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Bohr-Einstein debate: why did Bohr not simply say

  1. Feb 6, 2012 #1

    We've all heard of the Bohr-Einstein debates to some degree (the essence of them being: Einstein tried to convince Bohr that the uncertainty principle is not true by claiming to have found concrete thought experiments that seemed to violate it). Bohr countered Einstein's arguments.

    But what I don't understand is why Bohr not simply said "but you're using classical reasoning", since Einstein heavily depended on classical conservation laws, for example that of momentum. Okay conservation of momentum is still true for statistical averages in the QM formalism, but Einstein really used them classicaly: imagining one particle bouncing off a wall imparting momentum to the wall in such a way as to keep the total momentum fixed.

    For example, if Bohr hadn't given the conclusive counter-arguments which he did (incidentally also using classical conservation laws), would others have accepted Einstein's reasoning? Or would they simply have countered with "yes but you're using a classical reasoning"? I think the latter. Hence I'm confused why Bohr didn't immediately answer with it.

    NOTE: please don't reply with "the UP is merely a statistical statement that can be derived from the formalism and which talks about the standard deviations of the position and momentum distribution": I know this, but this is not in the least what the above question is about.

    EDIT: some people apparently, for some reason, interpreted my OP as inviting general comments about Einstein's realistic view, which is not what this thread is about, every sentence I wrote down was supposed to be specific to the two thought-experiments Einstein brought forth in the Bohr-Einstein debates and are not with a greater generality than that. My question is simply why Bohr thought Einstein's reasoning using classical concepts such as conservation of momentum (not averaged) was a serious threat, saying things like "it would be the end of physics if Einstein were right" etc, although these concepts are not really part of quantum mechanics and so can hardly be used to derive a contradiction.
    Last edited: Feb 7, 2012
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  3. Feb 6, 2012 #2
    Bhor probably did try to explain that to Einstein, but Einstein was so resistant to quantum mechanics (even publicly) that Bhor probably gave up trying to explain things in terms of QM and instead tried to find a way to explain how Einstein as wrong even in his own world.
    To quote Einstein,
    -"God does not place dice"-
    And he still said that even when we knew that he/she/it actually did.
    Einstein always thought there was hidden variables determining the random chaotic patterns of QM in order to fit his philosophy of a grand harmony of deterministic mechanism for the universe, but QM has discovered that is not possible, and he probably resented QM for suggesting that.
  4. Feb 6, 2012 #3
    I believe in one thought experiment Bohr replied to, his answer relied on the macroscopic object obeying Quantum Theory.
  5. Feb 6, 2012 #4


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    The disagreement between Bohr and Einstein is more subtle than that. Einstein famously said that the moon is there even if he's not looking at it; if Bohr had Einstein's capacity for one-liners he might have responded that as long as you're not looking at it, it doesn't matter whether it's there. That's the difference between realism and not, and that's way deeper than "classical" or not.

    Is it interesting to consider how Einstein might have changed his thinking if he had lived to see Bell's theorem? I think so... but Bohr might have rejected the question as meaningless because it didn't happen.
  6. Feb 6, 2012 #5
    I'm not satisfied with either replies, questionpost's post is way too speculative, and stevieTNZ seems to miss the point.

    @questionpost: First of all, I'm familiar with the history of things, and I think you're misrepresenting. Einstein wasn't as ignorant as you're describing him. Also, your statement that "QM has discovered that is not possible" is simply not true (look up "de Broglie Bohm pilot wave theory"). But let's not make it about that, it's too off-topic and hot enough to get lost in. What pertains to your on-topic answer: your guess that Bohr simply followed into "Einstein's world" to pleasure Einstein seems too speculative; Bohr gives no indication whatsoever of this view. I'm not saying you're wrong, but it's not very convincing as it stands.

    @StevieTNZ: Indeed he did, but that's not really the point. He and Einstein both used a (partially!) classical reasoning in the sense of using the classical concepts of particles bouncing around, obeying conservation of momentum. Indeed Bohr superimposed the uncertainty principle on it, a highly unclassical concept, but my problem is more with "why did Bohr toleratore the classical concepts Einstein used" even though they're not present in the actual quantum theory. One might even wonder why Einstein tried to use the classical reasoning (when trying to debunk QM) in the first place, since I'm sure he was aware QM doesn't use those concepts. It all makes me feel like I'm overlooking a big point.
  7. Feb 6, 2012 #6
    I'm aware of this; grosso modo Einstein was a realist and Bohr was a positivist, but I'm not sure how this answers my question? If it was intended to, can you clarify?
  8. Feb 6, 2012 #7


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    As I read the original question, you were asking why Bohr didn't end the debate by observing that Einstein was using classical reasoning. I'm saying that wouldn't end the debate, because the debate wasn't about classicism versus modern physics; it was about realism versus the Copenhagen interpretation. (I'm a bit reluctant to classify Bohr as a positivist because I suspect that a fair number of pre-20th-century positivists would disown Copenhagen; but Einstein's realism is indisputable, and a fair case could be made that Bohr was just accepting positivism without flinching).
  9. Feb 7, 2012 #8

    Ken G

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    I think Bohr didn't said that because it wouldn't have been very effective. It would be like someone today who wants to argue that quantum fields should not be considered real saying "but you're using quantum mechanical reasoning" to claim they are. It is natural to use the reasoning of the best available theory, and to not want to let go of that reasoning until it is absolutely necessary. I believe that is all Einstein felt he was doing-- saying "I will continue to use classical reasoning until I'm convinced it doesn't work because that's what got us this far." He had some pretty specific ideas about what physics should be, and it served him well in relativity so it wasn't really that much of a stretch for him to do the same in quantum mechanics. It just turned out that quantum mechanics was yet another seed change in what we think physics is (as was Newton's laws when they came around). And for proponents of the Bohm interpretation, they still think highly classical reasoning is fully appropriate, they seem the closest to Einstein's values about what claims on reality a physics theory should make.
  10. Feb 7, 2012 #9
    Ah now I'm getting your point (or at least more so). I agree that the bigger debate between Einstein and Bohr was about classicism versus modern physics, but restricting ourselves between their specific discussion about the UP and even more specifically concerning the said thought experiments, I don't see why "you're using classical reasoning" wouldn't be a valid counter-argument in this case. After all, Einstein is trying to prove QM wrong (with a specific example), but he's using non-QM reasoning; seems like a no-go.


    @ Ken G:

    I'm not sure if I'm following the analogue, possibly due to grammar? (I'm not trying to be hard-***!) Anyway, it would seem to me that if someone wants to find a flaw in a theory and proposes a thought experiment to explicitly show the flaw, certainly the thought experiment should use that theory, don't you agree? It seems quite straight-forward enough, but allow me to still propose an analogue of my own: it would be like someone trying to disprove relativity theory by using a galilean reasoning.

    Fair enough, but do you (personally) believe that his arguments (restricting ourselves to the UP ones, the thought experiments we've both heard of) would carry any weight even if Bohr hadn't countered them the way he had? I can't think of anyone saying "yes", but on the other hand it's Einstein we're talking about here, so I just want to make sure I'm able to appreciate his points to the best of my ability. Do you understand my struggle?

    I believe I disagree. Einstein is using the classical idea of conservation of momentum (and not statistically averaged by using the wave function). I don't think Bohmians do that?
  11. Feb 7, 2012 #10
    Sorry - my comment was more of a general comment than anything else.
  12. Feb 7, 2012 #11


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    Indeed it is. One of the issues is that people think Einstein disagreed with QM - he didn't. In fact he wrote a forward to a standard book on QM written by David Bohm that he greatly admired and recommended the book to all physicists. He believed it was incomplete in the sense that classical mechanics is incomplete but within its domain of applicability was correct. He was the first advocate of the ensemble interpretation of QM which is the one I adhere to and now days standard textbooks such as Balentines use it exclusivity and explain its advantages over Copenhagen.
    'The attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations, which become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems.—Albert Einstein'

    He disagreed with the Copenhagen interpretation because in trying to be a complete description it denied of an objective physical reality 'out there' independent of measurement - but he is hardly alone in that.

    In modern times we now know that QM is basically a variant of bog standard probability theory to allow continuous transformations of what are called pure states:

    I suspect Einstein would approve - it does not provide what he wanted - but at least, like the ensemble interpretation, it gels with his views of what is going on.

    Last edited: Feb 7, 2012
  13. Feb 7, 2012 #12
    Bhobba, although your post in itself is interesting, I feel it's off-topic, so I just want to request others not to go into the subject of your post and to stay on-topic, thank you :)
  14. Feb 7, 2012 #13

    It seems some people believe that Bohr indeed could have used the "well you're using classical reasoning" argument but that he simply didn't because he wanted to play along with Einstein and show that there can also be found a fault in Einstein's reasoning, without necessarily believing Einstein's argument held much weight as it stood.

    I strongly disagree and to back this up I want to quote Leon Rosenfeld, describing Bohr's initial reaction to Einstein's 2nd thought experiment (the photon in the box one)
    This clearly shows that Bohr thought Einstein's reasoning to be very valid, in the sense that he had no objection to using (for example) the conservation of momentum the way he did. Does anybody understand why? This is the core of my question.
  15. Feb 7, 2012 #14


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    Maybe, and this is just a guess, guys like Heisenberg had given similar semi classical reasoning in explaining the HUP to give it pictorial vividness. It follows very easily from the math of QM but in order for physicists to picture it they resorted to classical like 'analogies'.

  16. Feb 7, 2012 #15


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    I'd add this. Realism is indisputable because it is non-operational (faith vs empirical evidence). One often hears the comparison made between "Young Einstein" and "Old Einstein". Einstein's discovery of SR and GR, as seen in his early expositions, followed from the adoption of an operational re-interpretation of the meaning of space and esp. time. e.g. "Time is what we read off clocks, distance what we measure with measuring rods". This operational approach allowed him to explore the question of how to know if distant clocks are measuring synchronous times and thence to whether that even had absolute meaning.

    Quantum theory can be understood in an analogous way, by being totally operational with regard to the act of measurement and meaning of "state" one is led to the question of its meaning between acts of measurement and its relativization.

    But once this operational thinking leads to GR and once one formulates the most beautiful geometric model with space-time treated as a curved Riemannian manifold, (a "classical" reality model) I believe Einstein lost his youthful appreciation of operationalism. GR was so successful and the visualization of it in terms of a curved space-time manifold so compelling that it is difficult not to take the model for reality. (A problem, I believe with today's attempts to "quantize space-time" as if it were a real object.)

    In short applying GR via differential geometry was a distinct conceptual practice with a distinct mode of thinking to synthesizing it in the first place.

    I believe this turned Einstein into a more conservative realist who was unable to apply the same operationalism as it gets applied in to the relativization of state in QM.

    That's just my opinion, of course, and colored by my understanding of QM and appreciation of "the" CI. But one my wonder if Bohr, with the advantage of our hindsight and perspective, might have been able to convince Einstein by expressing it in such terms.
  17. Feb 7, 2012 #16


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    For the record, it was on-topic -- in the sense that your original post seems to be premised on a partial misunderstanding of what the Bohr-Einstein debate was about, which Bhobba tried to clarify.
    (I was about to post something similar to what Bhobba said in post #11, but he got in first. :-)
  18. Feb 7, 2012 #17
    I've heard that view many times before, but do you truly think that's convincing? The UP they are discussing so vividly seems to be of a different nature than the one we're nowadays proving in our books, but more importantly if they were just discussing analogies, why would Bohr give it such grave importance? "it would be the end of physics if Einstein were right" does not sound like someone finding a hole in your analogy.

    I understand it can be read that way, but I honestly just think I asked a specific question unrelated to Einstein's general position (which I incidentally believe I already understood) as I nowhere made any statement about Einstein's general point of view, everything was case-specific. But anyway, as much as I don't want to derail this thread by talking about Einstein's realistic position, I also don't want to make it about whether or not I was clear that I didn't want to make it about Einstein's realistic position :p

    @ jambaugh: please read post 12! You're telling me nothing new and are mainly discussing things which I've explicitly told (by now) that this thread is not about.
  19. Feb 7, 2012 #18


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    Sorry, I can't buy that analogy. A better analogy would be someone trying to disprove relativity theory by demonstrating that there are Galilean results that cannot be recovered from relativity theory when v<<c.
  20. Feb 7, 2012 #19
    Well, I'm just basing it off of what I know. I'm not saying Einstein was ignorant, but he didn't like QM, and QM implied determinism couldn't exist, which Einstein didn't like. Einstein just thought that things just couldn't possibly happen unless there were things causing them, even though in QM, there's nothing really "causing" a particle to show up in the place it does, there's no lower level of hidden variables, it just happens, undetirminstically, and I think Einstein had a problem with that.
  21. Feb 7, 2012 #20


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    Maybe because at that time those pictorially vivid analogies were considered the best intuition? As far as I can tell the UP Heisenberg discovered is exactly the same thing we have today ie an intrinsic property of non commuting operators. It most certainly is in Dirac's classic book which was around at that time and was Einsteins reference (he called it that perfect book). Interestingly Dirac eschewed the types of debates Bohr and Einstein reveled in and said shut up and calculate - although often attributed to him it is doubtful he said exactly that - but it did sum up his view.

  22. Feb 7, 2012 #21

    Ken G

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    Yes, I believe bhobba's point was that we should not overstate what Bohr and Einstein disagreed on-- they both agreed that we should retain every principle we already have, like conservation principles, unless we are forced to abandon them. To my knowledge, Bohr never abandoned the conservation of momentum, he did not say that momentum was only conserved in a statistical sense. That would have been a radical break from physics as we know it, and I don't think Einstein would have gone along with that (nor most physicists today, in my view). Instead, what Bohr was saying that we needed to let go of was the leap of faith that our everyday experience and intuition could be extended to the understanding of the quantum domain. Indeed, I feel his views are best encapsulated in his quote "there is no quantum world." So it really was about realism vs. positivism, not quantum vs. classical thinking. Einstein was saying that physics has succeeded by taking our everyday notions and applying them all over the place when we construct a concept of reality, and Bohr was saying there is no reason to think that is a true path to understanding nature as she is, instead of just a way we can try to understand her. By taking the latter position, Bohr stressed our inherent limitations in investigating nature, and his point was that a working physics must embrace rather than deny those inherent limitations. I believe his "end of physics" comment meant that his approach gives us a loophole: it allows physics to say that whatever is impossible to know does not exist (ironically similar to Einstein's views on the ether in special relativity), rather than having to allow that physics doesn't work.

    Now, that's the philosophical backdrop, but the question here seems to center on the specifics of the thought experiments, and what aspects of a thought experiment did Bohr think he would need to address to be able to hold his view. Bohr was not just espousing a philosophy that anyone could agree or disagree with, he was trying to formulate something powerful for predicting outcomes of experiments. He was basically creating a physical principle along the lines of, you cannot understand the outcomes of your thought experiments until you drop the literal realism you are trying to employ and embrace the inherent indeterminism of quantum physics. So he couldn't object to conservation laws, because those are the lynchpins of predicting experiments, but he could show why the things that he claimed were unknowable actually had to turn out to be unknowable in the thought experiments too.
    Last edited: Feb 7, 2012
  23. Feb 7, 2012 #22
    But why can/do you say that, when Einstein used a non-classical domain? He didn't restrict himself to the "v<<c" equivalent; he was specifically using conservation of momentum (partially, at least) one a miniscule system. If you, despite this, still believe in your analogy, can you give an argument for this? I see it as very "v -> c ".

    Again, if it's just an analogy, then surely Bohr wouldn't have minded Einstein's thought experiment. I'm trying my best to see it your point of view, but it just seems so very unlikely.

    Well it certainly looks the same, i.e. ``[itex]\Delta x \Delta p_x \geq \frac{\hbar}{2}[/itex]'', but when I hear Bohr and Einstein talk about it, the concept seems quite different. Okay, sure, maybe their talk was just an analogy, in which case it's without a doubt conceptually the same UP we know today, but as I have expressed above in this post it seems incredibly unlikely that Bohr would've said "it's the end of physics" if Einstein were correct if that were the case.
  24. Feb 7, 2012 #23
    Okay, thanks for the post, that is in the direction that I wanted to get responses. My apologies for bhobba if that were his point and I misunderstood.

    So even physicists today believe in conservation of momentum the way Einstein and Bohr used it in their debates? This seems unfounded. Is there any experimental or theoretical reason to believe this?

    On another note, if I understand correctly, your reply also seems to suggest that Einstein and Bohr weren't really talking about quantum mechanics specifically, but more about whether or not it is possible to emperically define precisely the concepts of position and momentum at the same time even in classical mechanics. Einstein (first) believed it was possible, Bohr didn't, the latter eventually proving his point. If that were the goal of the debate, it would indeed clarify to a great extent why their reasonings were not at all quantum mechanical(*). Hm, at first sight this makes a lot of sense (and actually seems quite obvious, come to think of it), but I have to let it sink in a bit. Have I understood you correctly?

    (*) the thing that still bothers me is that you apparently still believe their reasoning to be highly quantum mechanical, in the sense that for example their use of the conservation of momentum is sensible and even still common practice today (even by people who know what they're saying, i.e. not experimental physicists :wink:)
  25. Feb 7, 2012 #24
    Who moved this to "History & Humanities"? Even without notification? Come on, this is a topic only physicists with a knowledge of quantum physics can say anything sensible about, and although the question can be interpreted really literally as a historical question, a more decent look at the question shows that it's actually asking for a quantum mechanical answer. It's like putting a "why did Dirac write this and that in his book" in this forum just because it refers to something in the past...
  26. Feb 7, 2012 #25

    Ken G

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    I would say yes on both counts, but note that there is a difference between an observational reason to believe, and observational proof. The theory basically gives us two choices-- either momentum is conserved (a ramification of the translational invariance a la Noether's theorem, which continues to hold in Hamiltonian systems like standard quantum mechanics), or else our laws are not really laws, they are just statistical tendencies. Observations do not adjudicate those possibilities because the accuracy required is not achievable (we'd have to track quantized action in macro instruments), but they also do not force us to drop the contention that conservation laws are exact laws, at least in an idealized way.

    Of course, we can still be pragmatic, and say that all laws are only going to be approximate (and allow stochasticity to creep in that back door, which I believe is your approach), but the point is, physics has always been about idealizing reality in order to understand it, so the real question is, when we do idealize reality that way, what are the laws we get? I believe most physicists today, and Bohr and Einstein, would say that the law of conservation of momentum should correctly apply to the idealizations (like thought experiments), even in quantum mechanics.
    I think they were talking about nature herself, regardless of which theory we use to understand her. Bohr was saying that quantum mechanics was trying to teach us a lesson we had got wrong from classical physics (even though classical physics does not require anything contrary, most thought it had taught that contrary lesson and Einstein did not want to let go of that). So I would say you're right that Bohr must have felt we had overinterpreted the success of classical physics, whereas Einstein was loathe to agree.
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