Bell's Inequality

[@Eznid]

One of the rare, if not the only, good and clear explanation of Bell's Inequality

[@davidk9382]

Dont forget concise!

[@quahntasy]

*Who else is here in 2020 and still finds the video the very best.*

[@labidifaycal3185]

I am.

[@MrBendybruce]

I've known about the theory for a while but for some reason I found myself desperately wanting to properly understand the underlying logic because our fundamental understanding of reality hinges on this proof. So thank god for this video because I finally get it.

[@maxtabmann6701]

Well good that Bendy understood it. I did not understand it at all and so I watched it almost ten times to find out why I did not under stand it. The crucial point is at minute 15:30 where suddenly the experiment done was changed. Before, it was clear that A and B choose detector settings 1,2 and 3. So this should result in 9 different situations, because A can choose settings 1..3 and B likewise. 3x3 is 9 different situations. But now you switch to results being same or different for settings 1..3. When you say 1 is same, what does it mean? Is this the polarizer setting for A or for B or both? If its only A, then what is the setting for B? That's the confusing point.

[@MrBendybruce]

I see a big part of your problem. It is not a 3*3 Combination Matrix. It is a 2*3 Permutation Matrix, where on one side you have either it did or did not pass through the Polarizer (Yes No) and on the other side you have 3 Polarizers each set at different angles (1 2 3). If you list out all the possible Permutations you get a total of 8: YYY (1) YYN (2) YNN (3) NNN (4) NNY (5) NYY (6) NYN (7) YNY (8) There are no others. Also, notice (1) and (4) are statistically meaningless since either the photon will go through all 3 (1) or it will go through none (4). That leaves 6 outcomes that are of interest. But this table is meaningless by itself. We need to extrapolate another table that allows us to conclude the probabilities that yield a value of 33% To do that we need to remind ourselves of the actual experiment. Alice and Bob each receive one of a pair of matched photons that each have the same polarity. Alice and Bob will each choose a Polarizer at random (1 2 or 3) and then see if their photon goes through it or not. If they randomly choose polarizers that not from permutations (1) or (4) then we can see that 1/3rd of the time they will get the same results. eg 1/3rd of the time the photon will go through for both Alice and Bob. The other 2/3rds of the time they will get different results, eg it went through for Alice but not for Bob, or vice versa. This is the heart of Bells Inequality. It means if you run enough tests then 33% of the time Bob and Alice will get the same results if "Hidden Variables" is how the Quantum world works. But that's not what they get. they get the same result only about 25% of the time, which is what Quantum Mechanics predicts in a universe where entangled particles are in a super-position wave of possibilities right up until the moment you detect them (eg did it go through or didn't it). If you did this same experiment with pairs of gloves, which are left or right handed right from the start, Bells Inequality is not violated. This means we live in a fucked up universe where very small things don't seem to follow common sense.

[@MrBendybruce]

ps. To be explicit about the value of 1/3 lets consider permutation (2) YYN. There are 3 polarizers, 1 and 2 will let the photon pass through and the 3rd one will not. Alice and Bob must randomly pick one of the 3. If they pick the same one, then the result of the test will be meaningless, but if they pick different ones, then it will be useful. The only combinations that matter are 1 and 2 (YY) 1 and 3 (YN) 2 and 3 (YN). From this list of 3 possibilities 1 of them will yield the same result (YY) and the other two will yield different results (YN). Hence we get a value of 1/3 or 33%. This holds true for all the other permutations except (1) and (4).

[@JoseMoreno-hr3tw]

The only explanation that allowed me to finally understand in what way Bell's inequality enlightened modern physics. Real gratitude.

[@khyateeatolia9904]

Hello Sir, Thank you so much for this explanation. I have searched a lot of sources but no one has explained it in the concise and clear manner that you have. I felt like the conclusion was absolutely a natural consequence of your amazingly well laid arguments. You are a great teacher. Thank you for igniting more curiosity in me about physics and the strange nature of quantum particles. Keep doing your amazing work sir!!!

[@delightmysoul]

i had been searching explanation how we can devise experiment (The Bell's inequality) to prove argument between Einstein and Bohr. Tried most-viewed on KZbin still hard to grasp the idea. Until i found this (very underrated one - too few viewers). It's very clear and comprehensible. Thanks for your effort. Keep it up! God bless you!

[@michellespremich1813]

Thank you so much. I have been trying to understand Bell's Inequality for awhile outside of a formal academic setting). This is by far the best explanation especially for a non-scientist! I appreciate you!

[@valentinbernard8126]

@Little Monk cling

[@artymowski]

You have to be kidding? Clear as Mississippi mud!

[@akashitsme]

A big thank you...i was struggling to grasp bell inequality and its role to eliminate the hidden variables theory. Beautiful and lucid explanation👍

[@leonhard.doerflinger]

2022 here... and this is finally the one video that lets me understand what this year's Nobel prize was all about. DrPhysicsA has always been the best. Brought me through a third of my exams as well.

[@Vardaris]

There have been many years since that video was out and many other KZbinrs have made elegant videos about the Bell's inequality but this is still the one that helps you understand the whole idea more clearly and in depth. That is an unusual phenomenon by itself in the KZbin world.

[@robertparrott216]

Watched this video a half dozen times over the last few years and I still don’t get it, but I’m coming along! Definitely the best explanation around.

[@DrPhysicsA]

The polarisers in my example should be 60 or 120 degrees apart.

[@v.r.horowitz3907]

This is an important correction!!

[@freerehab2848]

I've watched so many videos trying to explain this. None made sense and I got lost. This was clear and easy to follow.

[@valariegoose6578]

Zero graphics... just a paper and a pen; yet I have understood it better than I did on any fancy channels.. Thankyou Sir 🙏🏼

[@vincentcausey8498]

Bells inequality was explained very well. The experiment was explained very well. What was lacking was showing clearly how the table of results can be mapped onto the 3 sets of Bells inequality. I basically am struggling to understand where A not B and B not C etc is in the table of results because none of that was mentioned in the end. Apart from that, it did help me to understand some of the issues.

[@liudas5377]

Fantastic lecture. Great, easy to understand explanation...

[@marciopocciotti]

Very Nice Video. Although there were some "bumps" on the road, still it was very clear. The best one I found on the web to explain well Bell's inequality. Thanks.

[@PJ-he5zk]

This was an extremely clear and unambiguous explanation, thank you!

[@TANTRASIUM]

The best concise and clear explanation i have seen of the Bell's inequality...plz make videos again........

[@fr57ujf]

I add my thanks to those of others. I've watched many videos on this subject, but this is the only one I've understood.

[@omsingharjit]

You are best teacher of both maths and Physics :)

[@docerex]

Thanks for making more clear what Bell's Theorem is all about. I've been struggling to understand what this is about for some time. Even though this is not a rigorous formalized presentation, I now can try to take on the more technical discussions with much more ease. Thank you once again.

[@michaellean]

This intro has been THE BEST explanation of the EPR paradox I've watched!! (and I've watched quite a few!!) Thankyou

[@-danR]

It starts _off_ as a good explanation, then becomes long-winded, errors creep in, they are corrected... And that's fine. But as the comments indicate, there is either something systematically wrong with the thing, or the S/N is getting incomprensibly low.

[@christianadler1297]

Brilliant- this simple analysis cuts through all the other confusing analogies and I FINALLY understand it! (Although, of course, I don't understand the true, quantum nature of those darned photons!)

[@alkemist1000]

I'm a 18 years old guy from Italy. Physics is my passion, and i found this video very clear and understandable. Thats great, thank you for the upload.

[@rafaelnogueira7982]

OMG! Thank you soooo much! I watched so many videos and couldn't understand why the hidden information proposition were not the right one. Such a good video!

[@Snipermac99]

I have a huge grin on my face because I understood! Thank you :)

[@RichardDLewis41]

I really enjoyed the video which explained the issues very clearly. There was one point towards the end of the video where the two possibilities were discussed and one of those possibilities was that the two particles are in constant communication. There is a third possibility, namely that the two particles must be treated as a single system extended over the space separation and that it is at the point of measurement of one particle that the whole system is affected, thus changing the possible outcomes of the measurement of the other particle. This third possibility explains the results and does not violate the rule that wave transmission in spacetime is limited to the speed of light. It does mean that a measurement of an entangled system can result in instantaneous effects over a distance but this cannot be used to transmit useful information faster than light. Richard

[@RichardDLewis41]

See also www.academia.edu/5927513/The_Spacetime_Wave_Theory Richard

[@simonruszczak5563]

Fourth possibility, the Electric Universe Theory is correct, faster than light communication is possible.

[@OpenWorldRichard]

@@simonruszczak5563 Hi Simon. I don't know about the electric universe theories but the observations of entangled systems suggest that the requirement would be for instantaneous communication. This is a more difficult requirement than 'faster than light'. This is why I prefer to think of the measurement of the entangled system as being the cause of the change of state of the entire distributed system. Actually the idea of an instantaneous effect acting over a distance requires the specification of the frame of reference (Ref: SR/GR) in which the instantaneous effect takes place and this frame of reference is the CMB rest frame.

[@DrPhysicsA]

I think you have it right. What I was trying to say was that altho Bob can obviously make a measurement he wont get a conclusive result if he measures the x component of the spin after Alice has measured the y component of the spin of the entangled particle.

[@krish2nasa]

Wow, what an explanation! Thank you very much.

[@DigitalAura]

I don't know if it's because I've spent hours watching Bell's Inequality videos already, but THIS one was my AHA moment. It just clicked the way this was presented. THX

[@lepidoptera9337]

You could have read Bell's original paper instead of wasting your time. At the very end of his paper Bell himself noticed that he wasted all of his effort because the actual physics of these systems is relativistic. Non-relativistic derivations will produce nonsense like Bell's inequalities. ;-)

[@rustysim]

I think this is the best explanation for Bell's theorem

[@valariegoose6578]

Best explaination so far...for years and I am surebfor years to come!. Thankyou!

[@chandanmazumdar1001]

Amazing, so neatly and so argumentative ly you have explained.. A big hug from me.. Thank 🙏 you

[@juangreen8194]

Thanks this is a very clear explanation, I think I'm beginning to understand it, must watch again.

[@DrPhysicsA]

Sorry about the confusion. I should have used letters instead of numbers. I dont use them as population values. I use the numbers as a shorthand for the number of a particular category within each of the numbered sections

[@huddybuddy8097]

u r the best sir.. how easily u r describing..

[@2222MalayalamElectronics]

Hellow Bob! 10 years ago.. and 10 years after... thank you for the simplified explanation ❤

[@vaibhavnakrani2983]

🫡 very well explained. You sir are indeed a great teacher!

[@patekswiss9521]

Great video as all of yours are. I think rather than saying that Bell's Theorem shows that quantum measurements cannot be explained by hidden variables, it would be slightly more accurate to say that Bell shows that quantum results cannot be reproduced if you impose (as EPR proposed) a requirement of locality. Bell acknowledges that Bohm managed to construct a hidden variable theory but points out that it is "grossly nonlocal."

[@galahadgarza6905]

Thank you for the excellent explanation of Bell’s Inequality and how it ostensibly proves that there are no hidden variables-as the EPR Paradox asserts.

[@anant0red]

I think I understand it now. For any *one* of the eight decision schemes (hidden variable) of the photon pair, the chance of (A and B) getting the same outcome is at least 1/3, since 6 of those schemes give a chance of 1/3 and the other 2 give a chance of 1. I was calculating the probability over *all* the combinations, which is the average (1/3 * 6 + 1 * 2)/8 = 1/2. Thanks.

[@windlesSpice]

Bravo. A great explanation even without fancy graphics. Thank you.

[@shaynemurray6351]

Thank You Dr Physics I was struggling to follow the written description of Bell's Inequality and until your most enlightening video!

[@matthijsgeerlings]

What a wonderful explanation. Small side note: only local hidden variable theories are ruled out, so Pilot Wave Theory (Bohmian mechanics) is not ruled out by this (Bohmian mechanics is a nonlocal hidden variable theory), and John Bell himself actually was a fan of Pilot Wave Theory. Also, an assumption that is made in drawing the conclusion about locality, is that there is no conspiracy (the particles don't somehow know in advance what your (random) measurement orientation is going to be) and no causal effects back in time (or those kinds of things).

[@UpNfamish2]

2024 29th of March - the lecture, the English, is very precise n clear. I think it is his native ( England) English that makes the difference. American English is very confusing. Students must need to spend a great deal of efforts to decipher Science n engineering books written by American authors. I discovered this by chance- one day I was reading a physics book from the Oxford’s series without knowing it is Oxford’s n found it’s English was precise, clear n succinct n I turned to the front pages n found out it is from the UK. Many of my good physics books are from the UK.

[@basics7930]

Great explanation 🙂.....A Good Teacher

[@vossmalone7034]

Thanks Doc - easily the clearest explanation on the web - and I have spent an afternoon searching - now I might go back to Prof Susskind (who was not so clear)

[@dikshakumari4286]

Your video is better than the lecture of prof.Leonard susskin

[@exalted_kitharode]

Crystal clear explanation. Thank you.

[@andreacausero4342]

Sir, you really keep on posting high quality content, even though you might have lost yourself a bit through the explanation it still remains THE BEST I´ve seen so far, and believe me, since the nobel price has been awarded, I´ve checked many... Thank you very much, keep up the great work you´re doing!

[@evelioguaperas]

Why do you expect 1/3 of the times to get the same result? Shouldn't it be 1/2? You can account for using the same polarizer experimentally but getting rid of the case were they go through every polarizer is something you don't know, you cannot possibly take the data and say "this are the same because is rule number 1 or 8", do you?

[@argfasdfgadfgasdfgsdfgsdfg6351]

Exactly my thoughts.

[@mananpanchal26]

Rule number 1 and 8 already added. Each one of the 8 rule has 1/8 probability of occurrence. 1 and 8 rule has 100% probability of same result. From 2 to 7 there is 1/3 probability of same result. So, total probability of same result is 1/8 + (1/8)(1/3) + (1/8)(1/3) + (1/8)(1/3) + (1/8)(1/3) + (1/8)(1/3) + (1/8)(1/3) + 1/8 for rule number 1 to 8 respectively. So it would be 1/8 + 1/4 + 1/8. And thus 0.37. 0.37 is greater than 0.33 and that is what the rule says. Probability of same result >= 1/3. Hope this helps

[@sinaggsina]

@@mananpanchal261/8+1/4+1/8 is not 0.37 ! It would be 0.5

[@sinaggsina]

Plz some body explain

[@GXD001]

Sina Gohary For each trial, no matter what the combination of polarizer is, the 2 photons share one set of hidden variables. For each combination of polarizer there are 4 sets of variables that can make a SAME observation, that makes a probability of 0.5, for each trial of the experiment. Based on this, We can forget about the probability for having each combination. So the whole statistical probability of SAME observations is 0.5. Yet In real life, the number of trials is limited, which means if the experiment never encounters hidden variables 1or8, (the number of SAME observations /the number of all observations) will be something like 2/6. Since The observed probability is less than 1/3, the hidden rules are already proved not likely to exist. I think DR might mean something like that. Please reply if you find anything suspicious.

[@swamiaman7708]

Great explanation Sir .....

[@pauljmn9135]

why isn't a misunderstanding of polarization a possible explanation (heavens knows the double split experiment is the root of all evil)?

[@mybluemars]

Thank You, this is the best video on Bell's Inequality I have found so far!

[@sreedharb.k6274]

BRAVO what a wonderful series of lectures. Please write a book on the contents of this site for thd benefit if humanity Thank you🙏

[@peterb9481]

A brave attempt. Another good explanation can be found in Brian Greens book: Fabric Of The Cosmos. One questions what percentage of results differ from predicted results, further on what distance and expected time variables, factoring in the limits of accurate measurement and possibilities to improve on this (at least theoretically!)

[@wouterserryn1384]

Maybe space is bendable, and are these bends all around, but not perceivable by us. So there could be a 4dimensional bend/tube/wormhole, which keeps both positron and electron at exactly the same place, where they entangled in 4dimensional space, but doesn't in 3dimensional space. So the 2 entangled particles are still one object/form/energy.

[@garymathis1042]

Show me the math.

[@plantsofwar8834]

you basically just stumbled upon ER=EPR (google it)

[@DrPhysicsA]

It is pairs of photons that are entangled rather than beams. But if photon A passes thro a 45deg polariser it would be possible for the entangled photon B to pass thro the horizontal polariser. There's a little more about this in my EPR Paradox video.

[@konstantinginzburg7422]

But is it a 100% certanty that photon B passes through the horizontal polarizer? Isn't there a fallacy assuming that all 8 scenarios have the same probability? As in if we have same filters then the photn passes 100% of the time. But if it's different filters then probability may not be the same in different combinations. Would that not possibly explain discrepancy between theoretical 1/3 and observed 1/4?

[@DrPhysicsA]

I shall do another video on this shortly during the series on quantum mechanics concepts. In essence, Bob can certainly get a result from such a measurement that since the entangled state of the two spins has been affected by Alice's measurement, Bob's result is not a true result that he would have got if Alice had not made her measurement first.

[@georgevendras5450]

Hi there. I am big fan of the way you deliver (convey) the message. But this time, I felt as if you had to give a try, indeed. It is not your fault of course. Things here start to become unstable, we may all more or less guess, so it is not your fault at all. Would you like to have a look at my Open Letter where I express question as for QM convincibility? Perhaps it will provide you some ideas. Thanx!

[@georgevendras5450]

Well, I pressed 'like'. Could not do otherwise.

[@dlaxmcm1414]

I think the problem is in describing the spin of an entangled particle at 2 different locations. The experiment is set up to determine discrete characteristics (spin direction >

[@lennydoyle7382]

Well explained. I watched the first three minutes of another 'explaination' of the same idea which was full of waffle and used a video dispaly. A 1/4 is greater than or equal to a 1/3. Mind bending.

[@tellme2381]

2:42 Why Bob cannot measure the spin in x direction? What prevents it and what happens if he tries? Does that imply ftl communication if Alice can encode information measuring the spins of particular particles and Bob can decode it by observing spins of which particles he can measure?

[@lepidoptera9337]

Bob can measure the spin projection in any direction. He will always get a random result, just like Alice. The only difference will be the amount of correlation between the two measurements.

[@sjzara]

Thank you. This is the first time I have seen this explained clearly.

[@1VirginiaL]

This helped me quite a lot, thank you DrPhysicsA.

[@nenhard]

Maybe I'm missing something. Hidden variables theorem should also imply that by definition of entanglement, possibilities for both particles are not independant - 2 and 3 should be mutually exclusive. S probability should be zero for combinations (2,3) and (3,2). What do experiments show?

[@QuantenMagier]

Doesn't matter if they are possible, what he wrote down are all 8 classical possible options and if you say options where polarizers 2 and 3 have same result are not possible this just means options 1, 4, 5 and 8 are not possible, but the remaining options 2, 3, 6 and 7 still have a probability of 1/3.

[@DrPhysicsA]

which is why the experiment is often described as measuring the spin of, say, an electron along the y axis. If one electron has spin up the other will be spin down.

[@MrIntelaravind]

I have watched many videos on this topic and I could not understand any of them... This one hits the right breadth and depth...

[@kristofru]

I can count twelve 'S' from twenty four possible {S, D}, therefore the probability of obtaining an 'S', if all the combinations (1 to 8) occur with the same frequency, should be exactly 1/2. I suppose that as we cannot establish the probability of the individual combinations, we have take 1/3 as the lower limit. Thanks for the clear presentation.

[@XTheDentist]

Hi, thanks for these great videos on physics very much appreciated. Now I've been trying to figure out what entanglement is for some time now & i think I've made some progress so if someone can help me out, just please dont bite my head off :) So, starting with the double slit experiment, a single slit produces random dots as would be expected of particle behavior whereas a double slit produces an interference pattern suggesting wave-like behavior..ok fine. Now, placing a detector at the slits, the interference pattern disappears & QM theory says that the "wave function" collapses as a result of the measurement effectively eliminating the superposed states etc. Now, if we generate a pair of entangled photons, A & B, and measure some property of A along an axis we shall have random results as would be expected, but when we measure its entangled partner we have 100% correlation with A (ie NOT random) implying that measuring photon A, and thereby collapsing its wave-function to a definite state, causes the wave-function of its partner, B, to ALSO collapse presumably instantaneously. According to Bell experiments all local hidden variable theories have been rendered useless, we have Bohmian non-local hidden variable theories which work as a valid interpretation but doesnt really advance our knowledge of QM. Now I have to ask this not because I think its correct but because the fact that im thinking it means theres something im not understanding. Why isnt the process of generating entangled particles the problem? I mean, during the process they simply acquire correlated polarization or whatever other property, why does there need to be this mysterious hidden variable or "pilot wave" or faster-than-light communication? Is there some experiments that have been performed that rule this possibility out even if we couldnt observe this process directly to tell for sure? Are there natural processes that produce entanglement? How do we know for sure that the entangled pair have not already experienced state-collapse as a result of whatever produces entanglement?

[@Patatmetmayo]

What you're asking about IS the idea of hidden variables. If the particles acquire definite correlated states at creation this means that there are hidden variables that define these states at the creation of the entangled pair. Bell's inequality experiments prove mathematically that this can't be the case however, so there must be something else at play here. Either instant and faster-than-light communication, or some other explanation where Bell's inequality can be violated.

[@FunkyDexter]

@@Patatmetmayo Bell's inequality confronts a linear probability distribution with a non linear one. It's frankly obvious that polarization experiments will violate the inequality, because the polarization follow's Malus' law, which is a cosine law (i.e. non-linear). It's just wrong to think about hidden variables as a predetermined outcome for ALL polarization angles. Also notice how Bell's derivation is purely a logical statement and has fundamentally nothing to do with quantum effects. Let's say our photons are created entangled with polarization on the z axis. Every measurement along that axis (detector A) will pass. Now put the detectors at an angle, like 22.5°: every photon will pass test A, but test B has a cos^2(22.5) chance to pass, that is 85%. If detector B is instead at 45°, it will have a 50% chance. See how the "hidden variable" refers uniquely to detector A, and how the outcome of the other measurement is not independent of what we measure at A. But there's no information exchanged between the two photons, each already had all it needed to produce experimental results, namely polarization aligned with A. The fundamental problem is that we think of the light going into the detectors as single indivisible packets, but that is not true: what is quantized is the EXCHANGE OF ENERGY to matter from the field. A photon is effectively one only at the moment of detection, before it's just a normal EM wave, subject to Malus' law.

[@ashhenriquez918]

Awesome explanation

[@georgechan2474]

the best video ever on Bell's ineq

[@aaronrheams2920]

In the simple case of recovering light from extra filters, I have a feeling these photons aren't blocked, but phase shifted to oppose its orthogonally polarized counterparts. The fact that you can recover the output with more filters is exactly what is seen in series resonant circuits when reactance pushes the voltage and current out of phase and then realigns it to eliminate the destructive interference of the opposing waveforms. Due to quantum wave effects, polarizing lenses probably act like the quantum equivalent of reactance rather than resistance.

[@lilianedubois2539]

Excellent presentation. The other explanation is that the 2 electrons do not communicate with each other but are both canceled immediately by an outside reactive force.

[@rikabernar]

This's amazing. Love it so much.

[@DrPhysicsA]

I don't think the experiment assumes that the hidden variables are random. But if they were not, then one would expect to find some consistency of results

[@SeedsofJoy]

i watched this video 3 times and Im none the wiser, surprised people thing this explanation is clear. What a bizarre thing quantum mechanics is.

[@FoosResearch]

Very sensible, thanks. The force of an EM wave does not imply that there is such a thing as photon particles. If the vibration of an electron emanates in all directions at once, then there is definitely a variable, not so hidden perhaps. However, if the electron is not even distributed throughout its orbital, there will be a very slight delay depending its location during the course of its vibration; hence, the probability will be slightly less than 1/3. This would be true for pairs of split photons as well, I guess. Eureka!

[@pritamroy4677]

dear DR physics, six year back i started to watch your videos, then i was doing my Bsc. now I'm doing my phd , and still watch your videos for better clarification whenever I need in any of my research topic. thanks Dr physics. you really cured my 'lack of control on physics ' diseases .🧖🧖🧖

[@creightonjones7318]

Has it been experimentally demonstrated/determined that, for an individual photon, the three possible polarization states that you chose for example each have equal probability of being measured? In other words, are individually emitted photons statistically weighted towards having a particular direction of polarization or another? Is that important to know for this example you have given? With that said, is it correct to say that in QM experiments and theory, that a single quantum element will have a different probability of producing a particular measurement than it will were it to be measured as part of a conjugate pair whose partner has been measured?

[@jorriffhdhtrsegg]

Yes I think that's the whole point. An individual one would display the component at that angle. Classically we actually get the component as a number between 1 and -1. Quantum particles instead show up as as probability of a qubit as in 1 or -1 discretely distributed per the component. Thus take this new state where the previous measurement outcome is erased. The recommended particle in an entangled pair shows correlation similarly as with measuring a particle twice, yes? Its worth mentioning of course that to infer a probability many repeated tests must be done the more the better. So it could never be done on a single particle much as without examining a coin used for a coin flip we would just have to flip the coin many times to get a probability. Although a coin could be examined and determined to be of a shape and weight distribution we can infer in many classical systems their pseudo random nature we cannot look at featureless particles in this way

[@TomTom-rh5gk]

When you say he cannot make a measurement you must say what happens if he tries first. Once you do not the listener is lost and cannot listen after that.

[@uvuvwevwevweonyetenyevweug7849]

did you find why bob cant male mesurments?

[@TomTom-rh5gk]

@@uvuvwevwevweonyetenyevweug7849 English isn't your first language. I don't think you understand my point. I do not know what you are saying.

[@uvuvwevwevweonyetenyevweug7849]

@@TomTom-rh5gk I acttually ansewred to wrong comment 😂 and yes english isnt my first language

[@TomTom-rh5gk]

@@uvuvwevwevweonyetenyevweug7849 No problem. I do the same thing. Go in peace my friend.

[@TomTom-rh5gk]

@@rafaelclp The point is that should be explained in the video. The video isn't wrong it is incomplete. The problem is when you know the answer you don't think you have to explain. You don't think you have to explain because the answer is obvious to you but it isn't always obvious to the viewer.

[@Paul-A01]

Now, why is it that when Alice measures one spin, bob can not measure the spin in the other axis of the other particle? Do we get scrambled data or something?

[@markyisser9004]

at first i checked the thumbs down. but i left it on and realized that it was actually pretty good. i guess i'm too used to animation and computer jazz to enlighten me.... you explained it better than everything else i've tried to watch i get the gist of the experiment now, i didn't before

[@CyrilleParis]

I would had that French physicist Alain Aspect was, in 1982, the first to do the actual experiment proving that Bell inequality was violated (Aspect found the 0,25 in the end of the video).

[@rainertheraven7813]

Thus we are free to use "Bells Hoax" instead of "Bells inequality".

[@1jdownes]

I think Clauser, Shimony et al did it in 1972

[@david_porthouse]

Bell’s analysis can be an intricate one, and hard to follow at times. In summary, if we want to do computer simulations of some quantum mechanical behaviour, then we must find a way to shift information faster than light. The information is like a one-time pad rather than a book, so it is useless for true superluminal communication. The pad cannot be accompanied by a second one-time pad such that there is a message hidden in the joint correlation between the two pads, but presumably we have an Exclusion Principle to take care of that. It’s not as bad as it looks. Modification of the received equations of quantum mechanics is prohibited, but with a nonlocal theory we get a new degree of freedom to play around with to circumvent this. I would note that there is more than one way to travel faster than light, and propose one way is associated with wavelike behaviour, and the other way with particle-like behaviour, or tachyonic Brownian motion which is orthogonal to the wavelike behaviour. It is at least imaginable that an entity can be both a wave and a particle.

[@saadijalal3877]

Thanks for you effort sir. I am missing one thing. Why do particles have to have same polarisation in pair production process...is this a postulate?

[@DrPhysicsA]

It is generally accepted that Bell's inequality coupled with experimental observations provide no explanation for how there could be hidden variables contained within the DNA of the particles produced. It doesn't mean that there is no theory that could account for this only that we haven't found one yet. Indeed, our current understanding of quantum mechanics suggests that we just have to accept that the quantum world is different.

[@haaardcoooreee]

i really fell in love with your videos, thank you, you made my modern physics student life easier.

[@dgphi]

I don't get this explanation. Bell's Inequality is presented in the video as "A not B + B not C >= A not C", and then 24:15 that becomes "Same outcome >= 1/3". There seems to be an explanative gap there. I know the A's turned into 1's and so on, but what happened to the not's?

[@gk89300]

Nice video! I still have a hard time understanding the Kochen-Specker addition to Bell's theorem. If you take any request for video's I would like to see you explain the KS theorem.

[@david203]

I don't see why there is so much resistance to accepting nonlocality in physics at the very smallest of scales. It explains the experimental results (such as double-slit and entanglement) without contradicting our intuition of locality at larger scales, where decoherence rules. We can even, in special cases, see coherence at large scales: superconductivity, superfluidity, Meissner Effect.

[@schmetterling4477]

It's trivially incompatible with relativity. All the people who think that physics is non-local simply don't understand physics. :-)

[@LamirLakantry]

Sorry if these questions where answered but I just misunderstood them: 1:54 Does Alice shouting the result to Bob have anything to do with Bob's readings? If Bob didn't know that Alice had even measured the z direction on her side, would he still be unable to measure the x direction? In the setts of children example, he says "A not B + B not C ≥ A not C". This is obvious if A = B ≥ C, but not true if A + B < C. Are we starting with the assumption that there are equal numbers of children wearing hats, gloves and scarves. Because he does not make that distinction.

[@alpratsibz]

Great explanation! Thanks!

[@m4inline]

I don't really get the first part about simultaneously measuring "spin in both X and z axes". Afaict electrons only have one spin direction, and if you try to measure it with magnets you have a certain chance of getting that spin or the exact opposite, depending on the previous spin. I.e. measuring with magnets reorientates the electron. In fact spin is just an emergent property of electrons in the presence of measurement apparatus

[@sherlockholmeslives.1605]

Thanks for these truly intelligent lectures, Phil! I love your knowledge but I don't understand the vast majority of your information! I wonder, just out of curiosity, if you know also Goethe's 'Theory of Colours', I think I can grasp that. Although sometimes it seems my mind is so slow I only understand in science Plato's 'Wax Tablet Hypothesis' and Aristotle's 'Theory of Everything'. Lol!

[@skylineuk1485]

Where mathematically does the 0.25 come from?

[@Serotta2001]

According to the lecturer, this is the experimental result.

[@alanjenkins1508]

For the curious the 0.25 probability comes from the equation Cos(120)*0.5+0.5. Substitute any rotation angle you want.

[@vispi1944]

Thanks for the clarification. I was wondering, where this 0.25 comes from. Can you be kind enough and explain where this formula comes from? Thanks a lot.

[@alkemist1000]

And i hope that my professors will be as amzing as you!

[@starsreflectingsky]

I've watched a lot of videos and this one does explain some things much more clearly for whatever reason. Kudos to the presenter. I'm still a little confused about certain observational realities that I need to know about to better understand this. I think I understand that if you measure spin, entangled pairs will always have opposite spin. This polarization aspect though seems different from the spin testing. When it comes to spin, you just measure it and you find out what the spin is. I'm not aware of a way to influence the spin. However when it comes to polarization, The experimenter can influence the outcome by virtue of using various polarization angles. I wonder if there is a way to do the bell test but with spin or some other quality that can't be influenced. Indeed when it comes to spin, I want to know if there are experiments that are able to maybe delve into the concept that maybe the methodology of measurement is influencing the spin. When it comes to things like measuring spin, you still have to measure the other entangled particle. It's not like it's superposition breaks down automatically and is freely detected. While it is true that you can measure one particle and know it's spin and therefore know the other one, you still have to measure the other one. This isn't necessarily verifying instantaneous communication. That experiment still could mean that something is happening at the entanglement process that we don't understand. What I don't understand about this particular polarization experiment is that it seems like the entangled photons don't have to successfully go through the angled polarization. Meaning that if I send one entangled photon to the left through a 45°, that it may or may not pass through. If it does or does not doesn't seem to have any impact on whether the one going to the right does or does not. Success of going through an angled polarization seems to be independent of entanglement. Does that make sense? I don't think the experiment is saying that if I get through then my partner definitely will get through. If you think about that a little harder, let's say that on the left and right side that the angled polarization is exactly the same. So you might intuit that if the left entangled photon can get through then the right entangle photon must also get through because they both have some quality that's entangled that gets them through the same angled polarization. I don't know the truth of that though. I would like that detail. But additionally, you are adding new variables by having two separate polarization materials that may not be exactly the same when they probably aren't. They may not be as precisely lined up on angle as is necessary at the quantum level. There are unfortunately some variables that have to be realistically considered when you are possibly dealing with details down to at least the plank length. It's probably safe to say that to some degree, our certainty on definite behaviors should be a bit reserved. Now if there is evidence of repeated testing that shows that the existing materials and method used already very consistent matching outcomes then that is meaningful and would address some of the issues I noted I can guarantee you that with real experts to do this up on the regular basis that they probably have tackled this stuff over and over and it's just people sharing that with the public haven't gotten to share it or don't notice share it because it's too small of an audience. Lol I saw a video of a guy finding all of these intricate connections between Disney movies and all I could think to myself was... Stop worrying about Disney and go work on quantum entanglement. Lol Put your brain power on reality. Hehe

[@riadhalrabeh3783]

There is a video under 'real time imaging of quantum entanglement' on utube and pdf by the same name on google that everyone should see. Online change in the polarization of one of a pair of entangled photons and shows the corresponding determined, repeatable and timely changes on the second of the pair. This does not need the Bell's inequality to accept and denies clearly the 'pair of gloves' or hidden variables argument. The easiest explanation to this is that action and reaction and the conservation of momentum are instantaneous and not affected by distance. There are two solutions to Maxwell equations. A dynamic one where photons travel across space at c, and a static one responsible for the static electric force (and the same for gravitational force), which is instantaneous and does not respect distance. QM being a good representative of nature must show these two effects- using a different route. Note that it is also 'probably' correct that no information can be carried by this method as energy travels only at c.

[@renedekker9806]

The video that you reference would have been an interesting video if they had explained a bit better what they are actually doing. But your explanation in terms of Maxwell's equations is just nonsense. The static electric force is NOT instantaneous, changes to it travel with the speed of light.. The same for the gravitational force, changes to that also travel with the speed of light. In all laws of physics, including QM, cause and effect travel maximum at the speed of light. It is just that, as shown in the video, there are cases that we can only explain by stating that information is traveling instantaneously.

[@prateekd4978]

at 12:50 if all three polarizers are those you've used in just earlier part of explanation 10:00 then why not the third (from left) polarizer placed in 90 degree. why it is 240 degree ?