Your Daily Equation #21: Bell's Theorem and the Non-locality of the Universe
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- เผยแพร่เมื่อ 5 ต.ค. 2024
- Episode 21 #YourDailyEquation: Albert Einstein and his colleagues Podolsky and Rosen proposed a simple way to rid quantum mechanics of its most disturbing feature--called non-locality--in which an action undertaken here can affect the result of a measurement undertaken there, even if here and there are far apart. John Bell came up with a way to test Einstein's vision of reality, ultimately showing that Einstein's vision was wrong.
Even if your math is a bit rusty, join Brian Greene for brief and breezy discussions of pivotal equations and exciting stories of nature and numbers that will allow you to see the universe in a new way.
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This video is just the best for someone like me, an amateur physics enthusiast, trying to understand the 2022 Nobel Prize. Thank you Professor Greene! I remember when I first saw The Elegant Universe on Nova in my bedroom as a kid in high school. You opened my mind to the beauty and wonder of the universe. It truly changed my life. I've also recently read The Hidden Reality. You are a treasure to humanity. I didn't expect to suddenly be gushing your praises, but you really have done so much for me when I think about it. So, thank you very much!
I couldn’t agree with you more. Also if we could bring Einstein into 2022 I have to think Einstein will accept the result from Quantum Mechanics but come to understand that quantum fields are one object, basically entangled particles aren’t truly particles they are messengers of a single wide object and they are the same object, and that their superposition is outside of our lightcone so to speak . We can’t see it, it doesn’t defy reality
Same, but I saw it as a kid
I've seen lots of attempts to explain this with fancy graphics but it never really clicked for me. Then this guy comes along and calmly explains it from his living room without any whizbang graphical aids and it all falls into place. Bravo, professor!
Wow thanks professor Greene, this is the first time ever that I have heard an explanation of this subject that I could get my head around. Have not missed one episode so far. Your Daily Equation is the best thing to have come out of this corona crisis for sure!
Yeah, it's really good. I think Sabina hoffstader could benefit from this. She is claiming there is no spooky action at distance and the state at start is simply not known, ie a hidden variable. In other words she assert EPR.
Yet Bells Inequality refutes EPR notion.
"Fine" example of MODERN PHYSICS: You have to BELIEVE guys like him. Running this experiment??? WHERE was it done, mate? And by WHOM? Jesus (if he existed) convinced a couple of dumb fisthermen to spread his strange ideas, nowaday physicists do the same on TH-cam. There too many theoretical physicists! But still they can't explain the magnetic moment of a proton. Physics is going down the drain!
Came here to make this comment. I have read and seen so many explanations but this is the first I have understood.
@@joeboxter3635 She doesn't say so if you watch the video more carefully.
@@joeboxter3635 i am stopping at 29 minutes in case he loses me can you say coward.
Sabina seems angr these days lol
honestly the truly tagic part is i get her anger and have it too. we dont need a biger lhc
Professor Greene that was amazing! I have never seen or read a better explanation of Bell's Theorem than you have just given. Very clear, very passionate. It is a joy to share your joy of physics.
It's absolutely scandalous that Jon Bell didn't win a Noble prize for this 🤯🤯🤯
I agree with you. He may not have won it because he died too soon after the Aspect experiments were won. You can't win the Nobel post mortem. I think the window between the confirmation and his death simply passed by.
Being able to bridge the abyss between the formulas and the easy to relate to observable reality is truly a gift, which often, as it did in this case, brings understanding and happiness to the audience. Even to, as in this case, someone with a PhD in elementary particle physics. Thank you!
Dear Prof. Greene,
Thank you again for this wonderful series. Like the previous “commenter”, I have not missed a single episode. And I also think that Your Daily Equation is one of the best things that have come about since the start of the Corona virus pandemic.
I would also like to say that, although I am a Science graduate, unfortunately I have never been very good at Math(s). However, this last episode of Your Daily Equation (21), is the first time that I have ever managed to understand Bell’s Theorem. Thanks again!
One small point that I would like mention, is that - during the Quantum Mechanics episode of the NOVA series : The Fabric of the Cosmos; you equated the EPR Entanglement proposal as being more like a pair of gloves, set in advance as left & right, rather than the standard QM picture of “Fuzziness” until measured. I thought that the pair of gloves analogy was an excellent way of putting it. I am only surprised that you did not mention it in yesterday’s episode.
And I still maintain that The Fabric of the Cosmos episode on QM is one of the very best explanations & visualisations of aspects of Quantum Mechanics, from Probability Waves to the Entanglement pair of gloves.
I am going to re-watch episode 21 of Your Daily Equation, to make sure that I can still understand Bell’s Theorem.
Many thanks again Prof. Greene.
with best regards,
Paul C.
1. I think the topic is easier for most people to understand using light passing through polarizing filters than using spin, since it's cheap and easy for laymen to perform experiments with polarized light.
2. The yellow thread connecting the entangled particles ought to be in the graphics (that start around 14:45) even before the state of a particle is measured, since Brian earlier said the thread continually connects entangled particles.
3. Brian neglected to emphasize an important logical point, which has been misunderstood by many physicists: although the Bell Test experiments have shown that Einstein was wrong about locality, they haven't shown that Bohr was right and Einstein wrong about whether quantum mechanics is a complete description of reality... for example there could be nonlocal hidden properties that both Bohr and Einstein denied, and if nonlocal hidden properties exist then qm is incomplete. For another example, if Many Worlds is true then qm is incomplete.
4. In a different sense, quantum mechanics is incomplete. As Brian noted near the end, the "measurement problem" isn't explained by qm.
Hi Professor. Thanks for the great video, but I couldn't get why quantum version of reality would give 50% probability of opposite spin observation on your set of detectors. The set of measurements on same axis such as {(1,1), (2,2), (3,3)} would always give opposite spins so thats 3 and the rest 6 would have equal probability of opposite spins come up, i.e. 3/6. So, net avg probability for entire measurement set from {(1,1)...(3,3)} would be 6/9 or 66.6% , which infact matches exactly with the Probability in Einstein's assumption. (Quick math - 5/9*75% + 9/9*25%).
Can someone please help me out here, where am I going wrong, because definitely Bell's inequality isn't
The key is that quantum mechanics predicts that if you don't measure on the same axis, you only have a 25% chance of getting the same result, instead of 33%. But I think it's been swept under the rug in the video because it really isn't that easy to explain.
But it isn't that important, actually, because the problem is how you explain that you always get the same result when you measure on the same axis : is it because the two particules have been sharing a certain property since they were separated, which is the straightforward explanation? The problem is that if this were the case, you'd expect that if you measure on a different axis, you should get the same result in at least 33% of cases. And experimental results show that you only get the same result in 25% of cases. Therefore the 100% success in the case were you measure on the same axis cannot be explained by the fact that both particles have been sharing some property from the beginning of the experiment, and there really has to be some non-local phenomenon. The fact that classical quantum mechanics accounts for the result is another (more technical) matter, but it's not necessary to understand it in order to understand the meaning of the result.
@@freyc1 " if you measure on a different axis, you should get the same result in at least 33% of cases".... that's an assumption that could be wrong as regards spin measurement. I can't help feeling that there is a massive flaw in this whole argument.
@@GravityBoy72 The assumption the inequality is meant to test is that there is something in the state of the two entangled particles at the beginning of the experiment which explains the ulterior measurement of their spins along each possible axis.
If it were the case (which is not true : that's the point), then we would have a situation that could be modelled by a story. Two people are going to be interrogated in two separate and very distant locations by interrogators who have had no contact with each other. But the ones being interrogated know in advance that each of them will be asked one question from a set of three questions (A, B and C). They can answer to each question either "yes" or "no". They can elaborate a strategy together before being interrogated. But there is one constraint (since we are"modelling" entangled particles) : if they are asked the same question, they must answer the same thing. So they have 2^3 possible strategies.
Then, each interrogator randomly asks to each person one and only one of the three questions. One time in three, they will ask the same question and get the same answer. We don't care about those cases. We only look at what happens when both interrogators happen to ask different question (the first one gets question A, the other one question B or C, for instance). What then is the probabiliy they will get the same answer anyway? If they have chosen to answer "yes" to each question, it's 100%. Same thing if they have chosen to answer "no" to every question. But if they have chosen to answer "yes" to 2 of the 3 questions (or the contrary), then there are two possibilities : either the first one answered "yes" (respectively "no") and the other one has a 1/2 probability of giving the same answer. That can happen for two different questions for the first one. Or the first one answers to the only question he had to answer "no" (respectivley "yes") to, and then the other one will always answer differently. In total, there is a 1/3 probabiliy that they will have the same answer to different questions (2x(1/2)+0)/3 if they have chosen to answer "yes" (or "no) to two questions out of three. Since it is 100% for the other strategy (answering "yes" to all questions or "no" to all questions), they will give the same answer when asked different questions at least 33% of the time.
As you can see, this is just logic. It has nothing to do with the particles we are dealing with or with spin. The fact that it doesn't work for measurement of spins of entangled particles along well-chosen axes simply chose that there can be nothing at the beginning of the experiment which may determine the result of the measurements.
Sorry for having answered too quickly the first time and then deleted the answer : I wasn't fully awake and wrote some stupid things!
@@freyc1 "The fact that it doesn't work for measurement of spins of entangled particles along well-chosen axes"... again, the assumption is that the same logic applies to spin.
What about degrees of spin?
What about "in between" spin?
BRAVO!!!!!!!!! To me, the layman that I am in your incredibly interesting field, you Professor Greene are an outstanding teacher enabling someone like me to understand in simple terms that which is complex and deeply non-intuitive. Thanks for doing this.
Dr. Greene's ability to explain complex phenomena in simple, understandable terms is so powerful it's akin to having your cognitive ability grow exponentially in the space of one hour. Watching his videos in various forums including this one is a treat to be cherished. How fascinating life has become with scholars like Brian Greene filling the internet with truly interesting, meaningful, and mind expanding information. Thank you, thank you, thank you.
Finally understood Bell's inequality. Thank you.
"Fine" example of MODERN PHYSICS: You have to BELIEVE guys like him. @40:00 Running this experiment??? WHERE was it done, mate? And by WHOM? Jesus (if he existed) convinced a couple of dumb fisthermen to spread his strange ideas, nowaday physicists do the same on TH-cam. There too many theoretical physicists! But still they can't explain the magnetic moment of a proton, after zillions and zillions of collisions at the LHC. Physics is going down the drain!
@@StellardroneMusic hahaha, science isn't based on trust.
I am not sure whether you still read the comments, but I want to tell you that you are great!! I am a yoga teacher and I don't know much physics but I like to read and here and there read and saw videos about Bell's theorem but never understood what they are talking about. You made it so clear and simple, and showed how the ideas behind it are just mind blowing and wonderful. I loved it!!! And I want to thank you for taking time and explaining these beautiful things. I feel like adding, I cannot believe it, I cannot believe nonlocal causality, it is spooky :-). Thank you from Israel - I go through the daily equations one by one - really love it.
As long as you don't follow or believe Deepak Chopra and his misuse of quantum mechanics and all his other bs, you are on the right corner, yoga teachers tend to love that charlatan. All his followers (Deepak's) should listen to THIS video several times to deprogram their heads from his bs, he uses a lot fo "action at a distance" etc to confuse people.
Just finished the video, engrossing! Can someone explain the whole third axis concept a bit more to me? Got a bit confused by the three arrows diagrams and what’s going on there, namely how we’re comparing them to one another?
15:20 I make this comment many times in various situations that describe Q events. The word "weird" is inappropriate. Indeed, using it is part of the problem. The universe is not that way; only incorrect thinking makes it. Einstein used the word "spooky" to mean that the explanation was wrong. Don't use "spooky" or "weird" (except when quoting) describing Q.
I was looking at nearly 10 videos to understand this concept and learnt 50% from those videos...
With this one video I understood the whole concept and so amazing and gifted Brian greene...
I dont mean I understand quantum mechanics which is always strange but the problem associated with EPR and Bell
Please continue to make such videos as others only make talks but you talk in mathematical , descriptive and imagery explanations and it is mind bogglingly simple
None could have thought that quantum mechanics could be so simply explainable
A wonderful, non technical explanation of one of the profound insights of quantum world. Have been a great admirer of your clarity of thought ever since I read The Elegant Universe and The Fabric of Cosmos. Thie admiration only grows after watching such videos. Keep sharing.
So far I ve watched so much of videos & read about Bell's theorem ,but description given by you sir is remarkable,now fully understood where the Einstein & Bohr world was conflicted .
Dr. Greene, you set the bar so high for these educational videos that when I watch other TH-camrs I inevitably scream at the screen stating, “Why can’t you explain it as clearly as Professor Greene’s video?” And I always come back to your channel. All I conclude is, you have a gift! Thank you for sharing it to the rest of the world!
He didn't explain it clearly, though. :-)
Your video, your explanation is the best! I finally understood what's really going on with EPR and QM and Bell's Inequality. Thank you so much. You needed every minute of this video to explain it well, and you did explain it well.
Wonderful ! I love these none formal lessons where I understand everything
Great intuitive, yet rigorous, description, of a complex physical phenomena, or in simpler terms, great teaching! Thanks for your time and effort in putting these explanations out in public.
Professor Greene with all due respect, my head almost EXPLODED !!!
Thank you so much Professor Greene! As a physician who was inspired to one day hopefully become a physicist by your books and others I can say that this series of yours came to fill a space not previously occupied. You are helping, brilliantly, people like me to do the leap between nonquantitative, nonmathematical physics books to the entire physics literature, I'm sure.
FINALLY somebody explains Bell's Theorem in a way I can understand it! Thank you! Thank you! Thank you!!! Liked and subscribed.
EDIT:
By the way, the way I've resolved the question of locality, personally, just for my own satisfaction, is that perhaps locality is real but not primary. Locality being primary would mean that locality is immanent and fundamental to reality. It being non-primary would mean that locality is a consequence of deeper physical laws. If locality was immanent and primary, a particle would not have to "know" where it is, whether in absolute terms, or relative to another particle, as space would exercise its influence directly. If locality was not immanent, non primary, a particle would have to somehow "know" where it is in order to "obey the law" (of locality), just as a driver needs to have a speedometer and know what speed it is going at in order to obey max speed signs; the signs alone are not enough.
If locality is primary, it is as if in some town there were no max speed signs, but rather speed bumps or physical arrestors that directly limit your speed without you needing to read signs or look at the speedometer ... Without you needing to even have a speedometer at all.
Now, let's assume locality is non-primary, and model it as two drivers driving in opposite directions from a spot in a city or town. The drivers only know where they are by looking at the address plates on the doors of houses and stores. But if they are driving very fast, they cannot read the addresses, and so they base their location reckoning on the last address they saw, which happens to be the address they had at the starting point. Therefore they keep thinking that they are both at the same place even as they speed away from each other. If they use their warped reckoning of their own location as input to the task of obeying the rules of locality, they will fail miserably at behaving non-locally (independently), as they were supposed to ... behaving as if locally instead (entangled), thus making it appear that the universe is non-local. Like a driver with a broken speedometer driving at MACH-3 in a 50 zone.
So, to me, the question of locality is not a to be or not to be, but rather whether it is primary or secondary.
That doesn't really make sense; what would it mean for two particles to “behave as if local” if not by entanglement as it's already defined? Particles don't have independent “behaviors” in isolation. Information only ever makes sense in the interactions between them. So the question of “what's real” is meaningless if it's not part of the axioms of the fundamental equations. If the reality is more detailed than predicted by the math, then the equations need to be updated. Philosophically it's that simple.
@@objective_psychology You don't understand what I say because you don't try to understand; you are here only to find fault. I never said anything that contradicts any equations. What I described is a way to understand how entanglement happens; not how it behaves. It's about interpretation. And if you don't believe in interpretation; only in calculation; good for you, but you then have nothing to contribute to a comment about how to interpret entanglement.
I'm still fairly confused Dr. Greene. I guess this isn't answered, but how does the data reveal opposite spin only 50% of the time? If we control the settings to the experiment with regard to axis, how does the data not reveal 55%? Is there any chance on live stream to go more in depth for this? Thanks again!
I have the exact same question Ryguy. If 5/9 is fixed like a ratio then it doesn't matter how many times you run the numbers you are always going to get some larger version of 5/9 or 55%. So why does the number become 50% just by running the experiment? Something else must happen. It would be great if Dr. Greene explained what that something else is. As it stands I agree with Ryguy this isn't fully answered. However I do agree with everyone else that this is a really good explanation of everything else. Or at least I followed it :) Very much appreciate and enjoy the videos Brian.
This is exactly where I lost the thread also (or I've missed something relevant before this point). If QM predicts 50% and that is what the experiment showed, but how is that? It would be useful to see the same step by step statistical explanation as for the EPR argument.
@@cottawalla When I consider all possibilities, I find the probability of detection of opposite spin is 48/72 (=67%) and the probability of detecting similar spin is 24/72 (=33%). This is the probability when the two detectors (left and right) can have only three angle (120 degrees apart). This analysis does not include the situation when the detectors are oriented in other random angles relative to each other. My guess is that may change these probabilities.
I find myself in the same place. What specifically are the conditions that we get 55% and what specific "change' in conditions resulted in 50%?
Same here, I didn't understand why standard QM predicts 50% (which is , eventually, the number the experiments return)
I keep coming back to this video because it’s by far the clearest on the subject. Especially the graphics.
Your Daily Equation is the best thing to have come out of this corona crisis for sure.
Thanks Professor Greene. I have tried to grapple with the Bell's inequality theorem for a long time. Your video was the first one that I've seen that explained everything so clearly. I'm down with Covid right now but I feel lucky to have used this opportunity to come across your video.
Regarding Pauli's comment about the dancing angels on the tip of a needle - the same applies to the Strong Copenhagen interpretation claiming that superposition is actual physical probability cloud, and that it really physically collapses instantaneous upon measurement.
Hi Dr. Greene. What a great explanation. Your ability to communicate complex physics in a way that's so graspable to us lay people never ceases to amaze me. In a future video, do you think you could give us a primer on what a "light cone" is? Thanks so much for these videos.
I am not qualified to say if this oversimplifies the essentials of this debate but it is at the very least a supreme example of how to communicate difficult scientific ideas. I think Prof Greene's students are very fortunate!
This is the best episode till now...
Can you make an episode on quantum eraser problem too!! Please 🤩🤩
I would like a similarly pedagogical explanation why the "true" outcome of the experiment is 50%. I whipped up a small program trying to replicate a non-hidden variables version of the setup but I always get around 66%.
66% is what you get from the deterministic "hidden variable" assumption (when he says that the measurements will be anti-correlated more than 55% of the time, he's neglecting to say that the actual amount is precisely 2/3, i.e. 66%), so your program that you wrote is just verifying the hidden variable assumption. The reason that Quantum Mechanics predicts a different number (in this case 50%) is very mathematical and there is no intuitive way to explain it (but I'll put it in words never the less: the expectation value of the product of the angular momentum operators for two unit vector a and b is given by -h/2 times the cosine of the angle between a and b ... not that you should understand that, but my point is that is highly technical, and cannot be simplified). It is important to note that quantum mechanically predicted value of 50% (in this particular example) is dependent on the choice of the detector orientations (as he mentions at 26:43) and so, for any other choice of detector orientations, the quantum mechanically predicted number can be wildly different from 50%. I mention this because people often hear the number 50% and think it has something to do with 'random chance, heads or tails, etc.', when in fact that has nothing to do with it, it's just a pedagogical choice because he thinks people like the number 50.
@@nspizz you mean it's 50% if we choose three axis at 120 degrees from each other? What are other setups that will not produce 50%?
wonderfully explained Professor Greene! I had NDE a few years back, experiencing three flatlines in a twelve hour period, and since recovery, regularly experience the Pauli Effect, as well as much Jungian synchronicity…been a fan of Bell’s inequality ever since I read Fritjof Capra and the work of the Fundamental Fysiks Group, which has all helped me maintain a semblance of sanity since I awoke from the coma!
gotta love this guy. i feel like i gained a lot of progress in my understanding of bells theorem although i still don't have the math figured out but that takes time. even still, it was a fascinating watch and i love his enthusiasm
It's a shame that John Bell wasn't awarded the 2022 Nobel prize in physics posthumously, because if it wasn't for him, Aspect, Zeilinger and Klauser would never have tested this idea in the first place.
Prof Greene is the Einstein of Physics explanations. If 'understanding' is quantised, then Prof Greene includes all the quantum steps. Many others (especially, and notoriously, my physics lecturers) leave essential quantum steps out - and your like, "but why, though?".
Potentially stupid questions:
1. Can detectors be put in series to see if the spin is constant once measured? (Or does detecting the spin destroy the particle?)
2. What happens if the particles do not reach the detector at the same instant? Will the spins still be opposite if they strike the detector 1 second apart? (I know this removes the "spooky" result. Just wondering if it would go back to random or if they would stay opposite)
In relation to 1, you wouldn't be able to know the result after each part in the series . You would only get the result of the last one it went through. The result would still be inverted on each.
Jeremy Hassen point 1: measurement on one axis, looses the entanglement for the subsequent measurement.
Jeremy Hassen i think ive seen experiments where the detectors were moved back and forth and the results didn’t change.
@@davidseed2939 and Bob Man, Thank you for your responses :)
QM, hidden variables , EPR paradox
I love them.
I love you Prof. Greene for Spreading scientific knowledge in this Trumponian Dark Age. I'm a biology student and you showed me the wonderful world of physics!
One of the best explanations of what I have never understood properly
good presentation but one major important thing is missing. How does quantum theory give a 50% prediction for the outcome? You explained completely the 55% prediction of ERP but didn't touch on how quantum theory makes a prediction.
This channel is the only one that I watch live, above working or parenting, because I simply can't wait a few hours and watch it later.
Thank you, Brian. I am slowly working my way through these so that maybe I can understand them better. I very much appreciate your efforts.
Best video of the series. Your passion, enthusiasm, and intellect ooze in this one.
Great video, thanks.
I have one doubt.
According to EPR hidden variables, anticorrelation should occur 5 out 9 times.
According to QM anticorrelation shoul occur 50 % of the times.
But according to QM every time the spin is measured along the same axis the result should be anticorrelated, one up and one down.
Did the experiments check if thatvwas obtained?
That's because if we assume that entanglement is lost as the particles start to fly the result will be still 50 % on average and it will also be 50 % on the same axis
Great question. Video was fantastic, but at the end we all are just expected to trust that the result is predicted by QM. I would like to see an explanation of why that is, along with how the result is not simply that the particles are no longer entangled once one is measured (or maybe that is the answer).
You would indeed have to also prove your particles remained entangled. That seems easy to do once you have the experiment setup and basic enough that I can't imagine it would have been overlooked.
Dear prof. Green your lecture is magnificent , it’s explains the most unintuitive result in such bright way we’re all just speechless. THANK YOU
There is a possibility that you missed. You assumed that each of the 5 cases of opposite spin and the 4 cases of same spin occurred with equal probability. but could we not assume that spin up in one axis makes spin down on the other axes less likely. To prove your point we'd need results of 50% occurring at every one of the 9 grid entries ( ie the 9 different combination of spin detector settings) Perhaps that has been shown.
Just watched this thru for the second time, and it really explains the logic and 'dilemma' brilliantly.
Where the EPR view could potentially be an over-simplification, I would suggest the underlying nature of entanglement to be as an oscillating dialogue between the pair of particles - as opposed to a single communication or signal prompted by measurement.
Either way, of course, the Bell tests as conducted appear to strongly demonstrate against locality and the EPR proposals.
Of course there still remain the potential loopholes as covered by you in detail Brian.
Of these, I'd suggest the most compelling might be those based around the assumptions or premises of the Bell tests - not all of which may be as reliable or as airtight as our reasoning may appear to demonstrate.
Fascinating topic!
You are fascinatingly wrong. ;-)
Thank you, Dr. Green, for such wonderful series. We are all grateful for your hard work.
I am a 14 year old, and this video summed up the complex result is a clear way. thank you
Easily the best description of Bell's Inequality. Many thanks Prof. Greene. Mind boggling implications. What however does choosing 4 axes or other numbers of odd or even axes say? Why are 3 axes the go to in this example? Just saying.
Thank you for making this subject clear it brings metaphysics to mind for me
I think I finally(!), years and years too late, got it :). Thanks. A great presentation.
I learned so much more about the EPR paradox from your description, than I did from studying the math. I say that grudgingly, but as a compliment. How did you get to the end without playing the super determinism card?
Seems he just ignored it
superdeterminism is even spookier than "spooky action at a distance" - maybe the last straw Calvinists are clutching at 🤭
The thing that crosses my mind here is that Bell found a way to disprove Einstein's idea by including 3 spins per particle instead of only 1. Do we know how many different spins a particle has and is it limited to 3 or perhaps on order of magnitude many more than that. It seems like the 3 spins Bell used for his paper is arbitrary and no one really knows how many differing spins a particle has.
There are three dimensions of space. A particle can spin in any of these 3 directions.
Trying for weeks to get my head around Bell's inequality. This one finally did it for me.
More mathematical one, but could you try to do something on the amazing theorem of quadratic reciprocity?
This goes straight through the head. Can't shake off what I've just learned.
I've run it by my cousin and he sounded kind of interested at my idea of measuring the one-way speed of light using quantum entangled particles.
If a light pulse is fired through both at enough distance, you should be able to measure the timing between the two reactions. He said it reminded him of Bell's work and here I am.
Maybe I misunderstand you, but I don't think what you said is related to Bell's Inequality. Bell's Inequality is about non locality, which means the measurement of one particle instantly has an effect on the other particle. It's not about the speed of light.
I am a firm believer of the many world's interpretation of quantum mechanics. I however do not believe it's the most radical solution to the EPR paradox, rather the least radical because it only applies to the whole universe the same principles it applies to quantum particles. Namely, quantum particles may be in a superposition state hence the whole universe may be in a superposition state, each state corresponding to a measurement outcome. I may however revise my beliefs if some experimental evidence shatters those as a good Bayesian. The reason, I believe, we aren't aware of the universe being in a superposition is that our own conscious experience of the universe is in that same superposition, each one being aware only of the universe it's entangled with.
Dear Brian, please tune your approach to my Q according to these details - I am an old non-English speaking woman with basic high school education and big passion for solving the mysteries of our existence and being. I just have seen your episode Daily Equation #21: Bell’s Theorem etc. - thank you for preparing that, I’ve learned a lot from your talk, and now have a Question:
“Couldn’t be - here and there - a world in its totality, which I understand as the non-organic-and-the-organic parts together, be a local in a sense the land and water (as a material form) are local everywhere around the Earth, and having (all the time and everywhere) the non-local qualities which the world’s inhabitants possess in the form of their minds?”
With regards,
‘hebel’ - in translation ‘plane’ - to plane planks was the job of my grandfather carpenter, to plane minds is my pleasure;
Brilliant explanation ,Professor Greene.I did physics at Queen's University Belfast 71-78 and John Bell was not even mentioned...once. Now that is spooky action at a distance!
A few questions come to mind :
Q1) How are entangled particles created? And do they stay entangled after measurement? If so, then could not multiple measurements be made assuming that the particles continue switching between up and down probabilistically forever?
Q2) Is it possible to entangle more than 2 particles? If so can we extend that to say all the particles in the universe can be/are entangled? What causes/stops the entanglement process from extending beyond 2 particles? If it were just a thought experiment I would just accept that as a precondition/setup of the thought experiment. However since this experiment has been done in a lab I would ask why only 2 particles and not more?
Q3) Why do we switch from only 1 paired property up-down in single axis of Einstein experiment to 3 paired properties in 3 axis of Bell experiment? Is the term spin "actual clockwise-anticlockwise physical spin" or a symbolic term?
Professor Greene is way better than I thought reading his books. Cool way to get things across!
Thank's for a clear explanation about this issue! Very many youtubers and other physicists try to explain it, but Brian Greene had it pretty well done. Thank You!
The last time I heard you talk was on that episode on the double split experiment that was uploaded on youtube 8 years ago. My perception of the world changed since, and it was due to your ability to communicate beautifully. Anyway I stumbled onto this and my brain has imploded yet again 😂😂 Thank you so much.
One of the Exellent explaination of Bell's inequalities on TH-cam
I know I’m late to the party, but I’d like to comment and question all the same. This is by far the best explanation of Bell’s Inequality that I’ve encountered. Spectacular ideas, indeed. Thank you, Dr. Brian Greene, and secondarily, Drs. Merman and Bell.
Question: What does “instantaneous” mean in the context of entanglement? In principle, as I understand, entangled particles can be at arbitrary distances when one is measured. Across the width of a lab, this would have no measurable consequence, but across, say, the width of the galaxy, then surely Einsteinian relativity would come into play. Particle A, measured at t1, affects Particle B … at what time? Suppose, in the extreme, that Particle B has just reached the event horizon of the galactic central black hole? Does “instantaneous” retain its meaning in that situation?
Nothing "instantaneous" happens.
Imagine that you are at the center of the galaxy and you send two physicists (A & B) in 2 opposite directions to the edge of the galaxy, each with 1 intricate particle. Then at time t0 you send a signal to them. They will receive it at time t1 and they will do just after a measurement on their particle. Now, if special relativity is correct (and all experiments ever done say it is), you can find a referential where the physicist A does its measure BEFORE B ; you can also find a referential where A does it AFTER B.
So if any instantaneous transmission happens between A and B, then causality would be broken, as A does its measurement BEFORE and AFTER B.
The solution is that correlations between the two particles exist because they share common past, and correlation can be non-local. But those correlations don't imply any instantaneous transmission between A & B. The difference with classical correlations is that the quantum ones can be stronger, and this is Bell's theorem.
Thanks Brian, definitely the best explanation and animation I've seen of the test of Bell's inequality. But I think at the end of the video where you committed to a conclusion that the universe is non-local, missed an important concept that you seemed to allude to throughout your video, that being that the observers have free will and the experiments are random. Superdeterminism also still seems to be a clean solution to the entire problem, that being the randomness of the experiments and the free will of the experimenters might not exist. I would love to see a video with your thoughts on Superdeterminism. Thanks again for the great video :)
The nobelist Zeilinger and his team used the frequency of photons from distant stars to adjust the measuring devices ( 2017), and another subsequent experiment used distant quasars.
So, the "free will loophole" is totally implausible. Moreover, local ( hidden variable) theories cannot explain the correlations in the experimental results ( that agree with standard QM ) without invoking contrived, infinitely fine tuned initial conditions. That's why most physicists do not take seriously superdeterminism.
@@dimitrispapadimitriou5622 This is not true, one local theory can explain the correlations and it's called quantum mechanics. What is non-local are the correlations, and correlations never need to be local (classical correlations are non-local). The difference with classical correlations is that quantum correlations are stronger, but the theory itself is completely local, as shown by the impossibility of transmitting any information through these correlations.
@@OliveVerte-d9m you're just using two different notions of locality in a confused way.
*I don't disagree of course that QM is local in the sense that is compatible with special relativity ( no signaling).*
In my previous comment " non locality" means " non separability" ( "weak non locality" if you like ).
Two different notions of "locality".
@@dimitrispapadimitriou5622 "Moreover, local theories cannot explain the correlations in the experimental results ( that agree with standard QM ) without invoking contrived, infinitely fine tuned initial conditions. That's why most physicists do not take seriously superdeterminism."
Locality means the same thing in QM and superdeterminism, i.e. one of the supposition to prove Bell's theorem. Superdeterminism leaves behind statistical independence whereas QM is non-realistic and local.
My response is not confused at all, I explicitly explained what is non-local in QM : the correlations. This kind of non-locality exists in superdeterministic theory (as well once again in classical physics).
@@OliveVerte-d9m In my older comment, "local theories" : local hidden variables theories.
These kind of theories have been ruled out by experiments ( Zeilinger et al).
Superdeterminism is an attempt to revive such theories ( free will loophole: no statistical independence), so , contrary to what you're saying it is kind of local hidden variables...
On the other hand , Bohmian mechanics is explicitly non local hidden variables theory ( Bell non local. Moreover , It has also problem with relativity)
The confusion has to do with the two different notions of "locality":
"Weak" non locality ( non separability ) is an experimental fact.
On the other hand, relativistic non locality belongs to the realm of fiction...
Thank you very much Dr. Green, I am absolutely delighted hearing you (indeed listening this more than one time)
The implications of quantum non-locality are still being debated by physicists. Some believe that it means that information can travel faster than the speed of light, while others believe that it means that the universe is fundamentally interconnected.
Yes, that was bullshit. ;-)
Great talk, thanks. I always assumed that Einstein agreed that the two particles were in superposition, but that they were just shown to be in opposite states when finally measured. It’s interesting that Einstein believed they had definite properties from creation.
I am fascinated with quantum physics and am loving your really well explained videos❤❤❤
Not easy to find a video where Bell's ideas are so easy to understand. Studrets of Physics should see this.Thanks a lot.
Excellent video, thank you. I finally understand the Bell's inequality! I've read there is another pre-requisite to validate the experiment: the selection of the axis between detectors must be truly independent of one another. What if they are not? Think of superdeterminism.
What an amazing science communicator Prof Green is. Thank you for sharing your insights into this fascinating intellectual discussion that has been ongoing since the last millennium.
Beautiful description of more complex ideas in simple words. Thanks professor
Very appreciated, thank you. Clearest explanation I've seen yet.
Mr. Brian, I'm interested in science. What are some things you suggest for people who love science and want to explore it more ?
Step 1: Learn mathematics.
Step 2: Learn mathematics
Step 3: Love Mathematics
Try reading a few books by John Gribbin, (former editor of New Scientist magazine). I can recommend "Science : a History" and "In Search of Schrodinger's Cat" and "In Search of the Big Bang". Bill Bryson's "A Short History of Nearly Everything" is also a good, fun read. Hope that helps. Good Luck!
Dear Prof. please continue your daily equation for eternity , we are all definitely enjoying it.
A great series. It was a major step for a person to step between two of the greatest scientists in the twentieth century (Bohr and Einstein) to settle an argument. The luck of the Irish!
Actually he was British. Prof green got it wrong.
Complete, simple, understandable, amazing explanation of the Bell’s article.
What if you were to add another particle detector to each side, with a large space between them. You could theoretically add as many detectors to each side as you wanted. You would be able to see if there's a change between point a and point b.. etc
What would you accomplish by that arrangement? Often, when spin is explained, it is stated that once you measure the spin of a particle in a particular orientation, if you measure again (later in the path), the measurement will be the same as before. Thus, spin is a property of the particle.
Thanks, Dr. Green! Great video...beautiful explanation of this surprising aspect of science!
The description was indeed incomplete and too speculative at the time. The problem is that the entanglement must obey the QM. paradigms of locality and unity, today this is what the experiment tells us.
38:00- A X-shaped matrix with each opposing pair being a mirror image of each other.
38:43- A tilted diamond spoon.
39:39- The center of a square.
THANK YOU , Pro Green Great fun you cleared up the entanglement, I did not know that the particles were in flux between spinning up or spinning down .
Fabulous explanation, thank you very much. Like others, I am a humble curiosity nerd, and I have struggled to "get" what Bell's Theorem meant that might affect me. Now I do. The next thing I want to know is what underlying mechanism, however accessible it may or may not be to my perception, affects consciousness. We get that, we get it all, we just might not be able to DO anything with the understanding.
Bell's theorem talks about things that don't exist. How can things that don't exist affect you? ;-)
Wow!! Best explanation of Bell‘s Theorem and non locality I‘ve heard ever!
I think you're great... every time I dont understand half of what you explain (but it his not on you, it is me) but I can never stop listening and watch the whole thing even if I got to come back to it later.
I'll never be a physicist but I love hearing you Greene and Sean Carroll speak... I love the concepts that I understand only half way, but I dislike the math in it, so I take you guys' word for it that all you say has been mathematically and experimentally verified.
Thanks for the video. What I've realized during watching this video is that at the time the EPR paper got out, "fuzziness" as you've described it, was not treated as necessary.
So, I wonder (pardon my ignorance) why we need the state that is fuzzy in Quantum Mechanics?
And if we do need it, how we can question it? So, I feel like we have two issues here, because the EPR paper is then questioning two things:
1. fuzziness - because it says it doesn't exist
2. how paired particles get out the fuzziness, i.e that exiting fuzziness of one particle affects it's pair "immediately"
Does really EPR paper tries to rule out superposition or entanglement, while trying to give explanation of the spooky action?
Thanks, Brian; you’ve just given me an idea how to emulate/simulate a 3-axis spin of my axiomatic virtual particles in my “gedanken simulation” of 3D digital color space… 👍
Amazing explanation, best one I've ever heard. One question to anyone that can answer it - do particles spin axis on every conceivable axis all at once? It seems to me that this is what Bell's experiment requires.
It's the wrong question to begin with. The first question is... do particles exist? The answer to that is negative, already.
I followed the first 40 minutes OK and it is the clearest exposition I have seen so far. So thank you for that. However, I now cannot understand why it is that the experiment disproves the idea of the spin directions being set at the start but is OK with them being set at the end.
I also have a question about the three axes. Why three axes? How do things change if you pick two or four, say?
This is an excellent presentation, very clear and easy to understand. But there is a question I have. Maybe this has been covered in the comments previously and, if so, apologies. There are two important features of Bell's experiment. Firstly, there are 9 possible combinations of axes from the two detectors. Secondly, there are 6 patterns of spin from the 3 spin axes, up up up, up down up, etc. Whichever of the spin patterns is detected this leads to at least 5 out of 9 of the spin combinations being different if spin is predetermined when the electrons are created. What is the difference between this and the situation when the spin is only known when measured at the detectors ? The spin pattern will be one of the 6 and there are still the same 9 axes combinations. Surely the only way to achieve less than 55% is if, at some time, the entangled electrons have the same spin on corresponding axes. This breaks the rules of quantum mechanics since energy would not be conserved. I would be grateful if someone can explain what I'm missing here.
There are no spin projections before a measurement has taken place. Entanglement simply means that (in this case) the total angular momentum of the unmeasured system is constrained. The only relevant physical question here is where the extra angular momenta come from after the measurements... and the answer is trivial: they are being supplied by the measurement systems.
So what happens if you repeat the experiment with 5 axis each at 72 degrees to each other?
That was a clear explanation of the Bell setup. I think what you'll find is that the correlation of measurements goes by the cosine of the angle between detectors. So COS(120) is one half, plus COS(-120) is one half, and that makes 50%. I don't see anything spooky here now. Ask the world whether it is day or night, and you should get 50-50, no matter what time, and who is asking.
This is the best explanation of the Bell's Theorem and the consequences of it that I've seen EVER!
I've been thru dozens of explanations (one just before this video, and some delivered from a scientists that I respect a lot for the logical and clear presentation ... of OTHER problems!)
One can see a common theme in many (perhaps even most of the other videos) - and that theme is "Bell Theorem disproves hidden variables" (and many use that to disprove the Pilot Wave theory) ... many of these other videos are actually called something in the lines "Local variables don't exist ... Bell's Theorem"
And while most describe more or less the same experiment it seems that (almost?) ALL of them draw the wrong conclusions from that - and they're not wrong because me, them or someone else like them or not (ie have some bias) but because they don't follow simple logic!
Disproving a system of assumptions only mean that ONE of these assumptions is wrong - not all of them, and it's very risky to pick one based on your taste.
One repeating pattern I see at the end of each Bell's Theorem presentation (though with different % in different videos) is that the prediction of the APR is for X% (when tested a way similar to Bell's Theorem), and the experiments done show a Y% - hence it's wrong, and Universe is intrinsically random ... so I'm thinking - Wait a minute you're telling me that experiment ALWATS show Y% and you're saying that's proof for RANDOMNESS!?!
How it ever can be more obvious that it's not random?! The prediction of APR (based on Bell Theorem) might be wrong (it seems so) but randomness is definitely not the logical conclusion!
It's a bit beyond this video why it's the Locality that has to take one for the team, but I've always intuitively thought so.
Actually while easy to see why Locality is a very classical view (and understandably assumed to be intuitive) for me (I can't explain why, since I'm not educated enough to argument myself with Math), but also Mr. De Broglie , and Mr. Bohm at least it was logical that it's more likely Locality was just wishful thinking rather than determinism.
Thank you Mr. Green for finally verbalizing my deep conviction!
You said:
"Wait a minute you're telling me that experiment ALWATS show Y% and you're saying that's proof for RANDOMNESS!?!"
Experiments always show Y% over many trials. That does not exclude randomness. Coin flips over many trials show the same behaviour, and yet a single coin flip is random.
So explain to me how Bell's Inequality can be violated if reality is non local AND it has hidden variables. According to the procedure Brian Greene used to explain Bell's theorem you would still get >= 5/9 if you have hidden variables, regardless of reality being local or non local. The only way I can see where Bell's Inequality would be violated in that scenario is if these predetermined spins (hidden variables) get changed non locally as soon as one of the particles is observed and in such a way that reality would conspire to violate Bell's Inequality despite the existence of hidden variables. And if this is the case then we would have no way to distinguish between hidden variables or no hidden variables. So we're back to Brian Green's pink hair argument.
So let's recap. A violation of Bell's Inequality proves that reality is non local, but it is impossible, even in principle, to determine if hidden variables exist. So this is like the equivalence principle in General Relativity where it is impossible to distinguish between inertial and gravitational mass, even in principle, and from this Einstein concludes that it is meaningless to say that they are different. In the matter of Bell's Theorem it is impossible, even in principle, to distinguish between hidden variables or no hidden variables, therefore it is completely meaningless to say that these two options represent different things. But what does it mean to say that hidden variables are equivalent to no hidden variables? I don't have the answer to this.
Maybe particle A has a definite spin up and particle B has a definite spin down and what is in a superposition is the location of each particle.
If the wave function describes the probability of finding a particle in a particular location, we cannot say that particle A goes to Alice and particle B goes to Bob.
We have to say that particle A and particle B are both in a superposition of being at both locations at the same time.
So, Alice cannot know if she has particle A or particle B until she measures and forces the particles to choose a location.
We can also say that all properties that are in a superposition have to be probabilistic until measured.
So, we don’t know if we have particle A or particle B and if particle A has a spin up or a spin down.
In fact, we can’t say that we have either particle until we take a measurement.
Alice does not have particle A or particle B, but a superposition of both A and B.
The wave function does not differentiate between particles when describing the locations of the particles.
All it says is that one particle is likely to be found with Alice and the other is likely to be found with Bob, but you cannot say that A or B is more likely to be found at either location.
In fact, until you measure, there are no actual particles, just probabilities.
james i agree. if they are a plank disance apart and their spin is also a plank difference then that could explain the 5%