review another source that is relatively accessible (again .. judging by my 12 year old son's ability to pick up the main points), take a look at: th-cam.com/video/U6fI3brP8V4/w-d-xo.html and th-cam.com/video/uK2eFv7ne_Q/w-d-xo.html . I truly respect your work and how it is having an impact on learners of all ages. You certainly have a big following amongst pupils at my son's school as he cannot stop talking about the wonders you have shown him.
jesus. cant imagine the youtube science community without you. I actually watched every single one of the videos(and some more) u showed at the begining (and agree with ur verdit 100%)and none of them gave me the satisfaction that this vid did. I'm donating in the very near future.
I am constantly amazed at how well you create highly educative yet very funny videos (I love all the twins appearances and recurring jokes like the energy conservation ^^!)! This one particularly helped me demystify a lot about my understanding of this experiment. Thanks so much for what you're doing :) !
Thanks again adds a lot of clarity and demystification to this well covered topic, just goes to show if I tune into Science Asylum there's always always something to gain.
10:37 "You can just as easily say the pattern on the wall is what determines what happens at the other detector". That doesn't explain why the pattern changes when you modify the other detector, though.
You're correct, but the point I was trying to make was that we shouldn't be so quick to jump to conclusions. Quantum mechanics tells us _a lot less_ than we (subconsciously) want it to.
Thank you, Nick Lucid, for your patience in replying to a question I had in another recent video. I had lost sight of the fact that you were dealing with motion/relativity; I hadn't yet put aside my (ancient) education in classical physics, and you very kindly set me straight. That really matters when someone's doing a scary thing - physics on their own!
Hi Nick! You're really clever. I've learned new things about the experiment. You show it in a different perspective. Well done, it's a pleasure to support you on Patreon.
This is probably the best, clearest explanation I've ever seen for the DSE, especially with the quantum eraser!! Kudos. I do feel the last bit was really rushed, and I see I'm not the only one. Could have definitely been worth the while to spend 2 more minutes on what actually happens there.
The explanation, by itself, is the best on the internet, But, the ANIMATIONS make this so damn accessible to laypeople and quasi-laypeople that it should be required as the first video ANYONE ever watches on this subject. Your gift of teaching/explaining really shines on this video!!
Well hopefully it didn't drive you insane (..or any MORE insane) because they really hammer the point home. As you mentioned, there are TONS of videos on this experiment and I've watch most of them, but nothing compares to this. THANK YOU!!
It put a big smile on my face every time Nick said QM is not magic! It was a lot of smiles... Then I laughed out loud through his conservation of energy chant! Thanks Nick! As with all your posts, my face hurts...
I'm sat here watching thinking it's about time someone talked about this with common sense ,but perhaps we had do to the mystery tour first😉 thanks for the video.
you know i watch other channels about quantum mechanics and i say to myself "i don't get it". then i watch Mr Lucid, then i say, "yeah okay, i get it". The explanation of the detectors makes all the difference here.
Nick, when you say "all those videos were designed to mystify you", I knew you gonna kill it. All those videos are twisting our minds to make us awwwww and owwwww, your videos makes us understand. You are literally the best science youtuber for education, the other ones are more in the show business. Exceptional.
Me: "how does that work?" Scientist: "We don't actually know because its all too damn small. here's some math that best fits the results of these experiments. It works so far" Me: "huh"
Yep, that's a pretty good summary of quantum mechanics. Just don't forget that we've already spent a century ruling out all the regular ways these particles could behave. That means, if what we have is wrong, then _the truth is even weirder._
@@ScienceAsylum Contrary to what I said in another comment, I'm not sure it actually _has to be_ so weird. Yeah, it _seems_ weird as heck, but what about the Pilot wave interpretation? I mean, sure it's _kind of_ odd to think there's a universal wave function that describes everything, but hardly any weirder than the idea that particles don't actually have definite properties until we measure them/they're "observed" in some manner. Why not assume we all the weirdness _is_ due to hidden variables like Einstein thought? We know it's not due to _local_ hidden variables, but even Copenhagen gives up the idea of locality anyway. If that's already out the window, why _also_ give up on determinism, as Copenhagen does? Giving up on determinism seems to largely be responsible for all the quantum weirdness, but we don't actually have to do that. We only have to give up _one_ of, determinism, locality, realism, or causality. Of the four, locality seems to be the least fundamental, though that's obviously debatable. But why give up _more than one_ as Copenhagen does, when it's not necessary? I still think the pilot wave interpretation of QM is the most plausible because it better fits with our observations of the macroscopic world in that it's deterministic. The biggest problem is that (as far as I'm aware, though I might be wrong) there's not an accepted relativistic version of it yet. But QM by itself is also not relativistic. QFT is though, so it's likely just a matter of someone (or more likely multiple someones) coming up with an analogous version of PWT. Of course, there are other deterministic interpretations, like many worlds, but if we're either postulating the idea of infinite universes or postulating the existence of a global wave function, the later seems a far simpler assumption and hence should be preferred via Occum's razor. What do you think?
Thanks for the best explanation I've ever seen. I've long had headaches trying to explain people that an "observer" is really just "anything that interacts with the particle, conscious or otherwise".
I love the down to earth approach! Reality check 🙂 let's not go down rabbit holes, thank you! Even some of the most brilliant scientists seem to like the "explain quantum mechanics as science fiction" approach in order to attract people, and I can understand wanting people to jog their minds and become creative. But let's beware of rabbit holes and wonderland 😧
Like a mental fog light, your video illuminated a subject that I could never see. QUANTUM MECHANICS IS NOT MAGIC. The double slit experiment has been misrepresented SO many times... THANKS NICK!
Two things I've been wondering about this: 1. According to Special Relativity photons don't experience time, so could that be part of the explanation? That from the perspective of the photons both detections happen at the same time? 2. If instead of a beamsplitter you used a switch that sends all of the photons either to the detectors or to the eraser, would that get you an obvious pattern on the screen before the data of the detectors become available? I expect not, but from the description of the experiment I can't find a reason why it wouldn't.
#1 - Special Relativity does not say photons do not experience time. Special Relativity is about LOCAL TIME etc but photons have a definite speed which is the speed of light. It takes eight minutes roughly for a photon to leave the sun and hit the earth. There's currently nothing that says they are outside of time or that the laws of causality can be violated. (however things might be vastly different at some deeper level but the observations by Einstein don't imbue photons with such properties as being outside of time etc...) I know there are thought experiments about "how would you experience time bla bla at the speed of light" but then jumping to the conclusion that photons are outside of time. That is simply not what's going on. #2 - Switch? Remember it is ONE photon at a time. If there's no beam splitter and you send to the screen then the results should be the same as the normal double-slit experiment. If you sent it to the detector instead of the screen it would just... detect them... of course, nobody truly understands how the SPLIT photon is aware of the other split photon especially when there's a time delay for detection but again without splitting them you're sending the single photon to one detector or the other. If you sent MILLIONS of photons to the screen only, then MILLIONS of photons to the detector and got different results from the screen alone that suggests.... it makes no sense actually. How could you send photons to a screen, build up a pattern, for say five minutes then do... nothing. Then repeat that but this time after you spent five minutes sending photons you start sending to the detector and...no wait, the screen pattern would need to change BEFORE you started because of... time travel? No... I get that there's some appearance of knowing the future but it can't actually be based on everything we know. It's possible that the speed of light is not the maximum speed and that there are much smaller, faster particles interacting with our environment in ways we can't yet know.
@@photonboy999 1. Okay, first of I haven't actually studied this stuff, I'm just a layperson who got everything I know about this subject from videos like this one and popscience books. But that being said, what I've learned about Special Relativity does indeed say, that photons (or anything else moving at the speed of light) do not experience time. Not that they are "outside" of time, but that time dilation becomes so great at the speed of light, that all of time appears as a single instant. And that's not just thought experiments, it's in the equations and there's also observational evidence from particles moving close to the speed of light. 2. That's what I was getting at. If you take the randomness out of it and all of the photons get either detected or erased, could you tell which is the case just by observing the screen? That would essentially mean seeing the future, so I expect there's something that prevents this, but I can't figure out what it is. Having thought some more about this, both these questions are a little beside the point though. The real question is: what is the physical difference between detection and erasure that makes the wave function collapse in different ways? That does indeed seem eerily like "the photons knew we were watching"
i think the best way to think about if light and time is that light experiences what we call time as space. light cannot tell the difference between what we call space and what we call time. so yes you are right light does not experience time but that is because the order of events is purely spatial. which has the same limitations of time.
Nick, I've said this before, but there is something about the way you explain things that inevitably makes me go "OHhhhhhhhhhh" at least once per video. This time, though, it was the whole damn video that made me go "Ohhhhhhhh!". Seriously, I felt like I understood the double-slit experiment pretty well, but suddenly it ALL MAKES SENSE. Thank you! Please keep doing these videos -- you have a peerless talent for explaining science
Good christ! FINALLY an explanation that doesn't resort to trying to make these experiments and their results seem more complex and mysterious than they actually are with 'woo' and over anthropomorphising. Of all the thousands of videos out there this is probably the most accurate and clear off all. Great job!
The explanation starting at 5:41 didn't just blow my mind, it reassembled it better than it was before. Everything clicked when i saw photons as probability waves, the "dot" you detect is just the random sample from that probability distribution
So why doesnt the detection of each single photon resemble a wave? Why does the wave collapse into a single point instead distributing a wave pattern onto the detection screen?
You really outdid yourself with this one. Easily the best video on the double slit I've seen. And I've seen my share. Good work demistifying the thing.
Quantum information isn't bound by time?! Literally, mind-blowing. This channel is AMAZING. Where have you been all my life??? This is right up there with Mark Rober and Smarter Every Day infotainment.
I think this is one of your best videos so far! I really enjoyed it! I think the reason why people love your channel is because you have a no bullshit approach to physics, you just say things as they really are and people appreciate it! Other channels just l o v e to mystify things because making physics more and more "misty" for some reason cranks up your view count. But channels like yours really are the best there are because you don't care about mystifying things, you just care about debunking all that "bullshit" that other channels and other people say about physics that are just not real. I have watched many double slit experiment videos online and I'm always left with some sense of mystery, but your video is as clear as it can possibly be, and I thank you for that because I learn a lot from this video and helped me clear up all these misconceptions that I've been gathering along all this time! Thanks for all that quality content :)
It's videos like this that make me feel that Nick will someday be able to explain to me why one sock always disappears in the dryer. Is it in a parallel universe? Do sock pairs have quantum entanglement? Is there some guy in a parallel universe holding up my other sock thinking that he's lost one or is the one that I have actually his and I have lost nothing? Can I take my cat out of the box now? Love me some Science Asylum!
Finally! A simple, easy to understand explanation of the double-slit & quantum eraser experiments. I've watched many videos about these experiments and none have given me a satisfactory answer until now. Thanks!
@@ScienceAsylum my grandson says to tell you, yes,it's ok to be a little crazy, we all are to some extent, lol! Till next time,remember,it's absolutely a/ok to be a little CRAZY. 😉😊😂 :p
@@undernetjack The universe just do what it does. Science tells us HOW is its behavior, not WHY, it has nothing to do with quantum mechanics. You can ask the same thing about classical mechanics: why bodies with mass attract each other, why energy and momenta are conserved, why the principle of least action is true? Any trying to answer these questions scientifically will only lead to circular arguments, it's just the way the universe works.
Whenever i try to explain concepts like the double slit experiment, entanglement, wave/particle duality, or other QM stuff to someone and they ARENT blown away... I just assume they dont understand and i need to try harder to explain it. You cant NOT be blown away by QM! Needless to say, this doesnt make me to popular with those i make listen to me babble... Lol
While the video's attempt to de-mystefy the issue is admirable it commits a number of critical errors: 5:30 "interaction with the polarizer caused the collapse" But the delayed choice experiment already refuted this idea. If you put a detector on just one slit the photons will still pass through the other slit as particles without having interacted with the detector. Even crazier, if the detector is placed further downstream from the slits the photons will pass as particles through the slits before having even interacted with the detector. 5:40 "humans are not important" But whenever and wherever there is ANY kind of "which way" detection we'll get particle pattern. Because the act of having knowledge is what causes the collapse then humans are the ONLY thing that matters. The type and placement of the detector is not the determining factor here, the observer is. 10:35 "the screen is also a detection" A screen cannot be a detector because all it registers is interference pattern. It cannot just decide to detect particles on its own then go on to influence the detector afterwards, that's not how it works.
Can you please provide me a link reference in where they explain that they use polarizer filters on the slits to produce the collapsed wave function (two stripes on the screen) part of the DS experiment?
"Do I feel any better about quantum mechanics..." I know the video was meant to help, but I don't feel like there's any non-magic way to explain why the photons are doing that. Like, WTF, it's like the photons already know where they're going to land before they've left and behave differently based on how we try to measure them? "I see two different options in my future, better act this way... oh, three different options? Better act this way."
The explanation on the double slit experiment was brilliant. The quantum eraser experiment still puzzles me though. Even if we think of the screen as a detector I don't see how the entangled "twin" can know if the other one went through the "which way" path or not after the fact and how it can be erased.
Yes, I am also struggling with this big time. Even if we consider the screen a detector, and the shape of the wave function as it hits the screen determines whether or not it will get detected at detectors A or B, or it will pass through to the quantum eraser box. This still makes absolutely no sense to me. If there is no polarizers at the slit, wouldn’t the photon pass through both slits, hit the BBO crystal and prism, one of the entangled twins would hit the screen dispersed in space as an interference wave function, the other entangled twin would pass through to the quantum eraser box in this case. That would happen every time if there is no polarizer at the slit and it is true the screen detection determines what happens at rest of the apparatus. Why would the photon ever pass through just one slit and hit the screen in the more localized single peak wave function and in turn be detected at detectors A or B if there are no polarizers at the slits to cause this to happen. I am struggling really hard with rectifying this one aspect of this explanation, it just doesn’t seem to make any sense to me if the screen is determining what happens at the rest of the apparatus, it would still be dependent upon if there is a detector at the slit or not (in this example the detector is a polarizer). Could you please explain this more clearly, please. @The Science Asylum
PBS space time takes too long to say anything. with something like this the shorter the distance between concepts the better chance that you'll hold on to one thing in order to connect it to the next thing.
His conclusion is, when you’re making ripples in a pool, and watching them go through a block of ice with two slits, and strike another block of ice, then you start adding thin sheets of ice everywhere to create new results, you cannot jump to too many conclusions about the nature of the ripples in the pool. Less you become confused, and lose sight of what truly makes sense. Your in a pool, studying ripples of water striking other items made of water.
@@paaao what you said makes no sense, and I don't think you think it makes sense, but 10 year olds do, even though they can't connect it to the question
The point - which some people would call magic - still stands; if we interact with (measure) a wave function it collapses into something different. Or by the definition of the most advanced interpretations; If you look, the universe renders part of its source code for you into "reality" .
This is THE double-slit experiment video. Almost everything else out there makes me feel like I'm watching some hysterical, overly-dramatic fluff on cable TV. Your channel is a huge contribution to the TH-cam community. Thank you! 8)
I think this is the most correct view I've seen on particle/wave theory of light. Light is a wave, but the interactions or events happen as particles. To understand the delayed choice experiment I think you must realize that Relativity says that time is relative to the observer, and that past, present future relationships of some events are relative to the observer. Even though the current view is that Quantum Mechanics and Relativity are incompatible I believe that they both take part in the Quantum Eraser experiment. Events only occur in space-time and different observers will not agree on the timing of the events. In our reference frame the experiment contradicts our perception of causality, but light does not see time!
I was rewatching the PBS Spacetime video about the quantum eraser and Matt kept talking about how it _seems like_ the particles somehow know we're looking and change their behavior. But I kept hearing you say "Quantum mechanics is NOT magic" and it made me want to rewatch this video. It's probably my favorite one of your videos. Definitely in the top 10.
In Matt's quantum eraser video, he says something similar at about the 1 minute 52 second mark... but then he moves on and it's easy to forget he said it.
@@ScienceAsylum I like that you make it seem less magical and more realistic. Your QM videos have influenced me a LOT over the past few years -- your channel is the thing that got me so interested in QM in the first place and I'm glad for it. Your view that what we call "particles" are really always waves is the only thing that's ever made QM really make sense -- knowing some of the math helps a lot too, but only so much. QM is still kind of weird, but thinking of it in terms if wave mechanics where the waves just localize and appear to be particle takes it from seeming magical to actually making a lot of sense. In your much earlier video about common misconceptions in QM, you said "Light is NEVER an object. It always a wave." and that phrase has stuck with me for years. I'm listening to a book on the philosophy of science right now and it's talking about wave particle duality and how paradoxical it seems. Everytime he talks about light as a classical particle I keep hearing you say, "Light is NEVER a [particle]. It's always a wave." because that's what makes the paradox stop being paradoxical.
I think I can safely say that nobody understands quantum mechanics.” It is one of the most repeated quotes of Richard Feynman (11 May 1918 - 15 February 1988), and is undoubtedly an unusual phrase coming from the mouth of a physicist.May 11, 2018
10:35 I am slightly confused here. Yes, this makes sense on a single-experiment scale, but the fact remains that the set-up of A and B (and D if present) changes what can happen on the screen. If you have just A and B, determining the positions of all photons, you get no interference. If you add in D and erase some of the information, you get two overlapping patterns - one with interference, one without. Yes, in either case it could be said that where a photon hits the detector screen determines whether the entangled photon hits A or B (or D) at this setup, but it remains that what exactly the set-up is (in this case whether you have the eraser or not) has an affect on what is *possible* at the detector screen in the first place. How is this explained?
It isn't yet. And that is why there are many interpretations. Because we do not have the math to explain this? the physicists are trying to make informed guesses of what is happening. Those are the interpretations.
But we, as observers, DO experience time. So for us, it seems like the photon went back in time. But of course, for the photons themselves there is no such thing as time, and do they can not go back in time.
Maybe it's because photons like PEANUT BUTTER? My point is that the ONLY important questions in physics are those that lead to predictions that can be tested. Unless you can conduct an experiment that proves photons are outside of time in the real world (which is doubtful) it's a pointless question.
To clarify: I understand the "detection" at the screen is a different kind than one that might happen at the slits or any other detectors. It was just meant as a segue/transition to the main conclusion of the video: that quantum mechanics is just a model. The way quantum mechanics works doesn't necessarily say _anything_ about the nature of matter/light. If we input some initial conditions and a few interactions, it will give us a result that matches an experiment really well... but only after we do the experiment with a _bunch_ of particles over time. That doesn't mean we can make any judgements _at all_ about what individual particles are doing at any given moment.
I disagree. The detection at the screen is in fact exactly like the detection at the eraser. The only difference is that it is "softer". When photons hit the screen, their position is consistent with both patterns, but each position has different probability for each pattern. Hitting the peak of the "detected" pattern, makes the second photon guaranteed to hit the detector. Hitting the troth of the "detected" pattern, makes the second photon guaranteed to hit the eraser. When the photon hits the screen somewhere in between, the second photon gets some probability of landing either in detector or eraser. The probability is proportional to the "consistency" of the first photon's position on the screen with given outcome.
@@KohuGaly ... Oh, yes? And what if I turn off or remove the detector after the 1st photon hit the screen in the "peak" zone?
@@adb012 It doesn't matter. The overall pattern on the screen is the same. It is the way it is, because the second photon leaks information into the wild.
Whether you actually happen to utilize that leaked information to figure out something about the first photon is irrelevant.
I actually thought that was the most important insight in the video. It is something most other Q.E. videos don't even mention. There absolutely is an interaction at the screen, and the pattern on the screen, taken alone, is the same either way. So the inference is that the detection at the screen can select for the result at the eraser without having to deal with anything going back in time. It's the simpler explanation of the two. Result at screen selects for the result at the eraser rather than the result at the eraser going back in time and selecting for the result on the screen. I was thinking about the Q.E. experiment all wrong until I watched this video (and I've gone through at least a dozen videos and two papers trying to understand it).
I'm really happy you did a video on the quantum eraser. You have a way of explaining things that allows one to home in on the parts that are most important.
-Matt
Another excellent physics video. It seems to me and a few others that the only non-hocus-pocus model is the de Broglie/Bohm pilot wave model. Its really too bad de Broglie let his contemporaries talk him out of further study of his insightful realistic model.
I love the “QM is not magic” mantra! Also particles being tiny waves makes a lot more sense than “they’re particle-wave magic hand-wavy particles!”. I’m so tired of being bamboozled by QM and double slits stealing my lunch money. The final part was a bit rushed for me though, I’d love to have the controversy around what’s going on addressed more meticulously maybe in another video.
dan fg Agree that the end did not clearly explain how the results stated where obtained.
man the quantum mechanics was not magic until the end when "models don't explain everything perfectly" soooo....... magic?
@@jurusco Nope. Still not magic.
Our inability to understand the universe with current models does not mean the universe is inherently not understandable. We just need better models.
Of course its possible that there is no model that can be completely understood by humans due to our inability to measure things beyond the limits of our own existence (multiple universes or dimensions, for example.) But that still doesn't make it magic in the sense of "there are no rules".. it just means we can't necessarily know all the rules.
@@altrag depends how you define magic. There are soft (no rules) magic and hard (with rules, like conservation of energy) magic systems in myths, legends and video games. You can also define magic as things that you don't understand. So, until we understand it better, one could argue that QM is magic (a system that has rules which we don't know about). The same way fire and thunder were regarded as gods until we devised tools not only explain, but understand at fundamental levels; enough to reproduce them (We still don't know a lot about Space, so that's why many people are into Zoroastrianism). Naming QM (or anything) magic should not prevent us from investigating it further though.
@@JamesSarantidis I'm defining magic not as "things we don't understand," or even as "things we can't understand due to the limitations of the human mind." I'm defining magic as "things that are fundamentally not understandable _even by God_ " (that is, by a being that has absolutely no limitations on their intellect.)
So kind of along the line of your "soft" definition, but in the most strict formulation of "no rules."
The best explanation of the double slit experiment ever. I actually picked up quite a few new pieces of knowledge.
Nope. He's full of crap. Photon. Now explain it with a protein molecule. He's also ignoring the wave collapse experiments that were done purely on thought. He's clinging grimly to his imagined materialist pseudo reality. lol
@@RAF71chingachgook Having looked at your channel I think I can safely ignore your opinion without losing any sleep over what I might have missed out knowing but thanks for throwing your 2 cents worth in anyway.
@@nybblesbytes4321 That's it buddy. Stay blind. This guy is preaching ignorance.
I would immediately forget it. This guys doesn't know what he's talking about.
RAF71chingachgook
RAF7qchingachgook
I agree.
If someone says physics is ...
- boring
- for nerds
- difficult to understand
- badly explained
- needs advanced math knowledge
- or any other misconception ...
Just send them to the Science Asylum and they 'll see the 'light' in all its beauty and clarity !
I can agree that these videos are great and they make "attempts" at what you claim
@@michaeld9682 What's the difference between an attempt and an "attempt"
@@stupidrainbo the same difference between difference and "difference"
@@michaeld9682 Is the difference between difference and "difference" the same as the difference between same and "same"?
@@stupidrainbo same difference
Whenever one of my classmates asks “so energy wasn’t conserved?” I always think of you:
CONSERVATION OF ENERGY SHALL NOT BE VIOLATED!! ⚡️
many people who try to explain the double slit experiment always do so in a very vague manner , its like they don't fully understand it .... but your explanation was very elaborate nick, you did the best way ... better than all other explanations.
Clearest quantum eraser description i have encounter.
I understand now!
Feynman: "If you think you understand quantum mechanics, you don't understand quantum mechanics."
Oh...
Ha!
I think the often quoted alleged Feynman statements are by far too pessimistic. I don't know whether he still believed that there must still be something "thingy" about elementary particles like the electron ("wherever it is, it is a point charge") not regarding this as a mere artefact of it being elementary and thus just interacting as a whole.
@@jensphiliphohmann1876 ,
How much Feynman have you read? He was never pessimistic in this way. Also, the idea that there is some "point charge" is meaningless to him and all advanced physicists. QED. Feynman diagram.
@@photonboy999 I read some lectures and the QED book where Feynman is not pessimistic at all. My statement refers to some alleged Feynman quotes like "shut up and calculate" or so, and the alleged Feynman statement that no one understands quantum mechanics.
Statements that don't really fit to Feynman's books.
Feynman also said very clearly in his NZ QED lectures that light is a particle... though I think I prefer Nicks view that the detection of a single photon is indeterminate about its particle or waviness properties.
This is a very useful and important video. The mystification of quantum mechanics is pedagogically horrific. We should always assume that everything can be understood, even if we don't fully understand it at this time.
Technically the more "radical" interpretations are not based on giving up on understanding but having a more nonsensical understanding.
Observing collapses the wave function...I want to try this test in water or some other medium to record the track to the interference pattern rather than detector at the slits...hmmm...I’m going to solve this...
Quite bold to assume that the human mind can understand everything. That’s like saying we are the supreme intelligent beings in the universe. There might be things in the universe that we can never comprehend.
@@aaambrosio3622 If that were true, then by contrapoint you have said there is a finite amount of knowledge we can understand. Which is as implausible as saying there are things we can never understand.
@@thorstambaugh1520 So, are you saying that it is more plausible that the human mind is capable of understanding everything in the universe?
Quantum mechanics might not be magic, but you have a magical gift for explaining things. You just demystified in mere minutes all the other videos and books I have read about the double slit excitement. I do believe Albert Einstein would clap for you.
I was having Science Asylum withdrawals, thank you for the fix 🙃
Yeeees! Fix! Ahhhh who needs beer when this this guy posts a vid. What a guy!
Al H. Me too. He had me at hey crazies
Yes I have a withdrawal too I need heroin please help
@@beri4138 Stop using heroine you junkie
@@AL-SH Lol you can't even spell heroin
The 6 dislikes are the ancient keepers of the light. They’re displeased that you know their deepest secrets.
We love your insight Nick and your ability to share it!
Seriously though, don't you sometimes just want to know who could possibly give this stuff a thumbs down? I can't figure out those people.
@@xyz.ijk. Maybe religious absolutists insisting God did it or tin foil hat types who think all science is nefarious shadow governments trying to control us for reasons or people who don't like his voice or the colour of his shirt. People are weird like that. Hahaha
Crazy topic, expertly delivered, and the longest vid yet😁, great animations nick, 'Waves'from UK 🙋
👋
check out pbs space time explanation too
I think for these topics video length around 10-ish minutes is more appropriate - otherwise (ie if shorter) explanations could be lacking (and thus understanding)
@Cameron McHenry 😂😂😂😂
@@blueckaym Yeah, that's why this video was longer. I couldn't make it any shorter without it being confusing.
Nick, I am drunk and I still understood your video, that's how good of a teacher you are. Everything so clear and still succint. Great job!
Nick just totally slam dunked the mysticism around the double slit experiment with extreme agression. This was VITAL, because as someone who have watched those vids, and been mystified, I needed this snap back to reality. Thanks!
This video is going to be another classic. You're a legend.
It was so much work! O_O
If there will ever be some video analogy for text book knowledge, this video will certainly be a standard explanation for the topic
Here's a mystery. If a photon is the smallest unit of light, how do you end up with two entangled half-a-photons? How do you detect half-a-photon? I think the last part of your video requires watching a few times.
@@crouchingtigerhiddenadam1352 It is the smalles packet of electromagnetic energy of a given frequency. That means the frequency is halved when the photon pair is created (it can also be any other proportion as long as it sums up to the original frequency) because energy is proportional to frequency. Such a process is also known as down conversion if you want to read up on it. The crystal needs some special properties that generate a so called nonlinear optical response. Most everyday materials only have a linear response, which merely alters the path of the light (reflection and refraction).
@@halbeard2996 there is a common definition given in physics which goes something like this: _the photon cannot be split. It has zero mass. It cannot decay. It can interact with another particle lose part of its energy and change wavelength._
... and then there is QED and the *vertex function.* Questions like, _if light is an electromagnetic wave, what is the relationship between a photon and an electron?_ or _how many times can you split a photon?_ _(why is the answer 3?)_ On TH-cam you would think Quantum starts and stops with the wave function. Yet the meaty stuff is in vectors.
You know it, all my life, I’ve been waiting I’ve been waiting for this
After watching lots of explainations on this, I finally understand it. Thank you, Nick!
"I think I can safely say that nobody understands quantum mechanics." -Richard Feynman, The Character of Physical Law (MIT Press: Cambridge, Massachusetts, 1995), 129.
@@costa_marco at the time he was probably right.. So what?
@@JB_inks Chill, man. No offense intended.
what explanation?
CJ the baka,
There are various levels of understanding. However, NOBODY is confident that they understand what's going on here completely. How do the particles change what they are doing based on a seemingly future detection? Nobody knows.
you're the ONLY ONE who mentions polarizing film! this explains everything!!!
It doesn't _have_ to be polarizing film. It could be anything that reveals the slit information. I just chose polarizing film as an example of something that could do that.
Nick, this is by far the best explanation of the double slit experiment on the internet. I have watched most of them without helping me understand the deeper details. Thanks!
You're welcome :-)
Great job Nick, after watching so many videos about this trying to figure out what kind of magic was happening here, it seems you are the only one that deserves the Nobel prize that has been put at stake! great explanation & a really conclusive model
Finally a good explanation!
The interference pattern was observed not only for photons but also for large atomic and molecular classical particles. All your explanation is valid only for photons which are not actually classical particles.
is anything a "classical particle"? it's all made of energy and waves... with larger amount of elements involved, the harder it gets not to collapse the waveform because chances of interactions are getting bigger. thats why we don't fall thru the floor.
@@CodepageNet
The very simple difference: classical particles have rest mass whereas photons do not.
1) The double-slit experiment can be done with all sorts of particles. The same rules/results apply to all of them. I just used photons in my video because it was a good transition from my previous video and because they're the first particle we did this experiment with.
2) Larger particles only behave "classically" because they're constantly interacting with things making them localize. If you could get a dust particle to be isolated for long enough, it... _should_ behave like a quantum particle too.
@@ScienceAsylum Thanks!
This was a fantastic explanation. I've been trying to wrap my head around the particle/wave duality thing for ages, not to mention entanglement. This video makes things so much clearer now. Keep up the great work!
the perfect explanation u may ever get on you tube as in present and in future
I think I've seen or read dozens of explanations of the topic along the years. This is the best one yet. You kicked the most famous QM experiment right in the mystification tender spot.
Lot to soak in with this, but I think the biggest thing that meant the most to me personally was understanding that tiny particles like photons are still waves. Just really tiny ones. I admit I still thought of them as, like... a very tiny ball. Of course it makes much more sense now that something so small wouldn't be represented that way. It's still kind of crazy to think of these photon waves as having probability fields and such. But you make another good point that it's just hard to accurately measure how such small things behave when we're so incomprehensibly massive compared to them.
I find quantum mechanics (and trying to understand them rationally) an engaging subject, maybe because I want to understand how the universe and physics work at its most basic, fundamental level. Also, kind of trippy to think everything that exists is fundamentally made of very, very tiny waves colliding and interacting with each other.
On wave/particle duality.
Sub-atomic particles very well may be point charges moving about in a wave-like pattern. The PROBABILITY is based on interacting with particles and finding the likelihood of them being in a certain point. It's not a provable "thing" in the sense of physicality. It's probably best to think of them as PARTICLES which locally vibrate in a wave-like pattern whether standing still or propagating through space.
I saw a theory today about this which was the particle is ' riding ' the wave much like a surfer . The wave splits into 2 and the particle goes one way and the two waves create the interferance, this I think would account for the interference but not the collapse when which way is detected
@@chique2008 That's the Pilot-Wave interpretation of quantum mechanics. I believe it's one of the refuted ones
Yay! 11 minutes 48 seconds of pure joy!
I can’t believe it was that long: it just breezed by, so absorbing it was.
Hi Nick. My 12 year old son wandered across your videos started asking me a lot of questions. He loves your videos and I must say that I am also highly impressed.
One part of your explanation, and that of other videos on this topic, show that when an observer is present (a measurement process that must in some way interact with the particle moving through the slits) the act of observing is "collapsing the wavefunction". The interference pattern changes quite significantly from the far-field pattern of a normal two slit case to two stripes. Given that the far-field pattern is itself due to the size of the slits, their spacing and that sufficient distance has been allowed to propagate the wave/particle to the screen to show a particular diffraction pattern, wouldn't the two individual stripes be overlapping, representing the overlap of the single-slit far-field diffraction patterns; one from each slit. However, if this is the near-field (to the slits) pattern of the "single-slit case" wouldn't the interference pattern of the "two-slit case" look more like the fresnel diffraction pattern with a higher-spatial frequency interference pattern than you present?
When you take into account that the BBO crystal is causing a degree of decoherence between successive photons entering the system, the interference pattern should wash out into a incoherent pattern.
You may want to review the comments I made and links I provided to Sabine Hossenfelder's video:
th-cam.com/video/RQv5CVELG3U/w-d-xo.html
This is a difficult experiment to use as a basis of explanation of the Delayed Choice Quantum Eraser but variations on it investigated by Alain Aspect offer less room for misinterpretation and highlight the weirdness of such quantum experiments. Using an experimental setup which does not use a BBO crystal to achieve entanglement limits the complexity of the analysis.
For those interested to review another source that is relatively accessible (again .. judging by my 12 year old son's ability to pick up the main points), take a look at: th-cam.com/video/U6fI3brP8V4/w-d-xo.html and th-cam.com/video/uK2eFv7ne_Q/w-d-xo.html .
I truly respect your work and how it is having an impact on learners of all ages. You certainly have a big following amongst pupils at my son's school as he cannot stop talking about the wonders you have shown him.
jesus. cant imagine the youtube science community without you. I actually watched every single one of the videos(and some more) u showed at the begining (and agree with ur verdit 100%)and none of them gave me the satisfaction that this vid did. I'm donating in the very near future.
I am constantly amazed at how well you create highly educative yet very funny videos (I love all the twins appearances and recurring jokes like the energy conservation ^^!)! This one particularly helped me demystify a lot about my understanding of this experiment. Thanks so much for what you're doing :) !
Thanks again adds a lot of clarity and demystification to this well covered topic, just goes to show if I tune into Science Asylum there's always always something to gain.
10:37 "You can just as easily say the pattern on the wall is what determines what happens at the other detector". That doesn't explain why the pattern changes when you modify the other detector, though.
You're correct, but the point I was trying to make was that we shouldn't be so quick to jump to conclusions. Quantum mechanics tells us _a lot less_ than we (subconsciously) want it to.
@@ScienceAsylum Are you going to make a video on Quantum Mechanics interpretations? I think you'd do it justice!
@@ScienceAsylum So does that mean that magic happens here?
Finally, someone who treats Science as it is: tentative. I love you
Thank you, Nick Lucid, for your patience in replying to a question I had in another recent video. I had lost sight of the fact that you were dealing with motion/relativity; I hadn't yet put aside my (ancient) education in classical physics, and you very kindly set me straight. That really matters when someone's doing a scary thing - physics on their own!
Hi Nick! You're really clever. I've learned new things about the experiment.
You show it in a different perspective. Well done, it's a pleasure to support you on Patreon.
Great explanation! Now I want a video on the interpretations.
This is truly legendary!! You really deserve a noble prize!! No words to thank you!!!
I appreciate your grounded explanations of these topics.
This video deserves 10 million views. Finally someone who does not try to mystify quantum entanglement. Thanks !
This is probably the best, clearest explanation I've ever seen for the DSE, especially with the quantum eraser!! Kudos.
I do feel the last bit was really rushed, and I see I'm not the only one. Could have definitely been worth the while to spend 2 more minutes on what actually happens there.
The explanation, by itself, is the best on the internet, But, the ANIMATIONS make this so damn accessible to laypeople and quasi-laypeople that it should be required as the first video ANYONE ever watches on this subject. Your gift of teaching/explaining really shines on this video!!
These animations were SO HARD! Not just for me, but also for my computer. It took 6 hours to render the video.
Well hopefully it didn't drive you insane (..or any MORE insane) because they really hammer the point home. As you mentioned, there are TONS of videos on this experiment and I've watch most of them, but nothing compares to this. THANK YOU!!
This is the clearest I've ever seen the double slit and quantum eraser presented. Thank you Nick!
You are something more than a physics teacher Nick. Thanks for your work and for ... being.
It put a big smile on my face every time Nick said QM is not magic! It was a lot of smiles... Then I laughed out loud through his conservation of energy chant! Thanks Nick! As with all your posts, my face hurts...
I'm sat here watching thinking it's about time someone talked about this with common sense ,but perhaps we had do to the mystery tour first😉 thanks for the video.
11:03 "In quantum mechanics, maybe the rational perspective is the craziest." Well, that would certainly be consistent with this channel. 😀
you know i watch other channels about quantum mechanics and i say to myself "i don't get it". then i watch Mr Lucid, then i say, "yeah okay, i get it". The explanation of the detectors makes all the difference here.
First video I watch from this channel. Subscribed before even finishing it, great job.
Nick, when you say "all those videos were designed to mystify you", I knew you gonna kill it. All those videos are twisting our minds to make us awwwww and owwwww, your videos makes us understand. You are literally the best science youtuber for education, the other ones are more in the show business. Exceptional.
The animation of probability waves moving and collapsing at 5:44 onwards were really helpful, something I haven't seen before -- thanks very much!
I've never seen it anywhere either. It's straight out of my brain. I'm just excited I finally have enough skill to animate it.
@@ScienceAsylum I was going to say they're excellent! It was a prudent move to plaster them with your logo ;)
I also like these ones th-cam.com/video/KKr91v7yLcM/w-d-xo.html
th-cam.com/video/RF7dDt3tVmI/w-d-xo.html
Me: "how does that work?"
Scientist: "We don't actually know because its all too damn small. here's some math that best fits the results of these experiments. It works so far"
Me: "huh"
Yep, that's a pretty good summary of quantum mechanics. Just don't forget that we've already spent a century ruling out all the regular ways these particles could behave. That means, if what we have is wrong, then _the truth is even weirder._
@@ScienceAsylum Or it could turn out to be super simple and we just slap ourselves on our foreheads for overthinking it for decades.
@@ScienceAsylum Contrary to what I said in another comment, I'm not sure it actually _has to be_ so weird. Yeah, it _seems_ weird as heck, but what about the Pilot wave interpretation? I mean, sure it's _kind of_ odd to think there's a universal wave function that describes everything, but hardly any weirder than the idea that particles don't actually have definite properties until we measure them/they're "observed" in some manner. Why not assume we all the weirdness _is_ due to hidden variables like Einstein thought? We know it's not due to _local_ hidden variables, but even Copenhagen gives up the idea of locality anyway. If that's already out the window, why _also_ give up on determinism, as Copenhagen does? Giving up on determinism seems to largely be responsible for all the quantum weirdness, but we don't actually have to do that. We only have to give up _one_ of, determinism, locality, realism, or causality. Of the four, locality seems to be the least fundamental, though that's obviously debatable. But why give up _more than one_ as Copenhagen does, when it's not necessary?
I still think the pilot wave interpretation of QM is the most plausible because it better fits with our observations of the macroscopic world in that it's deterministic. The biggest problem is that (as far as I'm aware, though I might be wrong) there's not an accepted relativistic version of it yet. But QM by itself is also not relativistic. QFT is though, so it's likely just a matter of someone (or more likely multiple someones) coming up with an analogous version of PWT. Of course, there are other deterministic interpretations, like many worlds, but if we're either postulating the idea of infinite universes or postulating the existence of a global wave function, the later seems a far simpler assumption and hence should be preferred via Occum's razor. What do you think?
Good job explaining difficult QM concepts without trying to mystify people.
Thanks!
This made me buy your textbook. Excellent as always
Thanks for the best explanation I've ever seen.
I've long had headaches trying to explain people that an "observer" is really just "anything that interacts with the particle, conscious or otherwise".
I love the down to earth approach! Reality check 🙂 let's not go down rabbit holes, thank you! Even some of the most brilliant scientists seem to like the "explain quantum mechanics as science fiction" approach in order to attract people, and I can understand wanting people to jog their minds and become creative. But let's beware of rabbit holes and wonderland 😧
Man this video urge me to finis the Feynman lecture vol 3 as fast as possible and to read the volume more than one time.
this is the clearest explanation of delayed choice quantum eraser I've seen. whenever I need to explain it to someone this is where I'll send them
Like a mental fog light, your video illuminated a subject that I could never see. QUANTUM MECHANICS IS NOT MAGIC. The double slit experiment has been misrepresented SO many times... THANKS NICK!
This is BY FAR the best explanation I've seen of the double slit experiment. This video should be mandatory in physics classes in schools.
Two things I've been wondering about this:
1. According to Special Relativity photons don't experience time, so could that be part of the explanation? That from the perspective of the photons both detections happen at the same time?
2. If instead of a beamsplitter you used a switch that sends all of the photons either to the detectors or to the eraser, would that get you an obvious pattern on the screen before the data of the detectors become available? I expect not, but from the description of the experiment I can't find a reason why it wouldn't.
#1 - Special Relativity does not say photons do not experience time. Special Relativity is about LOCAL TIME etc but photons have a definite speed which is the speed of light. It takes eight minutes roughly for a photon to leave the sun and hit the earth. There's currently nothing that says they are outside of time or that the laws of causality can be violated. (however things might be vastly different at some deeper level but the observations by Einstein don't imbue photons with such properties as being outside of time etc...)
I know there are thought experiments about "how would you experience time bla bla at the speed of light" but then jumping to the conclusion that photons are outside of time. That is simply not what's going on.
#2 - Switch? Remember it is ONE photon at a time. If there's no beam splitter and you send to the screen then the results should be the same as the normal double-slit experiment. If you sent it to the detector instead of the screen it would just... detect them... of course, nobody truly understands how the SPLIT photon is aware of the other split photon especially when there's a time delay for detection but again without splitting them you're sending the single photon to one detector or the other.
If you sent MILLIONS of photons to the screen only, then MILLIONS of photons to the detector and got different results from the screen alone that suggests.... it makes no sense actually. How could you send photons to a screen, build up a pattern, for say five minutes then do... nothing. Then repeat that but this time after you spent five minutes sending photons you start sending to the detector and...no wait, the screen pattern would need to change BEFORE you started because of... time travel? No...
I get that there's some appearance of knowing the future but it can't actually be based on everything we know. It's possible that the speed of light is not the maximum speed and that there are much smaller, faster particles interacting with our environment in ways we can't yet know.
@@photonboy999 1. Okay, first of I haven't actually studied this stuff, I'm just a layperson who got everything I know about this subject from videos like this one and popscience books. But that being said, what I've learned about Special Relativity does indeed say, that photons (or anything else moving at the speed of light) do not experience time. Not that they are "outside" of time, but that time dilation becomes so great at the speed of light, that all of time appears as a single instant. And that's not just thought experiments, it's in the equations and there's also observational evidence from particles moving close to the speed of light.
2. That's what I was getting at. If you take the randomness out of it and all of the photons get either detected or erased, could you tell which is the case just by observing the screen? That would essentially mean seeing the future, so I expect there's something that prevents this, but I can't figure out what it is.
Having thought some more about this, both these questions are a little beside the point though. The real question is: what is the physical difference between detection and erasure that makes the wave function collapse in different ways? That does indeed seem eerily like "the photons knew we were watching"
i think the best way to think about if light and time is that light experiences what we call time as space. light cannot tell the difference between what we call space and what we call time. so yes you are right light does not experience time but that is because the order of events is purely spatial. which has the same limitations of time.
Nick, I've said this before, but there is something about the way you explain things that inevitably makes me go "OHhhhhhhhhhh" at least once per video. This time, though, it was the whole damn video that made me go "Ohhhhhhhh!". Seriously, I felt like I understood the double-slit experiment pretty well, but suddenly it ALL MAKES SENSE. Thank you! Please keep doing these videos -- you have a peerless talent for explaining science
Best explanation of the Quantum Eraser I've seen to date :-)
Good christ! FINALLY an explanation that doesn't resort to trying to make these experiments and their results seem more complex and mysterious than they actually are with 'woo' and over anthropomorphising.
Of all the thousands of videos out there this is probably the most accurate and clear off all. Great job!
The explanation starting at 5:41 didn't just blow my mind, it reassembled it better than it was before.
Everything clicked when i saw photons as probability waves, the "dot" you detect is just the random sample from that probability distribution
So why doesnt the detection of each single photon resemble a wave? Why does the wave collapse into a single point instead distributing a wave pattern onto the detection screen?
both are detectors and it doesn't matter which it hits first, that statement actually made a lot of sense :)
You really outdid yourself with this one. Easily the best video on the double slit I've seen. And I've seen my share. Good work demistifying the thing.
Thanks! It was a lot of work.
Quantum information isn't bound by time?! Literally, mind-blowing. This channel is AMAZING. Where have you been all my life??? This is right up there with Mark Rober and Smarter Every Day infotainment.
Mark and Destin are powerhouses, so that's a huge compliment.
I think this is one of your best videos so far! I really enjoyed it!
I think the reason why people love your channel is because you have a no bullshit approach to physics, you just say things as they really are and people appreciate it! Other channels just l o v e to mystify things because making physics more and more "misty" for some reason cranks up your view count. But channels like yours really are the best there are because you don't care about mystifying things, you just care about debunking all that "bullshit" that other channels and other people say about physics that are just not real.
I have watched many double slit experiment videos online and I'm always left with some sense of mystery, but your video is as clear as it can possibly be, and I thank you for that because I learn a lot from this video and helped me clear up all these misconceptions that I've been gathering along all this time!
Thanks for all that quality content :)
Clear as "it possibly can be" is still very "misty"
It's videos like this that make me feel that Nick will someday be able to explain to me why one sock always disappears in the dryer. Is it in a parallel universe? Do sock pairs have quantum entanglement? Is there some guy in a parallel universe holding up my other sock thinking that he's lost one or is the one that I have actually his and I have lost nothing? Can I take my cat out of the box now? Love me some Science Asylum!
11:14 The answer is: No, Maybe and Yes, all at the same time, depending when you ask and if you erase the answer or not.
Finally! A simple, easy to understand explanation of the double-slit & quantum eraser experiments. I've watched many videos about these experiments and none have given me a satisfactory answer until now. Thanks!
You're welcome :-)
My grandson & I love your channel. We watch your videos every night before bedtime. He'll be 9 in April.
That's wonderful! 😊
@@ScienceAsylum my grandson says to tell you, yes,it's ok to be a little crazy, we all are to some extent, lol! Till next time,remember,it's absolutely a/ok to be a little CRAZY. 😉😊😂 :p
I agree. We're all a little crazy 🤪
At last someone is talking rationally about the double slit experiment.
Hear hear, I was about to say the same.
No quantum consciousness or other woo.
I've always never fully understood this experiment but now I do.
>pulls rabbit out of hat< He still just glosses over the WHY with some hand waving.
@@undernetjack
Then you explain it better!
@@undernetjack The universe just do what it does. Science tells us HOW is its behavior, not WHY, it has nothing to do with quantum mechanics. You can ask the same thing about classical mechanics: why bodies with mass attract each other, why energy and momenta are conserved, why the principle of least action is true? Any trying to answer these questions scientifically will only lead to circular arguments, it's just the way the universe works.
Whenever i try to explain concepts like the double slit experiment, entanglement, wave/particle duality, or other QM stuff to someone and they ARENT blown away... I just assume they dont understand and i need to try harder to explain it. You cant NOT be blown away by QM!
Needless to say, this doesnt make me to popular with those i make listen to me babble... Lol
While the video's attempt to de-mystefy the issue is admirable it commits a number of critical errors:
5:30
"interaction with the polarizer caused the collapse"
But the delayed choice experiment already refuted this idea.
If you put a detector on just one slit the photons will still pass through the other slit as particles without having interacted with the detector.
Even crazier, if the detector is placed further downstream from the slits the photons will pass as particles through the slits before having even interacted with the detector.
5:40
"humans are not important"
But whenever and wherever there is ANY kind of "which way" detection we'll get particle pattern. Because the act of having knowledge is what causes the collapse then humans are the ONLY thing that matters.
The type and placement of the detector is not the determining factor here, the observer is.
10:35
"the screen is also a detection"
A screen cannot be a detector because all it registers is interference pattern. It cannot just decide to detect particles on its own then go on to influence the detector afterwards, that's not how it works.
...
My brain is even more broken
Bravo one of the best (probably the best) explanations of the DS experiment.
Can you please provide me a link reference in where they explain that they use polarizer filters on the slits to produce the collapsed wave function (two stripes on the screen) part of the DS experiment?
easily, best explanation of quantum eraser, even better than PBS SpaceTime.
Thank you grand master Lucid 👌
"Do I feel any better about quantum mechanics..."
I know the video was meant to help, but I don't feel like there's any non-magic way to explain why the photons are doing that. Like, WTF, it's like the photons already know where they're going to land before they've left and behave differently based on how we try to measure them? "I see two different options in my future, better act this way... oh, three different options? Better act this way."
@@BreytnerNascimento No, it's not.
The explanation on the double slit experiment was brilliant. The quantum eraser experiment still puzzles me though. Even if we think of the screen as a detector I don't see how the entangled "twin" can know if the other one went through the "which way" path or not after the fact and how it can be erased.
Yes, I am also struggling with this big time. Even if we consider the screen a detector, and the shape of the wave function as it hits the screen determines whether or not it will get detected at detectors A or B, or it will pass through to the quantum eraser box. This still makes absolutely no sense to me. If there is no polarizers at the slit, wouldn’t the photon pass through both slits, hit the BBO crystal and prism, one of the entangled twins would hit the screen dispersed in space as an interference wave function, the other entangled twin would pass through to the quantum eraser box in this case. That would happen every time if there is no polarizer at the slit and it is true the screen detection determines what happens at rest of the apparatus. Why would the photon ever pass through just one slit and hit the screen in the more localized single peak wave function and in turn be detected at detectors A or B if there are no polarizers at the slits to cause this to happen. I am struggling really hard with rectifying this one aspect of this explanation, it just doesn’t seem to make any sense to me if the screen is determining what happens at the rest of the apparatus, it would still be dependent upon if there is a detector at the slit or not (in this example the detector is a polarizer). Could you please explain this more clearly, please. @The Science Asylum
Best explanation of the quantum eraser I have seen.
To be honest... I find your explanations to be better than "PBS Space Time"... please keep up the awesome work.
PBS space time takes too long to say anything. with something like this the shorter the distance between concepts the better chance that you'll hold on to one thing in order to connect it to the next thing.
"the humans are not important" is the best quote for 2019 😂😂😂
So it seems like you have no conclusion at all about the quantum eraser.
True, he explained everything else perfectly though.
His conclusion is, when you’re making ripples in a pool, and watching them go through a block of ice with two slits, and strike another block of ice, then you start adding thin sheets of ice everywhere to create new results, you cannot jump to too many conclusions about the nature of the ripples in the pool. Less you become confused, and lose sight of what truly makes sense. Your in a pool, studying ripples of water striking other items made of water.
Wouldn't that make empty space a "wave?" Also it's "lest" not "less you become confused"
@@paaao what you said makes no sense, and I don't think you think it makes sense, but 10 year olds do, even though they can't connect it to the question
AltCapRight, for real! Everything is light.
Wow, now that you have explained I can see that even the "particle" pattern is simply a wave
The point - which some people would call magic - still stands; if we interact with (measure) a wave function it collapses into something different.
Or by the definition of the most advanced interpretations; If you look, the universe renders part of its source code for you into "reality" .
@@kolkolak I am an app developer and this source code reference was so on point 😅
HOW IS IT YOU HAVE
This completely just clarified my understanding of probability waves. Wow. That's awesome.
"Quantum mechanics is not magic".
The lady doth protest too much, methinks.
The latter half of this video was in no way convincing that it's not magic.
This is THE double-slit experiment video. Almost everything else out there makes me feel like I'm watching some hysterical, overly-dramatic fluff on cable TV.
Your channel is a huge contribution to the TH-cam community. Thank you! 8)
Oh rawr! Just what i Needed! Tanglements
I think this is the most correct view I've seen on particle/wave theory of light. Light is a wave, but the interactions or events happen as particles. To understand the delayed choice experiment I think you must realize that Relativity says that time is relative to the observer, and that past, present future relationships of some events are relative to the observer. Even though the current view is that Quantum Mechanics and Relativity are incompatible I believe that they both take part in the Quantum Eraser experiment. Events only occur in space-time and different observers will not agree on the timing of the events. In our reference frame the experiment contradicts our perception of causality, but light does not see time!
I was rewatching the PBS Spacetime video about the quantum eraser and Matt kept talking about how it _seems like_ the particles somehow know we're looking and change their behavior. But I kept hearing you say "Quantum mechanics is NOT magic" and it made me want to rewatch this video. It's probably my favorite one of your videos. Definitely in the top 10.
In Matt's quantum eraser video, he says something similar at about the 1 minute 52 second mark... but then he moves on and it's easy to forget he said it.
Also, thanks. Lots people hate on me in the comments when I remove the "woo woo" from QM. It's like they're taking it personally or something.
@@ScienceAsylum I like that you make it seem less magical and more realistic. Your QM videos have influenced me a LOT over the past few years -- your channel is the thing that got me so interested in QM in the first place and I'm glad for it. Your view that what we call "particles" are really always waves is the only thing that's ever made QM really make sense -- knowing some of the math helps a lot too, but only so much. QM is still kind of weird, but thinking of it in terms if wave mechanics where the waves just localize and appear to be particle takes it from seeming magical to actually making a lot of sense. In your much earlier video about common misconceptions in QM, you said "Light is NEVER an object. It always a wave." and that phrase has stuck with me for years. I'm listening to a book on the philosophy of science right now and it's talking about wave particle duality and how paradoxical it seems. Everytime he talks about light as a classical particle I keep hearing you say, "Light is NEVER a [particle]. It's always a wave." because that's what makes the paradox stop being paradoxical.
Dude, I finally understand quantum mechanics! Thank you for simply explaining what other scientists couldn’t!
Glad I could help 👍
I think I can safely say that nobody understands quantum mechanics.” It is one of the most repeated quotes of Richard Feynman (11 May 1918 - 15 February 1988), and is undoubtedly an unusual phrase coming from the mouth of a physicist.May 11, 2018
10:35
I am slightly confused here. Yes, this makes sense on a single-experiment scale, but the fact remains that the set-up of A and B (and D if present) changes what can happen on the screen.
If you have just A and B, determining the positions of all photons, you get no interference.
If you add in D and erase some of the information, you get two overlapping patterns - one with interference, one without.
Yes, in either case it could be said that where a photon hits the detector screen determines whether the entangled photon hits A or B (or D) at this setup, but it remains that what exactly the set-up is (in this case whether you have the eraser or not) has an affect on what is *possible* at the detector screen in the first place. How is this explained?
It isn't yet. And that is why there are many interpretations. Because we do not have the math to explain this? the physicists are trying to make informed guesses of what is happening. Those are the interpretations.
Science Asylum: "Quantum Mechanics is not magic"
Also Science Asylum: "The Quantum Eraser is a time machine".
Very few youtubers are able to blow my mind, you're the first place on my top 5, what a genius explanation, just bought your book on gumroad.
as usual this channel presents a topic as if it’s going to be very simple - only to explain it in the most convoluted way I can think of.
If you find his explanations so confusing, then why do you keep watching his videos in the first place?
Maybe there is no true 'delay' because photons do not experience time?
That’s soooo trueeee!! Never thought about that!!!
But we, as observers, DO experience time. So for us, it seems like the photon went back in time. But of course, for the photons themselves there is no such thing as time, and do they can not go back in time.
for photons, that is true, but the experiment works with pretty much anything.
However, these experiments work the same with particles with mass, that do experience time. Just remember, it is probability, not magic!
Maybe it's because photons like PEANUT BUTTER?
My point is that the ONLY important questions in physics are those that lead to predictions that can be tested. Unless you can conduct an experiment that proves photons are outside of time in the real world (which is doubtful) it's a pointless question.
Best explanation of all of these topics that I've seen - well done.