The EPR Paradox & Bell's inequality explained simply

This video is on Quantum entanglement, Bell’s inequality, EPR paradox, nonlocality, determinism vs nondeterminism and probability. Bohr and Einstein argued passionately about their views on the essence of reality. And for 30 years, both views were considered equally valid. Then in 1964, Irish physicist John Bell devised a way to prove whether Einstein’s view of a classical, deterministic view of reality was correct, and he put this in a simple elegant equation – called the Bell Inequality.

The weirdness of quantum mechanics can be demonstrated with a dice. If the dice was a quantum system, it would be in superposition. It would be a 1, 2, 3, 4, 5, and 6 all at the same time. It’s value can only be known once it is measured. Einstein, was bothered by this interpretation of quantum mechanics. Einstein along with Boris Podolsky and Nathan Rosen came up with what they thought disproved the Copenhagen interpretation. The crux of their argument rested on the idea of a phenomenon in quantum mechanics called entanglement. EPR argued that since nothing can travel faster than light according to special relativity, this should invalidate the Copenhagen interpretation. This was the EPR paradox.

in 1964, John bell proposed an equation to determine who was right. In a universe where local hidden variables are true, when the two particles are emitted, they know what their state is going to be in all three directions, Z, X, and Q from birth. And there are only 8 possibilities of spins that each particle could have.

what is the probability that Alice measures in the z direction, gets a positive spin, and Bob measures in the X direction and gets a positive spin? Well, if the above case is for Alice, there are 4 events where Z is positive. In order for Bob to get X positive, Alice would have to have measured X as negative. So these would be in event 3 and event 4. To get the probability we have to divide by the total number of events, 8.

Let’s do this for two more scenarios. What is the probability that Alice measures positive in the Z direction, and Bob measures positive in the Q direction? In this scenario, it would be event 2 and event 4. Again we divide by 8 to get the probability.

And the third case is: What is the probability that Alice measures positive in the Q direction, and Bob measures positive in the X direction? This would be event 3 and event 7, divided by 8 for probability.

P: Z+, X+ = E3 + E4/8
P: Z+, Q+ = E2 + E4/8
P: Q+, X+ = E3 + E7/8

So these are the three probabilities given the hidden variables theory. Now here is big insight that John Bell had:

If I take the total number of Events, and multiply that by the probability that Alice measures Z positive and Bob measures X positive, this has to be less than or equal to the total number of events times the probability that Alice measures Z positive, and bob measures Q positive, plus the probability that Alice measures Q positive, and bob measures X positive.

P:Z+,X+ less than or equal to P:Z+,Q+ + P:Q+,X+

I can prove this is true by doing simple math:
E3 + E4 is less than or equal to E3 + E4 + E2 + E7

This makes total sense, because E3 and E4 are on both sides of the equation. And E2 and E7 have to be positive. So this inequality absolutely HAS to be true for any hidden variables theory to be true.

But what happens in a universe where the laws of quantum mechanics are correct, and not hidden variables theory?

And that probability of Bob measuring Q to be positive, after Alice has measured Z to be positive, is given by the following equation:
P: Z+,Q+ = sin^2 of 45 degrees/2

This is the critical difference between quantum mechanics and hidden variables theory. The probability is not linear but looks like sine wave. When you plot this out, this is what the probabilities look like: So you can see from the graph that at 0, and multiples of 90 degrees, the two systems are in agreement. But in between, like at 45 degrees, the probability is 25% for hidden variables, and about 14.6% for quantum mechanics.
#bellsinequality
#eprparadox
#bellstheorem

But the proof is in the pudding, because in test after test, the sine function correlation has been confirmed. The particle does not behave linearly, and so the hidden variables theory cannot be correct.

So most theorist do not think special relativity is violated, because we can’t communicate using this seemingly faster than light phenomenon.

[LydellAaron]

The understanding of this material doesn't really come from one video, it comes from watching a whole bunch of them, listening to all their perspectives, and then slowly merging together a coherent interpretation. It's very exciting! Thank you for producing your videos and adding perspective to this problem.

[GlorifiedTruth]

"The universe has rigged the game against me." I've known this all my life... SIGH.

[planpitz]

The most outstanding thing about this video by Arvin Ash is throwing in the explanation of the Mathematics in a way for the Layman to understand ...no other big media company Science show has ventured into this realm..not even the BBC .Congratulations!

[laserbeam]

I have no idea what he said but I enjoyed listening to him.

[chriskaplan]

continually stunned at his ability to take incredibly complex concepts and topics and make them accessible in a conversational and layman-friendly way. truly setting the standard for content in these genres.

[brendanfan]

good teachers make a huge difference, thank you!

[basedbax]

"local hidden variables" sounds like computer coding

[jrausa]

Bravo to the actors playing Alice and Bob. It’s very difficult to stand motionless in front of a camera for a long period of time and not lose focus / concentration.

[johnjay]

I have seen many of these explanations but this is the best. You hit a few key points that are always overlooked and those key points made it very clear. The meat that most explanations don't explain is at 14:47. That was the missing part that you nailed! Good

[TinHatRanch]

Just imagine if the scientist who has the probability of solving quantum mechanics is alive right now but doesn’t get into the school necessary because “diversity, equity, and inclusion” bar him or her from acceptance. #science.

[macgumby]

This video really got me I should quit smoking so much weed

[Velodan]

I can tell Arvin takes his role of teacher quite seriously. Loved this video and the subject matter is always edgy. That is Arvin explains current accepted science as simply as possible with mysterious difficult to comprehend topics.

[marin]

Best video about entanglement I've seen .

[danjdavison]

How can a particle have a Q+ spin and a - spin for X and Z, when X and Z are components of the vector Q?

[hisholiness]

"Truth is, the game was rigged from the start."

[augustofretes]

Bell inequalities do not disprove hidden variable theories. It rules out local hidden variables, but you can still have non-local hidden variables, like in pilot wave theory.

[ang]

hi arvin, I just wanted to say that I really really appreciate your videos. they make me so happy and teach me a lot, keep up the great work ❤

[tarzanautowala]

Einstein is right :
When I was asleep at home during physics test at school, I got a call from my school, the school existed .

It was worth skipping the test .

According to neils Bohr
My school shouldn’t have existed cuz I was asleep ( not conscious) .

[PhysicsHack]

I've never seen a good explaination of this. This was clear, thank you.

[jackhill]

This is far and away the best explanation of Bell's inequality I have ever seen/read. Arvin has truly hit the nail on the head, not too hot, not too cold, just right. I actually think I understand exactly what Bell's inequality is all about. Thank you Arvin!!!