Bohr-Einstein debate: why did Bohr not simply say...

Nugatory

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.

questionpost

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.

bhobba

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.

Thanks
Bill

Ken G

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.

nonequilibrium

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.

nonequilibrium

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 )

nonequilibrium

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...

Ken G

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.

nonequilibrium

Isn't the only theoretical proof of momentum conservation (in QM) a statistical one? I don't see this reflected in your response, or am I missing it?

Ken G

No, the theoretical proof of momentum conservation is not statistical, unless you start with a state that does not have a well defined momentum from the get-go (in which case your initial conditions are statistical, so no theory can get rid of the statistical character of such a situation). Einstein's thought experiments started from a situation of well-defined momentum. Perhaps you are saying that it is impossible for a macro system to have a well defined momentum, and Bohr could have simply made that argument whenever there is coupling, but it would have been viewed by Einstein as circular reasoning. Bohr had to show that adopting Einstein's view that all systems could have well defined momenta still resulted in experiments where there were not well defined momenta (or whatever version of the UP was under dispute). Bohr was not trying to argue a fringe philosophy, he was trying to show that the alternative was internally inconsistent. Neither Bohr nor Einstein at the time seemed aware of the possibility of the Bohmian interpretation, and I do wonder what each of their reactions were when they did find out about that.

bhobba

The modern viewpoint is via Noethers Theorem and it is conserved just as much in QM as in classical mechanics. But in QM it is the momentum operator that is conserved - the statistical aspect is the usual statistical interpretation of operators - ie provided the system is in an eigenstate of momentum and it posses spatial symmetry then it remains in that eigenstate. Actually you do not need to invoke Noethers theorem in QM because it is strongly related to the displacement operator:
http://en.wikibooks.org/wiki/Quantum...ntum_Mechanics
'However, such a plane wave is invariant under a displacement, except for the multiplicative phase factor, which has no physical consequences since it disappears when the probability distribution is obtained. Thus, we see that invariance under displacement of the wave function and a definite value of the momentum are linked, in that each implies the other: Invariance under displacement ⇔ Definite momentum'

Thanks
Bill

bhobba

Although off topic Einstein was evidently not amused:
http://arxiv.org/ftp/arxiv/papers/1007/1007.0769.pdf
'Have you noticed that Bohm believes (as de Broglie did, by the way 25 years ago) that he is able to interpret the quantum theory in deterministic terms? That way seems too cheap to me. But you, of course, can judge this better than I.'

Thanks
Bill

Ken G

Interesting article. By "cheap", I suspect Einstein might have meant that it was the form of determinism without the substance-- Bohm allows outcomes to be truly deterministic but effectively non-deterministic (FAPP, as Bell would say), but that makes it a very sterile form of determinism, of no use to a physicist.

StevieTNZ

I read this thread and get a tension vibe... just saying.

jambaugh

IBYP, I'd missed your qualifications and edit as I was composing. I appreciate your desire to rein in the thread and prevent a devolution into debates of interpretation.

But you asked potential respondents to get into the head of Bohr (and by implication Einstein) with the form of your question and that does, absent your qualifiers, invite a broad range of speculation. Rereading I still am not clear what answers you are seeking.

It would be helpful if you further qualified what you understand as the alternative to "your using classical reasoning", though that again may open the interpretation can-o-worms.

One point w.r.t. conservation laws. Note that energy-momentum conservation is typically absolute in quantum interactions. One sees this in the mechanisms for producing entanglement, e.g. two quanta are entangled via anti-correlation of their momenta by exact measurement of their total momentum as zero. Typically this is done right before they interact in a way made uncertain by their imprecise positions (e.g. symmetric elastic scattering).

If one is invoking only statistical conservation then the entanglement doesn't occur and one only has classically statistical correlation of the subsequent measurements of the two components.

Uncertainties in the effect of an interaction arise (and this was the gist of Bohr's counter arguments to Einstein) due to (logically)a priori uncertainties in the constituents. To use Einstein's earlier thought experiments to violate UP he is, as Bohr points out, invoking a circular argument, you must first negate UP to disprove UP.

The EPR thought experiment, of course, moves beyond the issue of uncertainty in the state of the apparatus.

edguy99

One of Einstein's argument:

"Suppose two particles are set in motion towards each other with the same, very large, momentum, and that they interact with each other for a very short time when they pass at known positions.

Consider now an observer who gets hold of one of the particles, far away from the region of interaction, and measures its momentum; then, from the conditions of the experiment, he will obviously be able to deduce the momentum of the other particle. If, however, he chooses to measure the position of the first particle, he will be able to tell where the other particle is.

How can the final state of the second particle be influenced by a measurement performed on the first, after all physical interaction has ceased between them?"

He seems to make a lot of sense.

bhobba

Yes - but remember the hidden assumption - namely they actually have properties independent of measurement. If not you have strange correlations but influence is probably not the appropriate word.

Thanks
Bill