Bell's Inequality

A basic introduction to Bell's Inequality which shows that there cannot be hidden variables (a form of inbuilt DNA), as postulated by EPR, to explain how entangled particles behave.

[@Eznid]

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

[@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!!!

[@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.

[@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👍

[@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.

[@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.

[@PJ-he5zk]

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

[@windlesSpice]

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

[@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!

[@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

[@valariegoose6578]

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

[@valariegoose6578]

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

[@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!

[@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!

[@happylittlemonk]

Did this explanation ring a BELL then?

[@valentinbernard8126]

@@happylittlemonk cling

[@artymowski]

You have to be kidding? Clear as Mississippi mud!

[@1VirginiaL]

This helped me quite a lot, thank you DrPhysicsA.

[@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.

[@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...

[@greggoryrice7046]

Thanks for scrapping the intial explanation, the revised explanation was infinitely more clear. Cheers.

[@liudas5377]

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

[@exalted_kitharode]

Crystal clear explanation. Thank you.

[@DrPhysicsA]

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

[@v.r.horowitz3907]

This is an important correction!!

[@shaynemurray6351]

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

[@juangreen8194]

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

[@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.

[@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.

[@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. ;-)

[@huddybuddy8097]

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

[@krish2nasa]

Wow, what an explanation! Thank you very much.

[@omsingharjit]

You are best teacher of both maths and Physics :)

[@tremolando6066]

Brilliant teacher. Also love the bit where he interupts himself, to make the polarized pairs clearer :-)

[@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.

[@vaibhavnakrani2983]

🫡 very well explained. You sir are indeed a great 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)

[@winch1126]

This is a GREAT presentation of the material. It makes the main points accessible to non-mathematicians. I do have one question - what if all of the 8 rules are not feasible or, more realistically, do not apply to the photons in equal percentages? That would explain why the measured outcome is less than 1/3

[@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!)

[@mybluemars]

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

[@chandanmazumdar1001]

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

[@rikabernar]

This's amazing. Love it so much.

[@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.

[@TANTRASIUM]

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

[@rustysim]

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

[@haaardcoooreee]

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

[@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.

[@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."

[@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.

[@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

[@sudippatra1289]

Thanks DrPhysics...but is there any reason that why the result is always less than 1/3? if you please explain...

[@alpratsibz]

Great explanation! Thanks!

[@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!

[@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.

[@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.

[@rocknrolladube]

First thing that popped in my mind when I saw your illustrations was: the slits interact with the particle if it passes through. So really the table might be sorted into a layers…rules to pass through plus rules of how to interact with slits once passed through. Making the slit more like a game of mini-put. We just can’t see the hidden windmill yet.

[@2222MalayalamElectronics]

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

[@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.

[@happylittlemonk]

Be careful. That is how a religion starts.

[@plantsofwar8834]

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

[@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.

[@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.

[@basics7930]

Great explanation 🙂.....A Good Teacher

[@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?

[@AD6043]

Thank you for the video. What is the importance of the angles of the polarizers, if any?

[@ronaldderooij1774]

To distinguish between polarisations of photons.

[@georgechan2474]

the best video ever on Bell's ineq

[@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).

[@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.

[@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.

[@ramymagdy6896]

It's a great video but i want to ask two questions first why we try to explain the polarization by saying that light consists of photons ? because we explain interference of light by consider it as a wave not as a beam of light and second if we explain polarization by QM , by doing the measurement , the wave function is collapsed when you draw a vertical , then tilted then horizontal polarizers in the same order you explained it by saying that a photon may be a vertically or may be horizontally polarized until it enters the polarizer and then find out say it was vertically after passing through the first one how when it passes through the second polarizer the wave function arises again after it being collapsed ? or precisely , Is their a wave function that describe the photon that exist only when the photon is not observed , and hide when the photon is observed ?

[@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 >

[@travelman8361]

very well explained!!! thaanks

[@1mlister]

Hi. Thanks for the clear video. When you say Bob can't measure in the x-axis. What do you mean? What would occur if bob and Alice tried to measure x and y spin simultaneously?

[@sinaggsina]

My exact question plz some one explain

[@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.

[@sjzara]

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

[@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🙏

[@EnDotter]

Hey! I got a question since i am "a bit" confused. When A and B are doing experiments on entangled photons using random slits (/, \ and |). How come can the 1st line of the table be all true (T)? If one photon can pass through |, it means other one will certainly pass through | and maybe pass through / or \. I think i understood this enough to say it's fine. On the other hand if one photon passes through / i can for sure say other one cannot pass through \ since its 90º rotation and that it could pass through |. Why is there a given case that A and B could register same result no matter which combination they choose? If my reasoning is right, will it shift the probability of getting the same result to be greater or lower than 1/3?

[@Plystire]

6 years or so late, but the answer is... the 3 slit options are all 120 degrees from each other. 1st slit is 0 degrees, 2nd is 120 degrees, 3rd is 240 degrees. The real trouble with this experiment is, similar to your assumption that a photon passing through one can't pass through the other, there's only a CHANCE the photons pass through the differing polarizer selections. There are bound to be instances where the photon passes through one but not the other, which is not to say that COULDN'T have passed through it, just that it didn't in that particular instance. And for the instances when this happens the experiment ASSUMES that the photon COULDN'T have passed through it, marking it as such in the data. Similarly, when a photon pair successfully passes through a pair of polarizer selections, the experiment assumes the photons HAD TO pass through them, supposedly due to this "hidden variable determining outcome" hypothesis.

[@swamiaman7708]

Great explanation Sir .....

[@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.

[@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.

[@user-tc7ki9gp5h]

Hello Sir, thank you very much for this explanation. I think I am very close to understanding. I hope I can ask a questions for clarification. From a symmetrical point of view I am missing a vertical polarizer. But it looks to me that the (relative) angles do not play any role. It looks like A and B can chose randomly from arbitrarily many devices as long as the device has 50% chance of letting a proton pass. This cannot be true. What am I missing?

[@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.

[@alun101]

Great explanation. I now get it.

[@Sheriour]

Excellent video! I would just like to make one thing clear for myself. The "same" result is in no way priveleged in comparison with "different"? What I mean is that the 0.25 probability applies regardless of whether we focus on getting the same or different results?

[@amarrao95]

Seems to me the trials are not testing spin direction [which is the 50/50 proposition,] but instead identical polarization [which apparently should always be the same]? Also, in many cases [when Alice and Bob pick different polarizers,] the trial is not measuring spin in the same plane, which I thought was a condition for the spins of entangled particles to always be found to be opposite?

[@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.

[@mohteeti6022]

Thanks for this clear explanation. I'm not a physicist but love it. My question is as follows. According to the experiment setting, It seems to me that ruling out the existence of any hidden variables is based on the entangled particles, i.e., quantum entanglement is taken for granted. What would happen if the quantum entanglement itself does not always happen? Could the entanglement phenomenon be also probabilistic?

[@Serotta2001]

Not unless you are prepared to throw out conservation of angular momentum and many other conservation laws. One example of entanglement is the consequence of a particle with no spin decaying into 2 particles with spin, such as an electron and a positron. Conservation of angular momentum requires that they have opposite spin. Yet, uncertainty requires that each one's spin is all possibilities until it is measured. Only then does the wave function collapse. Entanglement means that it collapses for both particles at the same time. Put another way, these properties are probabilistic until they are measured. The spin of an electron could end up either way depending on when it is measured. It didn't start out with an up spin and it's partner a down spin. Measuring it makes the spin definite and makes its partner's spin definite at the same time.

[@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.

[@ericreiter1]

I was trying to find Suskind's video where he shows the result of Bell's ineq. Could not. Please can you send or show a link? Also, my understanding of Heisenberg's uncertainty principle is not about dimensions of x y and such, but rather is about position * momentum

[@chrisofnottingham]

+Eric Reiter There are several (pairs of) properties exhibiting complementarity and a full knowledge of x and y spin is one. It isn't really my strong point but as I understand it, any pair of properties that could tie an electron (say) down to being fully a wave or fully a particle will exhibit uncertainty. At the risk me of being totally wrong, I would suggest that full measurement of the pairs you mention would result in the electron being reduced to all particle, whereas fully knowing the spin characteristics would result in it being all wave.

[@thefastestcat]

Bell had a lovely statistical explination.... but what does the result of .25 same mean... what produces ... or predicts / explains that outcome?

[@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.

[@--Artur--]

I have a question! Please someone who understands this - answer , Im so curious. In the video we assume that the photons will have EQUAL probability (12,5%) of any of the 8 possible combinations . Why is that? Why cant the porbability of the combinations 1-8 differ? Why do we assume this? Could perhaps the experiment be faulty so that some angles of polarization were more probable? Or was the experiment conducted with some other particles and spins were measured and it still came out this way?

[@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?

[@eliaswrwf5826]

I understood for the most part though you lost me at the 1/3 part, I understand that's the probability of obtaining a different polarizer, though 1/3 seems like the probability of each individual case, all of them together on the other hand should be 6/8 or 3/4, this while taking in count the ones that are the same, what am I missing?

[@kellyorator9007]

I think the individual case (1/3) and overall case (3/4) yield the same result because the overall case is accounting for every combination of hidden variables while in an actual experiment you would perform it once with 3 polarizers in 3 different pairs with any hidden variable combination governing that one particular experiment

[@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 ?

[@nisaxaxa123]

I had a question, what about the cases when alice and bob picks the same polarizers (1,1) (2,2) and (3,3)? isn't that a key part of testing hidden variable?

[@vwcanter]

This is great, but toward the end you say that when you run the experiment, the two measurements get the same result "greater than 1/4 of the time" and you say that contradicts the inequality relation from the beginning. Did you possibly intend to say "less than 1/4 of the time"?

[@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.

[@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

[@iam774526]

What if we get photon pair of only 2nd and 3rd hidden variable (alternatively) and Alice and Bob chooses 23 pair more? The %-age of getting "same" will automatically decrease, won't it? What point am I missing?

[@daniratm]

Ok I understood fairly enough. Just one question, What is the relation between polarization and spin direction? Ain't a physicist so...

[@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