• Wayfarer
    12.1k
    If, like me, you've heard about, and read up on, the famous Bell Inequality experiment, then my guess is, unless you're physics grad, that you won't understand it. I, too, don't understand it, but my understanding of it is now a little better than it was before listening to the presentation below. It's by science writer Jim Baggott, and it was given in an address to the Royal Institution. He's a legit writer and it's a legit institution, it's not 'quantum quackery'. (I'm very much aware of spurious interpretations of quantum mechanics, and I'm not trying to encourage them, but I think this is genuinely educational.)




    I'm not really posting this for discussion, but because it has been said that the Bell inequality experiments are among the greatest science experiments ever, and because it's a very difficult subject to get your head around. This presentation helped me to understand it for the first time.
  • Gregory
    3.1k


    If they can rule out any hidden variables, then Bell's theorem proves there is randomness in the universe. This makes sense with regard to entropy anyway
  • Wayfarer
    12.1k
    The point of Bell’s inequality is not that it proves ‘randomness’. ‘Randomness’ is associated with the unpredictable nature of the ‘quantum leap’. This experiment concerns the apparently non-local causation implied by correlation of spin states of separated particles. It’s ‘spooky action at a distance’, not ‘God playing dice’.
  • fishfry
    2.2k
    If they can rule out any hidden variables, then Bell's theorem proves there is randomness in the universe. This makes sense with regard to entropy anywayGregory

    I am not an expert, and for whatever reason, Bell's theorem makes my eyes glaze over every time I try to make a run at it. At some level it's just something I care about.

    However, I have heard one thing relevant to your comment, and it's important. Bell's theorem rules out local hidden variables, but not hidden variables in general. What's the difference? I have no idea. But Bell's theorem does NOT rule out determinism or prove that the world is random, no matter how many "Internet experts" think it does. Say, weren't we just talking about that?
  • Gregory
    3.1k




    Spooky action at a distance was proved in Einstein's day I thought. It all pretty confusing. Faster than light action can be explained, or explained away, by geometry and super-dimensions. But determinism would be hard to disprove given that people could subscribe to compatibilism if needed
  • fishfry
    2.2k
    Spooky action at a distance was proved in Einstein's day I thought. It all pretty confusing. Faster than light action can be explained, or explained away, by geometry and super-dimensions. But determinism would be hard to disprove given that people could subscribe to compatibilism if neededGregory

    My only point was that when people say Bell's theorem disproves hidden variables, what I've heard is that it only disproves LOCAL hidden variables. Leaving the question of non-local hidden variables open. And this is only something I've read, I have no understanding of any of this.

    Compatibilism isn't a physical theory, I don't see how it would make any difference to the debate unless there are some mathematical models and experiments to back up the models.
  • Banno
    12k
    Interesting that at thread on good physics so quickly became a thread on bad physics.
  • Wayfarer
    12.1k
    Spooky action at a distance was proved in Einstein's day I thought. It all pretty confusing.Gregory

    'Action at a distance', or entanglement, was implied by Schrodinger's equation. That's why Einstein objected to it, but at the time it was purely conjectural, it had never been demonstrated. However, that was his motivation for the so-called EPR thought-experiment which you can read about here. The upshot is, that what Einstein thought would certainly disprove 'spooky action at a distance' ended up by proving it instead - although, of course, Einstein had died by the time the conclusive experiments were conducted.

    The narrative history is not confusing, but concepts of quantum mechanics sure are.
  • Shawn
    11k
    Personally, I believe decoherence happens whilst the wavefunction evolves during that event.

    That's what has got me pondering on the truth that Bell's Theorem might be true for low entropic states only.
  • Gregory
    3.1k


    I've seen videos saying Bell's theorem is about randomness. Maybe it's too complex to explain to non-physicists. The PBS spacetime series said Bell himself thought fatalism was consistent with his theorem but didn't say anything about faster than light travel.
  • Wayfarer
    12.1k
    I've seen videos saying Bell's theorem is about randomness.Gregory

    Interesting that at thread on good physics so quickly became a thread on bad physics.Banno

    :up: :sad:
  • Gregory
    3.1k


    They pick random samples sometimes in physics but if our will is predetermined nothing we do is random like that. Non local variables are ones we can't detect in the system yet i think
  • fishfry
    2.2k
    They pick random samples sometimes in physicsGregory

    I think there was a recent thread on randomness, I don't remember if I participated. There's no evidence that anything in the world is random. Or the contrary. We don't know. Quantum mechanics tells us that our measurements are random. There's an x% probability that I'll find it here and a y% probability I'll find it somewhere else. QM tells us nothing about where it really is. In QM that question is unknowable. The Copenhagen interpretation tells us it's in both places till we look. Many-worlds says it's in both places in different worlds. That's as good as it gets in physics these days.

    Randomness is a very tricky business.
  • Gregory
    3.1k


    I had a thread on randomness and I think you making a joke lol, because you did participate with some great observation.

    I don't know how they can rule out all non-local variables because with "many worlds" couldn't another world influence the entangled pair? And if the communicate faster than light, to save relativity they posit wormholes and these might be connected to other worlds and so there is then even more territory we have to rule out
  • fishfry
    2.2k
    I had a thread on randomness and I think you making a joke lol, because you did participate with some great observation.Gregory

    No joke, just didn't remember.

    I don't know how they can rule out all non-local variables because with "many worlds" couldn't another world influence the entangled pair? And if the communicate faster than light, to save relativity they posit wormholes and these might be connected to other worlds and so there is then even more territory we have to rule outGregory

    I am afraid that's all way over my head physics-wise.
  • Shawn
    11k
    Something about QM:

    If the wavefunction evolves, then temperature gradients determine how it does due to lower entropy.
  • T Clark
    5.1k
    Interesting that at thread on good physics so quickly became a thread on bad physics.Banno

    It's been a while since there has been much good physics on the forum. I'm thinking about starting a thread to prove that force does not really equal mass times acceleration. Or that Ursus Americanus don't urinate in forested areas. Or that people are mistaken about the Pontiff's religious affiliation.
  • Wayfarer
    12.1k
    I’d be obliged if the comments in this thread were merged into the Bad Physics thread. I shoulda known better. :yikes: (Although I still say the presentation at the head stands on its own feet.)
  • Tom Storm
    968
    I'm thinking about starting a thread to prove that force does not really equal mass times acceleration.T Clark

    I'm thinking of starting a threat that proves Yahweh/God is real because quantum mechanics was predicted in Bible prophesy - taking Genesis, you just need to use a higher power decoding equation I have developed based on Lurianic Kabbalah and the tree of life as a functional reorientation of the chart of Standard Model particle physics.
  • Banno
    12k
    'Do not give dogs what is holy; and do not throw your pearls in front of pigs, or they may trample them and then turn on you and tear you to pieces.

    (see - I can do bible quotes, too!)
  • Wayfarer
    12.1k
    I don't suppose you have any reflections on the actual topic in the video? It is quite an inteliigible presentation, by a well-known science writer. The presentation in its entirety is basically a promotional for his current book, Quantum Reality: The Quest for the Real Meaning of Quantum Mechanics - a Game of Theories. It's well worth listening to the whole thing. The jacket copy of the book says:

    Quantum mechanics is an extraordinarily successful scientific theory. It is also completely mad. Although the theory quite obviously works, it leaves us chasing ghosts and phantoms; particles that are waves and waves that are particles; cats that are at once both alive and dead; and lots of seemingly spooky goings-on. But if we're prepared to be a little more specific about what we mean when we talk about 'reality' and a little more circumspect in the way we think a scientific theory might represent such a reality, then all the mystery goes away. This shows that the choice we face is actually a philosophical one.

    It seems that even the mention of the subject elicits a lot of nonsense. That says something, I guess.

    ;-)
  • Banno
    12k
    Action at a distance is Newtonian gravity. And with it, the demise of materialism - the notion that all causation is the banging of things one in to the other.
  • Wayfarer
    12.1k
    Action at a distance is Newtonian gravity.Banno

    Well, you can say that gravity seems to require action at a distance, about which Newton famously 'proposed no hypothesis'.

    (Relevant articles on that in Philosophy Now here and also Noam Chomsky's comments on it here.)

    But the non-locality falls out of the equations of quantum mechanics goes further - it suggests that you measuring something here causes an instantenous change over there - and 'there' might be a light year distant. So something that is done in one places causes a change that seems to propogate at greater than the speed of light, which is impossible - hence, 'spooky action at a distance'. However, now this principle is proven, and it is actually being used to secure encrypted communications. (I penned a brief article on this, for no particular audience, which can be found here.)

    I don't want to offer any kind of interpretation, as it plainly baffles a great many far more educated minds than myself. i just wanted to draw attention to Jim Baggott's presentation, because I think it is the best, non-specialist representation of the experiment.
  • T Clark
    5.1k
    I don't suppose you have any reflections on the actual topic in the video?Wayfarer

    Geez, now you're asking us to respond to the actual subject of the thread. That's pretty unreasonable.

    I just looked at the video and I see it's an hour long. I will watch it, but it might be a bit before I respond. I'll try to do it tomorrow.
  • Wayfarer
    12.1k
    not trying to burden you with work. :yikes: Besides, it’s bookmarked to the specific topic I’m referring to and that section about <10 minutes.
  • T Clark
    5.1k
    not trying to burden you with work. :yikes: Besides, it’s bookmarked to the specific topic I’m referring to and that section about <10 minutes.Wayfarer

    I started out watching the first five minutes. Then, at your suggestion, I forwarded to the section on Bell's inequality and watched through the end. Let me say first off that the part that always amazes me the most is the technology that allows these types of experiments to be performed. For example, in this video, a light source that allows you to shoot out one photon at a time, the camera that allows you to record one photon at a time, or, most of all, the clocks that allow you to measure the incredibly short periods of time. Another example is the gravity wave detection in the LIGO experiments. The detectors allow measurement of distortions of space by gravity waves much less than the diameter of a proton. How is that possible?

    Back to the question at hand. I've read about Bell's equality and entanglement before. I have a real hard time understanding the geometry and statistics of the experiments. Sometimes I can grasp them for a second, but when I stop concentrating, I lose it. Upshot - I trust what the physicists say and leave it at that. I joke and say "I understand quantum mechanics completely - it's just the way things are."

    Actually, I don't think that's really a joke. I think that's what they mean when they describe the Copenhagen Interpretation. Don't ask why or what it means, just ask how the world behaves. Shut up and calculate. The narrator calls that the anti-realist position, but I don't see it that way. I think, at bottom, none of our science tells us why. It just tells us how.

    Which brings us to what he calls the realist interpretations. For me, the big question, the only question, is whether or not there is an empirical method to determine which is correct even in theory. I believe that is a pretty controversial subject now. It is my understanding that no method for testing the interpretations have been developed. My intuition is that no testing is possible, although I can't justify that scientifically.

    That means that there is no difference between the interpretations. That pleases me. I find the Copenhagen Interpretation very satisfying. As I said, it's consistent with how I see science in general. What we call reality is a story we tell ourselves. I think interpretations that can't be told apart even in theory are, I was going to say equivalent but that's not right. They are meaningless. Which is consistent with my preference for the Copenhagen Interpretation. It was always meaningless.

    The narrator calls the differences between the interpretations "metaphysics," but I don't see it that way. For me, metaphysics is the stage we build on which physics plays out, the conventions we have established to allow us to talk about reality. I've written many lectures about that, so I won't go into that again.

    One more problem, in the first five minutes, one of his basic graphics is wrong - the illustration of the two slit experiment at 3:40. There were others I thought might be wrong, but I was confused about what he was trying to show, so I'm not sure. Sloppy errors like hat undermine my confidence in the narrator.
  • Wayfarer
    12.1k
    Sloppy errors like that undermine my confidence in the narrator.T Clark

    Long time back, I mentioned his earlier book, Farewell to Reality, which got a pretty good review from yourself:

    Baggott writes very well and has a very direct and clear understanding of the metaphysics of science.T Clark

    Same narrator!

    Which brings us to what he calls the realist interpretations. For me, the big question, the only question, is whether or not there is an empirical method to determine which is correct even in theory. I believe that is a pretty controversial subject now. It is my understanding that no method for testing the interpretations have been developed. My intuition is that no testing is possible, although I can't justify that scientifically.T Clark

    Agree. And there can't be an empirical method to decide on the differing interpretations - because they're interpretations! They're debates about what the observations mean, even given that all sides on the debate see the same results. That's why it spills into metaphysics. I'm reading a textbook by a physicist named Shimon Malin, 'Nature loves to hide', which attempts to create an interpretive framework from elements of Whitehead's metaphysics, along with Plato and Plotinus. Not far into it yet, but very much liking it so far.

    metaphysics is the stage on which physics plays out, the conventions we have established to allow us to talk about reality.T Clark

    I don't see it like that. The cosmos is the stage on which physics plays out - provided you confine physics to the observable, which I think is proper. Metaphysics considers the implications of physics in terms of what must be the case in light of certain observations.

    I think the question of the nature of the wave-function is a metaphysical question, or even THE metaphysical question implied by modern physics. A lot of the controversies revolve around that point.
  • T Clark
    5.1k
    Same narrator!Wayfarer

    I'd forgotten the book and Baggett. What can I say. I was much younger then.

    And there can't be an empirical method to decide on the differing interpretations - because they're interpretations!Wayfarer

    As I noted, there are physicists who believe there may be testable differences. We'll see.

    I don't see it like that. The cosmos is the stage on which physics plays out - provided you confine physics to the observable, which I think is proper. Metaphysics considers the implications of physics in terms of what must be the case in light of certain observations.

    I think the question of the nature of the wave-function is a metaphysical question, or even THE metaphysical question implied by modern physics. A lot of the controversies revolve around that point.
    Wayfarer

    Disagree, but that's a different discussion.

    I'm glad you asked me to respond. I enjoyed putting what I think about this issue in writing.
  • fishfry
    2.2k
    Noam Chomsky's commentsWayfarer

    I'm a big fan of Chomsky and Newton and I'm really enjoying the linked article and video. Thanks for posting.
  • Wayfarer
    12.1k
    hey you’re welcome. I found it very interesting also.
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