QFT made Bohmian mechanics a non-starter: missed opportunities?

[Demystifier]

Well, the question is, where the collapse conjecture is ever used in the application of QM. Usually you have a system somehow prepared (particle beams in an accelerator) and then the outcomes are measured (particle detectors of different kinds) and then analyzed using statistical methods. That's all that's needed and that's indeed the typical "standard" procedure.

In most applications, you are right. But in some applications you measure the same system more than once, at different times. In such applications, the collapse rule is used in a practical sense.

 

[Demystifier]

I don't think that this is really a problem.

Ballentine on the quantum Zeno effect, in his book:
"... we have been led to the conclusion that a continuously observed system never changes its state! This conclusion is, of course, false."

 

[gentzen]

Ballentine on the quantum Zeno effect, in his book:
"... we have been led to the conclusion that a continuously observed system never changes its state! This conclusion is, of course, false."

Oh, so you want me to review Ballentine's "analysis" of the quantum Zeno effect, and either confirm that you are right and he made a "mistake" here, or else defend his "analysis"? But even if Ballentine should have made a mistake here, that doesn't disprove his statistical interpretation. My guess would be that Ballentine is indeed wrong here, because of his telltale remark at the end of that subsection:

It is sometimes claimed that the rival interpretations of quantum mechanics differ only in philosophy, and cannot be experimentally distinguished. That claim is not always true, as this example proves.

In this case, I should better review what other people have written about his specific opinion on the quantum Zeno effect. I guess the following thread should be a good starting point for me:
https://www.physicsforums.com/threads/errors-in-ballentine-qm-textbook.998385/

 

[vanhees71]

In most applications, you are right. But in some applications you measure the same system more than once, at different times. In such applications, the collapse rule is used in a practical sense.

But this should then be also describable by quantum dynamics. It's also rare that the collapse assumption really holds, even in a FAPP sense.

 

[vanhees71]

Ballentine on the quantum Zeno effect, in his book:
"... we have been led to the conclusion that a continuously observed system never changes its state! This conclusion is, of course, false."

And what do you think is wrong with this?

 

[Morbert]

Re/ the QZE: An ideal physicist would write down a suitable model Hamiltonian for the unstable atom + detector + EM field + environment, and then compute the relevant decay rates/probabilities for some time of interest. This "shut up and calculate" procedure isn't contingent on the acceptance or rejection of any interpretation, statistical, minimalist, or otherwise.

What makes QZE interesting is the challenge of shutting up and calculating: the construction of the model Hamiltonian and the computation of decay rates etc. From https://arxiv.org/pdf/quant-ph/0307075.pdf:

The latter condition explains why the QZE was not obtained for exponentially decaying systems by theories based on the projection hypothesis. By applying the projection operator, the quantum coherences between |x, 0〉 and |g, k〉’s are destroyed regardlessly of the energy of the emitted photon. Therefore, the projection-based theory corresponds to ∆ → ∞ [20]. In such a limit, however, inequality (10) cannot be satisfied and the QZE never occurs. Since ∆ of any real detector is finite, such a limit is rather unphysical.

Naively projecting here and there is not a feature of an interpretation. It's a feature of poor application of a theory.

 

[PeterDonis]

The purpose of interpretation is not the truth, the purpose is intuition. Intuition is a thinking tool.

I agree with this, but I'm not sure all physicists who are proponents of particular interpretations do. Many of them appear to think that their preferred interpretations are "true", not just useful thinking tools.

 

[Demystifier]

I agree with this, but I'm not sure all physicists who are proponents of particular interpretations do. Many of them appear to think that their preferred interpretations are "true", not just useful thinking tools.

Yes, and I believe I can explain why is that. At the beginning, when one hears about a particular interpretation for the first time, nobody thinks it's "true". At best, it looks like "OK, maybe there is some truth in it". But with this state of mind, the tool is not yet fully efficient, one must create a stronger bond with it to exploit its full potential. For many people, the thinking tool may work the best when one imagines that it is "really true". But when one comes to that psychological state of imagination, the problem is then to later step out from this state of mind. So after a while the imagination may start to look like reality. The map may start to look like the territory.

My own way to cope with this is changing the roles. For example, when I speak with very strong proponents of the Bohmian interpretation, I often present them the counterarguments, some of which I've learned in this forum.

 

[Demystifier]

And what do you think is wrong with this?

Let me put it this way. Do you know a reference where the real quantum Zeno effect (which is a measured effect) is explained theoretically without reference to any kind of collapse? I would really like to see how one can avoid it, in actual calculation.

 

[vanhees71]

Can you refer to a reference, where "the real quantum Zeno effect" is demonstrated? I could imagine it's some atom in a cavity with a laser kicking it somehow to stay (maybe in principle for ever) in an excited/metastable state. I'm not sure, whether one can treat the full quantum-time evolution of this exactly.

The main objection against the hand-waving arguments using 1st-order perturbation theory for the transition rate for "infinitesimal times" and then treat the process as if it were Markovian is at least misleading.

The more we think about open quantum systems in our research work the more we come to the conclusion that Markovian approximations are making more trouble than good. Particularly it's not clear, how to ensure proper thermalization in the long-time limit, but that's a different topic.

 

[Demystifier]

Can you refer to a reference, where "the real quantum Zeno effect" is demonstrated?

https://arxiv.org/abs/quant-ph/0104035

 

[martinbn]

Let me put it this way. Do you know a reference where the real quantum Zeno effect (which is a measured effect) is explained theoretically without reference to any kind of collapse? I would really like to see how one can avoid it, in actual calculation.

Does collapse make sense in an ensemble interpretation?

 

[Demystifier]

Does collapse make sense in an ensemble interpretation?

It depends on who you ask. In my opinion, yes.
https://www.physicsforums.com/threa...on-is-neither-minimal-nor-statistical.998661/

 

[martinbn]

It depends on who you ask. In my opinion, yes.
https://www.physicsforums.com/threa...on-is-neither-minimal-nor-statistical.998661/

Then what is collapse in an ensemble interprwtatoon?

 

[vanhees71]

Does collapse make sense in an ensemble interpretation?

Collapse doesn't make sense to me, because it claims that there is dynamics not described by QT and then you ad hoc assume some magic that violates relativistic causality. I have no clue, what the collapse postulate is needed for.

 

[martinbn]

Collapse doesn't make sense to me, because it claims that there is dynamics not described by QT and then you ad hoc assume some magic that violates relativistic causality. I have no clue, what the collapse postulate is needed for.

That is not what i am asking. In a non ensemble interpretation it is clear what collapse is, whether it is needed or not is a separate question. But my question is what is it in the ensemble case?

 

[Demystifier]

Then what is collapse in an ensemble interprwtatoon?

Information updwatee.

 

[vanhees71]

But your information update should relate to what you look at. You cannot say that whenever you measure an observable the correct information update is that then the system is in an eigenstate of the measured observable with the eigenvalue given by the measured value. It depends on, how the system interacts with the measurement device, how to describe its state after the measurement. Sometimes the very object you were measuring is destroyed in the measurement process (like a photon hitting a photoplate). What sense then does the collapse postulate make?

 

[martinbn]

Information updwatee.

No, i mean how is it formulated if we have ensembles and the wave function describes them, not the idividual system?

 

[Demystifier]

But your information update should relate to what you look at. You cannot say that whenever you measure an observable the correct information update is that then the system is in an eigenstate of the measured observable with the eigenvalue given by the measured value. It depends on, how the system interacts with the measurement device, how to describe its state after the measurement. Sometimes the very object you were measuring is destroyed in the measurement process (like a photon hitting a photoplate). What sense then does the collapse postulate make?

https://www.physicsforums.com/threads/difference-between-collapse-and-projection.998545/post-6445809

 

[Morbert]

That is not what i am asking. In a non ensemble interpretation it is clear what collapse is, whether it is needed or not is a separate question. But my question is what is it in the ensemble case?

If you have some experiment involving a sequence of measurements, then an example of a "collapse" would be a partitioning of the ensemble of experimental runs into subensembles in accordance with the outcome of one of the measurements.

 

[vanhees71]

What has this to do with the collapse postulate?

 

[Demystifier]

No, i mean how is it formulated if we have ensembles and the wave function describes them, not the idividual system?

Then the collapsed wave function describes a subensemble, in which all members show the same measurement outcome.

 

[martinbn]

Then the collapsed wave function describes a subensemble, in which all members show the same measurement outcome.

How is it a subensemble? In any case it is a different ensemble, so what is collapse exactly?

 

[Demystifier]

What has this to do with the collapse postulate?

If you ask me, the last formula describes a generalized collapse rule for general POVM measurements.

 

[Demystifier]

How is it a subensemble? In any case it is a different ensemble, so what is collapse exactly?

Suppose that you prepare 100 particles, each in the state
$$|\psi\rangle =|+\rangle +|-\rangle$$
This means that you have an ensemble of 100 particles in the state ##|\psi\rangle##. Now suppose that you perform a measurement in the ##|\pm\rangle## basis and find that 48 particles (out of 100) are in the state ##|+\rangle##. This means that these 48 particles constitute a subensemble (it's a subensemble because it does not refer to all 100 of them), each in the collapsed state
$$|\psi'\rangle = \pi_+ |\psi\rangle = |+\rangle$$
where ##\pi_+=|+\rangle\langle+|## is the projector.

 

[vanhees71]

I thought POVM descriptions give probabilities as also do the usual idealized von Neumann measurements, which are a special case of a POVM. I still don't understand what either kind of measurements should have to do with the general collapse postulate, which claims (for a von Neuman measurement) that, when you measure an observable ##\hat{A}## with the outcome ##a## (eigenvalue of ##\hat{A}##), of a system prepared in the state ##\hat{\rho}## then after the measurement you have to describe the system as being prepared in the state
$$\hat{\rho}'=\frac{\hat{P}_a \hat{\rho} \hat{P}_a}{\mathrm{Tr} \hat{P}_a \hat{\rho} \hat{P}_a}$$
with ##\hat{P}_a## the projector to the eigenspace ##\text{Eig}(\hat{A},a)##.
Some similar "collapse postulate" can for sure also be formulated for general POVMS.

In either case, I don't understand, why should this be true for all measurements? For sure it's not true for the drastic example of a photon being absorbed in the measurement process.

 

[Demystifier]

In either case, I don't understand, why should this be true for all measurements? For sure it's not true for the drastic example of a photon being absorbed in the measurement process.

All measurements are POVM measurements. In the linked post above, I explained how it works for photon absorption. You even liked it back then, and it looked as if you understood it.

 

[martinbn]

Suppose that you prepare 100 particles, each in the state
$$|\psi\rangle =|+\rangle +|-\rangle$$

No, absolutely not. If you say each in the state, that is not an ensemble interpretation!!!

This means that you have an ensemble of 100 particles in the state ##|\psi\rangle##.

No, I have 100 representatives of that ensemble.

Now suppose that you perform a measurement in the ##|\pm\rangle## basis and find that 48 particles (out of 100) are in the state ##|+\rangle##.

Again if the particles are in the state, you are not talking about ensemble interpretation.

This means that these 48 particles constitute a subensemble, each in the collapsed state
$$|\psi'\rangle = \pi_+ |\psi\rangle = |+\rangle$$
where ##\pi_+=|+\rangle\langle+|## is the projector.

I have no problem understang collaps in non ensemble interpretations. I want to know if it.makes sense in the ensemble interpretations.

 

[vanhees71]

But there you explained it as a transition matrix, which is quantum dynamics and not collapse, and that's why I liked it. Maybe I've misunderstood you then, because now you claim it's a collapse, i.e., some "update rule" outside of quantum dynamics.

 

[Demystifier]

But there you explained it as a transition matrix, which is quantum dynamics and not collapse, and that's why I liked it. Maybe I've misunderstood you then, because now you claim it's a collapse, i.e., some "update rule" outside of quantum dynamics.

Yes, you misunderstood me. It's a collapse (though not a projective collapse), it's not quantum dynamics.

 

[Lord Jestocost]

Freeman Dyson in “THE COLLAPSE OF THE WAVE FUNCTION” in John Brockman’s book “This Idea Must Die: Scientific Theories That Are Blocking Progress (Edge Question Series)” (New York, NY, USA: HarperCollins (2015)):

“Fourscore and eight years ago, Erwin Schrödinger invented wave functions as a way to describe the behavior of atoms and other small objects. According to the rules of quantum mechanics, the motions of objects are unpredictable. The wave function tells us only the probabilities of the possible motions. When an object is observed, the observer sees where it is, and the uncertainty of the motion disappears. Knowledge removes
uncertainty. There is no mystery here.

Unfortunately, people writing about quantum mechanics often use the phrase “collapse of the wave function” to describe what happens when an object is observed. This phrase gives a misleading idea that the wave function itself is a physical object. A physical object can collapse when it bumps into an obstacle. But a wave function cannot be a physical object. A wave function is a description of a probability, and a probability is a statement of ignorance. Ignorance is not a physical object, and neither is a wave function. When new knowledge displaces ignorance, the wave function does not collapse; it merely becomes irrelevant.”

 

[Demystifier]

No, absolutely not. If you say each in the state, that is not an ensemble interpretation!!!

No, I have 100 representatives of that ensemble.

Again if the particles are in the state, you are not talking about ensemble interpretation.

I have no problem understang collaps in non ensemble interpretations. I want to know if it.makes sense in the ensemble interpretations.

Then I have no idea what do you mean by ensemble interpretation, so I cannot answer your question.

 

[vanhees71]

No, absolutely not. If you say each in the state, that is not an ensemble interpretation!!!

No, I have 100 representatives of that ensemble.

Again if the particles are in the state, you are not talking about ensemble interpretation.

I have no problem understang collaps in non ensemble interpretations. I want to know if it.makes sense in the ensemble interpretations.

Of course, you must be able to say (to some accuracy at least) that each single realization of the system is prepared in this state. Otherwise you cannot build the ensembles we are talking about to begin with. The probabilistic meaning refers to the outcome of measurements, the state itself refers to a preparation procedure.

E.g., if you want to do a double-slit experiment with, e.g., an electron, you must prepare the electron with a sufficiently accurate momentum running towards the double slit, i.e., then it's prepared in a state ##\hat{\rho}=|\psi_{\vec{P}} \rangle \langle \psi_{\vec{P}}|## with
$$|\psi_{\vec{P}} \rangle=\int_{\mathbb{R}^3} \mathrm{d}^3 \vec{p} g(\vec{p}-\vec{P}) |\vec{p} \rangle,$$
where ##g## is some normalized ##\mathrm{L}^2(\mathbb{R}^3)## function, sharply peaked around ##\vec{0}##.

Then you ask, where the electron will be detected behind the two slits, and of course QT gives you only probability-density distributions and not a determined spot, where the electron will be registered. To test this, you have to prepare an ensemble of equally prepared electrons, justifying that this ensemble is described by the function ##g##. This is realized (with sufficient accuracy) by the "electron source".

 

[martinbn]

Then I have no idea what do you mean by ensemble interpretation, so I cannot answer your question.

If you dont know what an ensemble interpretation is, then you shouldnt make statments about it.