New experimental proof of wave-function collapse?

[bhobba]

As other have said spontaneous emission is not explained by normal QM - if its in an eigenstate of energy it should remain so. That's my whole point - one has to go to QED to explain it. My suspicion is that is the rock bottom reason for the randomness we see in things like the phase of photons in decoherence.

Thanks
Bill

 

[bhobba]

spontaneous emission looks indeed like a random interruption of a coherent time evolution (Rabi oscillation). But if you use a full quantum description like the Jaynes-Cummings model, the randomness in the time evolution goes away.

But is there any way to predict when it will spontaneously emit? If that's not the case then we know why photons have random phase.

Thanks
Bill

 

[kith]

But is there any way to predict when it will spontaneously emit? If that's not the case then we know why photons have random phase.

Maybe I didn't understand completely what the problem is, you think the vacuum fluctuations may solve.

Let me set the stage: when two separate quantum systems interact, we get entanglement and therefore decreased coherence if we look at one system only. Usually, the unitary time evolution of the combined system may lead to increasing entanglement / decreasing coherence in the subsystems, as well as decreasing entanglement / increasing coherence in the subsystems.

In measurements, we don't observe coherences between the possible final states of the system at all, which implies that the interaction with the measurement apparatus is such that the coherence is suppressed (a) strongly and (b) in a long-lasting manner. This can be referred to as approximate decoherence.

Is the problem now how to derive this approximate decoherence or do you want to show that the decoherence is more permanent and that there's no recoherence? Or is it something else?

 

[bhobba]

Is the problem now how to derive this approximate decoherence or do you want to show that the decoherence is more permanent and that there's no recoherence? Or is it something else?

Its to do with some decoherence models requiring a random environment eg:
http://quantum.phys.cmu.edu/CQT/chaps/cqt26.pdf

Now to me its a very obvious reasonable assumption that the phase of the photons (for example as in the above) is random and doesn't require any explanation simply due to the fact the number of disorderly phases is much much greater than orderly ones. I personally wouldn't even count it as a formal assumption - but that's just me - it is an assumption. Now Ruth who has been referred to in this thread thinks it needs explaining - in fact she believes this assumption really assumes what you are trying to show so its circular. I don't believe that - but that's her argument.

My view is there seems to be a natural randomness in photons from QFT due to spontaneous emission - any photon we observe likely has been randomly emitted by spontaneous emission eg:
http://www.famaf.unc.edu.ar/~vmarconi/moderna1/emision_estimulada_AJP.pdf

As the above points out the modern explanation is vacuum fluctuations of the quantum EM field that permeates all space. My understanding of QFT is not as good as I would like it but I do know something of it. The explanation of vacuum fluctuations I have seen is the Heisenberg uncertainty principle - you can't say it has a definite value for the same reason. My suspicion is this is the cause of the randomness. Its inherent and removes any possibility of circularity.

Thanks
Bill

 

[vanhees71]

Where is there a problem? Everyday matter provides such randomness. Just the cosmic microwave background radiation, which is the most accurate realization of black-body radiation ever achieved (literally in the universe ;-)), is sufficient to make objects like the moon behave classically FAPP.

For "quantum research/applications" the opposite is a problem, namely how to avoid decoherence over a sufficiently long time and keep quantum coherence stable long enough!

 

[bhobba]

Where is there a problem?

Your preaching to the converted. There is no problem. But Ruth will not be dissuaded. I am simply trying to come up with an argument with no holes that can be exploited. She believes even statistical mechanics has this circularity. First I have heard of it - the only issue I have read about is actually proving the ergodic hypothesis.

Thanks
Bill

 

[kith]

Its to do with some decoherence models requiring a random environment eg:
http://quantum.phys.cmu.edu/CQT/chaps/cqt26.pdf

Now to me its a very obvious reasonable assumption that the phase of the photons (for example as in the above) is random and doesn't require any explanation simply due to the fact the number of disorderly phases is much much greater than orderly ones. I personally wouldn't even count it as a formal assumption - but that's just me - it is an assumption. Now Ruth who has been referred to in this thread thinks it needs explaining - in fact she believes this assumption really assumes what you are trying to show so its circular. I don't believe that - but that's her argument.

I think it depends on what you want to show. Approximate decoherence can be explained by arguments along your lines (namely statistical reasoning which is very similar to Boltzmann's molecular chaos argument). But because of things like Poincarés recurrence theorem, many arguments in the foundations of QM get weakened if decoherence is only approximate. For example, the intuitive picture of splitting worlds whenever decoherence occurs makes less sense if you keep in mind that after a certain time span -which can be really big- recoherence and therefore the merging of worlds occurs.

If Ruth is talking about this, she is correct. If you ask yourself, "how can decoherence in a system be permanent?" and get the answer "because of the random phases of photons which interact with the system" the immediate follow-up question is "how can the phases of photons be permanently random?". After all, the randomness of phases is essentially equivalent to decoherence in the field. So the question about permanent decoherence has been shifted from the system to the field but not answered.

It is really a pattern in these discussions that people are talking past each others because of this.

 

[bhobba]

It is really a pattern in these discussions that people are talking past each others because of this.

Yes.

I have always said regarding this stuff more research is required.

Thanks
Bill

 

[kith]

I have always said regarding this stuff more research is required.

I don't think I agree with you here. To me, it looks like fundamental irreversibility is the key issue and I think this question has essentially been settled by statistical mechanics: there is no fundamental irreversibility. The world looks irreversible to us because it started in a special state and we are experiencing it in a coarse-grained way.

 

[atyy]

I don't think I agree with you here. To me, it looks like fundamental irreversibility is the key issue and I think this question has essentially been settled by statistical mechanics: there is no fundamental irreversibility. The world looks irreversible to us because it started in a special state and we are experiencing it in a coarse-grained way.

However, that is only true for classical statistical mechanics. If one uses quantum mechanics as the basis for statistical mechanics, it is less clear (unless one is not using Copenhagen, but maybe some version of BM).

 

[kith]

However, that is only true for classical statistical mechanics. If one uses quantum mechanics as the basis for statistical mechanics, it is less clear (unless one is not using Copenhagen, but maybe some version of BM).

I think Copenhagen fits in because it is about people doing science.

 

[atyy]

I think Copenhagen fits in because it is about people doing science.

What I mean is that in classical statistical mechanics, irreversibility is not fundamental, because we take Newton's laws as fundamental and statistical mechanics and thermodynamics as emergent. However, in quantum mechanics, in Copenhagen, we need an observer to decide when an irreversible macroscopic mark has occurred. Since the observer is fundamental, irreversibility is fundamental.

 

[kith]

What I mean is that in classical statistical mechanics, irreversibility is not fundamental, because we take Newton's laws as fundamental and statistical mechanics and thermodynamics as emergent. However, in quantum mechanics, in Copenhagen, we need an observer to decide when an irreversible macroscopic mark has occurred. Since the observer is fundamental, irreversibility is fundamental.

The "irreversible macroscopic mark" is left in a classical system, so I don't think it is fundamentally irreversible. Sure, Copenhagen includes measurements as key elements but whether a measurement has taken place is a matter of practice and not of principle. As you say, it is a decision which the scientist makes. I think it is misleading to call this "fundamental irreversibility".

 

[TrickyDicky]

To me, it looks like fundamental irreversibility is the key issue and I think this question has essentially been settled by statistical mechanics: there is no fundamental irreversibility. The world looks irreversible to us because it started in a special state and we are experiencing it in a coarse-grained way.

Isn't this arguing against your #77? Since when is irreversibility not fundamental? Last I checked the second law was still alive and well, both in cosmology/GR and QFT. This has never been settled by statistical mechanics to my knowledge. A special initial state IS a way to define irreversibility as fundamental.

 

[atyy]

The "irreversible macroscopic mark" is left in a classical system, so I don't think it is fundamentally irreversible. Sure, Copenhagen includes measurements as key elements but whether a measurement has taken place is a matter of practice and not of principle. As you say, it is a decision which the scientist makes. I think it is misleading to call this "fundamental irreversibility".

But the classical world in Copenhagen is not fully lawed - in particular, it is not fully lawed by Newton's laws or classical relativity, which are falsified by quantum mechanics. So in Copenhagen the decision a scientist makes is fundamental. For the observer to be not fundamental, one needs an interpretation in which the observer is not fundamental, ie. BM or MWI.

 

[Derek Potter]

Yes, that's among the papers I know about. I have tried to read almost all your papers with great interest! I guess I'm not enough of an expert to evaluate its correctness by myself, and I don't know if there is consensus about whether it really works, at least not the way Bohmian Mechanics for non-relativistic quantum mechanics has been examined for all sorts of tricky situations, and really does seem to work. Would it be fair to say that this is still pretty much at the frontier of research, rather than textbook knowledge? I have the same reservations about MWI - is it really an alternative interpretation to Copenhagen - or is it still an approach that it is unclear whether all the problems have really been worked out?

So would it be fair to say that at the consensus level - eg., what one can teach to undergraduates - Copenhagen is still the only interpretation of quantum mechanics?

(Consistent histories, maybe - but it essentially has collapse and all the same problems as Copenhagen, just declared not to be problems)

Well I understand "elegance" and I understand "mathematical consistency" but this is the first time I've come across "being able to teach it to undergraduates" as a criterion for accepting an interpretation :)

Consistent histories can, I think, be formulated without collapse. it then becomes a many histories theory, which only needs a small dash on ontology to turn it into a Tegmarkian world.

 

[atyy]

Well I understand "elegance" and I understand "mathematical consistency" but this is the first time I've come across "being able to teach it to undergraduates" as a criterion for accepting an interpretation :)

Consistent histories can, I think, be formulated without collapse. it then becomes a many histories theory, which only needs a small dash on ontology to turn it into a Tegmarkian world.

No, what I said was that being unquestionably right was a criterion for teaching it to undergraduates.

 

[kith]

But the classical world in Copenhagen is not fully lawed - in particular, it is not fully lawed by Newton's laws or classical relativity, which are falsified by quantum mechanics. So in Copenhagen the decision a scientist makes is fundamental. For the observer to be not fundamental, one needs an interpretation in which the observer is not fundamental, ie. BM or MWI.

The decision a certain observer makes is not fundamental because another observer can make a different decision. If different observers disagree whether a measurement has been performed, they also disagree about whether a process is irreversible. So what Copenhagen needs is the observer and his subjective notion of irreversibility, which is the second kind in my post #79.

 

[atyy]

The decision a certain observer makes is not fundamental because another observer can make a different decision. If different observers disagree whether a measurement has been performed, they also disagree about whether a process is irreversible. So what Copenhagen needs is the observer and his subjective notion of irreversibility, which is the second kind in my post #79.

Yes, I agree. But there is no fundamental reversibility either.

 

[kith]

Yes, I agree. But there is no fundamental reversibility either.

Yes, in the sense that Copenhagen doesn't try to remove the observer and his subjective notions.

 

[vanhees71]

What is unclear concerning quantum statistics? The H theorem is most naturally derived from detailed balance which follows from the unitarity of the S matrix, i.e., the (generalized) optical theorem, which is at the heart of quantum-many body theory. Ironically, it's much harder to do classical than quantum statistical physics. Even if you try to do everything in terms of classical theory, you need to introduce quantum ideas to make everything clear. Although thermodynamics and statistical physics survived the quantum revolution best, many "clouds on the horizon of classical physics" were solved by the discovery of quantum physics and triggered its development. One must not forget that quantum theory started with Planck's solution of the black-body radiation problem, a typical statistical-physics problem, and Einstein's idea about "wave-particle duality" (although obsolete now) came from his analysis of this solution.

 

[kith]

What is unclear concerning quantum statistics?

There isn't something unclear about what you probably would call the physics. The last couple of posts were just concerned with how the Copenhagen interpretation fits in with what I wrote in post #79. This could be a starting point for another fundamental discussion about interpretations but I don't want to lead such a discussion right now.

 

[Demystifier]

The H theorem is most naturally derived from detailed balance which follows from the unitarity of the S matrix, i.e., the (generalized) optical theorem, which is at the heart of quantum-many body theory.

That sounds like the Weinberg's proof of the H-theorem, in
S. Weinberg, The Quantum Theory of Fields vol I, Sec. 3.6, pages 150-151.
Can you explain why at the left hand side of Eq. (3.6.19) we have dt and not d(-t)? The sign of t should not matter in a T-invariant theory. On the other hand, with d(-t) in Eq. (3.6.19) we would eventually "derive" that entropy decreases with time, contrary to what we wanted to obtain.

My point is, you cannot really derive the H-theorem without assuming some form on time asymmetry from the beginning.

 

[Derek Potter]

No, what I said was that being unquestionably right was a criterion for teaching it to undergraduates.

Well, it was a light-hearted comment but if you want to be serious about it, perhaps you can say which version of CI you regard as unquestionably right. CI tends to be an umbrella for all sorts of interpretations including Heisenberg fuzziness which inspired Schrodinger's Cat. However as far as I know, CI always has some sort of randomness built into it, whether as a projection postulate or a slightly simpler wavefunction collapse. MW manages without any such thing. So it would seem that CI actually has redundant hypotheses making it pretty unlikely to be right at all, let alone unquestionably so.

 

[vanhees71]

That sounds like the Weinberg's proof of the H-theorem, in
S. Weinberg, The Quantum Theory of Fields vol I, Sec. 3.6, pages 150-151.
Can you explain why at the left hand side of Eq. (3.6.19) we have dt and not d(-t)? The sign of t should not matter in a T-invariant theory. On the other hand, with d(-t) in Eq. (3.6.19) we would eventually "derive" that entropy decreases with time, contrary to what we wanted to obtain.

My point is, you cannot really derive the H-theorem without assuming some form on time asymmetry from the beginning.

This is precisely the one and only correct proof of the detailed-balance relation for the most general case. You don't need time-reversal or parity invariance at all. I also don't understand your question concerning dt vs d(-t), because there's no time integral involved in (3.6.19). You just integrate (3.6.19) over ##\mathrm{d} \alpha##. Then both integrals are equal, and thus ##\int \mathrm{d} \alpha P_{\alpha}## time-independent.

However, your final statement is correct: Of course in deriving transition-matrix elements you assume a directnesses of time, the socalled "causality time arrow". The H theorem just proves that this "causality time arrow" is the same as the "thermodynamical time arrow", defined as the direction of time, where entropy doesn't decrease.

Of course, a system in thermal equilibrium doesn't admit the determination of any time direction, because it forgot any history. In other words, if you make a movie from a equilibrium system, you cannot tell whether you show it forward or backward running, as long as you look at the macroscopic state only, i.e., averaged/coarsegrained quantities.

 

[LaserMind]

Note that EPR, Bell's inequality and entanglement don't demonstrate "nonlocality" (though this is the common word for it) so much as it confirms the initial "superposition of states" as predicted by quantum mechanics. In other words, the initial state of the photons are not polarized in a particular direction, the initial spin of the fermions are not in some specific x-y-z direction. The "nonlocalitiy" has to do with those states being 100% correlated antisymmetrically, as required by standard quantum mechanics.

Like others in this thread, I'm not seeing anything that looks like "proof of wave function collapse". It's called "proof of existing quantum theory." There is an unfortunate tendency in physics to conceive of the math as being the reality. The math is the description of the reality, the quantitative language we use to communicate about the reality, subject to experimental verification.

Or to use an analogy from the Matrix, the quote of "There is no spoon." There is no wavefunction. There are phenomena that we measure that are described by math we call "wavefunctions", which aptly predict our measurements. The notion that you can "prove" that a mathematical construct has objective material behavior (collapsing or otherwise) is absurd.

 

[LaserMind]

The wave function collapse is a value exchange - a numerical event!

 

[Dale]

The OP is long gone and now the discussion is just going in circles. Thread closed.