Quantum Mechanics 10b - Bell's Inequality II

Here we conclude our discussion of Bell's inequality and the implications of its experimental violation.

Comments, including questions, suggestions and constructive criticism are always welcome.

My cartoon Bell inequality experiment was largely motivated by the "flying nail gun" presented in Chapter 12 of "The Cosmic Code" by Heinz Pagels, Dover, 2011, ISBN 9780486485065

[learnerlearns]

For decades, I've read about and seen countless videos on these subjects. This video (along with part a) is the clearest, most concise explanation yet.

The graphics are appropriate to demonstrate these concepts. They are neither overly produced nor over simplified. The narration is intelligible, at a comfortable pace and intonation that is neither frenetic nor soporific. No distracting music! (Yay!) The material is logically organized and presented in complete sentences. There is no fluff or "filler."

I also GREATLY appreciate how there is no attempt to "dumb down" or "popularize" the subject by omitting math. Indeed, the math HELPS anyone who wants to comprehend the incomprehensibility of non-local reality. The math here is presented with explanations of specialized or unfamiliar symbols, so basic algebra and a glancing familiarity with trig and statistics is enough to understand everything. This math is both a necessary and welcome ingredient.

All too often, YT videos or TV shows emphasize the "spooky" aspects of these subjects. They add mystery-movie music and an actor's vocal inflection to dramatized dialogue in the attempt to evoke an emotional response. The astounding facts of QM are mind bending enough without adding emotional manipulation.

Such "pop" dramatization leads to bogus interpretations of experimental results in terms of consciousness or invisible super beings affecting reality. QM is already strange enough. It does not need or benefit from magic.

Here, the author explains everything needed to understand Bell's Inequality and how it is violated in the "real" world by experimental results. The demonstrable weirdness of QM is neither avoided nor interpreted. It just "is."

The ONLY thing I can think of that could possibly make this series better would be to include video of the actual experiments with the real instruments in real laboratories. The graphics are necessary, but I would LOVE to see actual experiments. The combination of these explanatory graphics WITH the physical apparatus doing actual experiments would make this the best possible presentation on this subject.

All students or inquistive fans of physics should view impressive and valuable work! Teachers too will benefit from this lucid paradigm.
THANK YOU!

[eiver]

I have never seen a better explanation of quantum mechanics to the general public. Thanks to your series I now have at lest a rough idea of qhat QM is all about. You should teach teachers how to teach.

[argfasdfgadfgasdfgsdfgsdfg]

This video is GREAT. I have seen a lot of videos about Bell's inequality now, but most of them lack major points. Your video is a complete step-by-step explanation. Thanks a lot! Please keep making these videos!!!

[WanJae]

I've watched and read many dozens of explanations of Bell's test and inequality, and none of them are as well explained as ViaScience's video.

Many explanations competently illustrate the inequality alone, but then leap to a conclusion, "therefore, because a set of numbers do not agree, reality is nonlocal." We are left to scratch our heads and wonder how they got from A to B. Worse, many explanations understand Bell's work only through the lens of entanglement.

Here, we all the dots are connected. Bell's work is a simple statement about reality that could apply to anything you may ask three yes/no questions of, be they particles, students, or playing cards. A violation of Bell's statement means that some of our test subjects changed their answers at some point during the test, thus breaking the airtight statistics. Quantum particles are shown to break Bell's inequality, thus demonstrating that they are "changing their answers" as they are being tested. Finally, in an attempt to "fix" our particles' test results, we begin testing entangled (cloned) copies of our particles so that we may give a particle a yes/no test, but have a handy copy available for a second yes/no test that would be (in theory) unchanged from the first test.

This, however, does not fix the situation -- even our entangled particles violate Bell's statement exactly as before, thus implying that our entangled particles, too, are being changed as they are being tested. That is, a test on one particle does indeed, without a doubt, alter its entangled partner.

Since this test can be done over arbitrarily large distances, and the "change" is demonstrated to happen instantly, locality itself must be violated, and some spooky action at a distance is our only explanation.

No other explanation I've watched or read has strung together those key elements and packaged them so well for consumption. This is brilliant. Further, as another commenter pointed out, ViaScience does not dumb down the hard science, not does it attempt to scare the audience into thinking quantum is more complicated than it is.

Thank you for this. This channel deserves some sort of award ...

[rubbeldiekatz]

Best explanation of Bells theorem i have seen so far. Great work!!!!

[ScottStratton]

Wonderful series, thank you! I can only imagine how much work it takes to figure out how to present these topics to a general audience without throwing rigor out the window (hah - not to mention the time to *produce* them once you figured if out!). Thanks to this and the prior video, I think I understand Bell's inequality and the EPR paradox enough to ask a question I have had since I first encountered these ideas (question requires a whole setup, of course ;-)): as your comments section nicely shows, its natural as soon as you get the profound nature of these ideas to start speculating about FTL communication. I know I did that the first time. It appears that the nature of QM is such that while non-local, it does not, in fact, allow "instantaneous communication" across galaxies or whatever. I certainly don't claim to understand it all, but the reasoning seems to make sense to me: though Bob's particle's "local reality" is indeed dependent in some sense on Alice's particle, Alice can't sneak the score of the FIFA-Andromeda finals into the system so Bob can know the winner before the information could travel at the speed of light.

But - is there any experimental setup similar to the one described in your videos, such that:
• given the system being created and used, and
• the particular choice of what to measure and how to measure it,

would

(1) give us the same kind of experimental proof of non-locality and entanglement as we really get in the Aspect-type experiments - IF both groups (Alice and Bob or actually take the measurements as is normal; and (2) if one group elects to take a holiday and do nothing, the other group would then get results that are still consistent with QM, but different from what they would have gotten in case (1)?

Now that I ask it in writing, I can't see why I was so sure It was important, lol. ah well! again, thanks for the great vids. I agree with the other commenter - I can't believe you don't have more views/likes/etc. I hope that changes! cheers.

[brendanboyle]

These two videos on Bell are just excellent -- astonishingly clear and thorough. Thank you for the time and effort that you put into producing them. On the basis of the comments below, that time and effort was anything but wasted. I am eager to watch other videos that you've made. Thanks again.

[danzuck]

Though I still do not fully understand, this video has more clarity than I have found on other videos on this topic. This video is most excellent. Thanks.

[dendrites]

9:50 What if the effect on the "bar" is not random, but reorients the bar in a consistent way depending on the angle of the bar with respect to the filter. This would suggest two entangled photons would still pass or fail together, and that local interactions explain the behavior.

[makeit]

your videos are really helpful. you give the big picture and you explain it easy way to understand.

[colindupee]

Love the videos; thank you.

There is a part of this video that threw me for a long time, though: it might be really helpful to show the plot of P(a~b), P(b~c), and P(a~c) for the cartoon example. The plots are linear instead of sinusoidal, and the time I spent without that realization were pure mental agony! :)

[jayashrishobna]

This is an amazing video. But could I please clarify - why is it that a violation of the inequality is what shows that reality is non-local? Wouldn't the fact that you get equal results whether you measure just one photon or two entangled photons be what proves that reality is non-local? Couldn't you have a situation that doesn't violate the inequality but where the single-photon and double-photon experiments both give the same results, and wouldn't that in itself prove non-locality? Why is the inequality itself so crucial? Thanks!

[kostuek]

well, I didn't understand a thing, but it all sure sounds significant

[AntonioPereira-vtwb]

Great! Finally I realize I don´t understand Q. Mechanics, but now I know Bell´s Inequality!

[protocol]

Non-locality is only an issue if it violates the speed of light and/or doesn't have a very good explanation.  The entangled photons, under Special Relativity, have traveled zero distance in zero time (infinite length contraction and time dilation) from their own reference frames no matter how far away they end up or how long it seems to take from an observer's reference frame.

There's a paper on this that even covers entangled sub-light particles like electrons (involving magnetic coupling) but I'm not good enough to spot any potential flaws.

Search for "EPR Paradox solved by Special Relativity" by J. Lee.and published in the Polish Academy of Science's Acta Physica Polonica A volume 125 in 2014.  Careful, there are two versions and I'm not sure what differences there might be -- the newer published one is formatted in two columns.  PDFs of both show up in a Google search of the title.

[stefanostramigioli]

I would like to express my great appreciation to the author of such bueatiful lectures! I tried to track the name which I cannot find, but these lectures are fantastic! Congratulations!

[Theotheruniverse]

Best video I have ever seen on this topic.

[davidgiardini]

Excellent video! And there is something here that is, at least, curious for me: you derive the violation of Bell's inequality without any quantum assumption; just using a well known result of electromagnetism; that is the intensity of electromagnetic field proprtional to square of the electric field. Is like Maxwell is allways right :-)
Thank you for this :-)

[jackcasali]

Amazing. What is most surprising to me is that you showed the violation of Bells Equality without even needed to invoke experimental results. This is quite incredible because I think that shows the issue is not outside, as in some external phenomenon, but a logical one.

[jessstuart]

9:00 That is a really good demonstration. Getting some light through by placing a filter in front of the first filter that was blocking 100% of the light is not what I would have expected to happen. Surprise!