I The Extended Wigner's Friend Scenarios

[gentzen]

I hope the rest of what I said is now clearer too. If not, please ask what specifically isn't clear.

It is unclear to me why an example with a single electron should convince me that decoherence can be reversed in Bohmian mechanics.

 

[Demystifier]

It is unclear to me why an example with a single electron should convince me that decoherence can be reversed in Bohmian mechanics.

Because decoherence refers to the wave function (not to Bohmian particle positions and velocities), which in Bohmian mechanics is the same as in other interpretations. Hence, decoherence can be reversed in Bohmian mechanics if and only if it can be reversed in standard QM.

 

[gentzen]

Because decoherence refers to the wave function (not to Bohmian particle positions and velocities), which in Bohmian mechanics is the same as in other interpretations. Hence, decoherence can be reversed in Bohmian mechanics if and only if it can be reversed in standard QM.

Let me remind you that this is a discussion about AoE and it relation to decoherence:

Yes, but that assumption requires that decoherence cannot be reversed, because decoherence is (part of) what makes the observation an "objective fact" that all observers must agree on.

If decoherence can be reversed, then it is impossible for AoE to be true.

Since AoE is true FAPP, decoherence cannot be reversed in standard QM, FAPP. And depending on how you expand AOE, it may simply be true, not just true FAPP:

The specific LF assumption that the theorists hone in on is what they call Absoluteness of Events (AoE). This is the idea that once something has been measured, it becomes an absolute fact about the state of the objective universe.

That's not what "Absoluteness of Observed Events" (AOE) means.

In addition, you seem to claim that AoE would remain true in Bohmian mechanics, even if decoherence could be reversed. Here I am unsure. This seem to depend on how you interpret the de Broglie-Bohm theory. Or maybe AoE is true in that theory, indepedent of how you interpret it. I am currently not convinced, but open for good arguments.

 

[Sambuco]

It certainly was not obvious to me. Overall, I found PeterDonis' arguments slightly more convincing than yours, but was still unsure.

I'm not entirely sure what the objection is. If it is about the idea that decoherence can be reversed (at least in principle), that is something that is assumed in all the papers we are discussing about Wigner's friend thought experiments. The core idea behind these kind of thought experiments is that Wigner has complete control of his friend's lab, including the system and the friend, in the same way that the experimenter has complete control over what happens to an electron (to its wave function, of course) during a Stern-Gerlach experiment.

Lucas.

 

[gentzen]

I'm not entirely sure what the objection is. If it is about the idea that decoherence can be reversed (at least in principle), that is something that is assumed in all the papers we are discussing about Wigner's friend thought experiments.

Neither Schrödinger, nor Wigner made that assumption. For both, sufficient personal testimony is available to known which points they wanted to make with their thought experiments.

Some later papers seem to make such assumptions. But neither do those papers succeed in justifying those assumptions, nor are their conclusions generally accepted.

I'm on record since a long time that you must justify the possibility of such a reversal (and that macroscopic superpositions might not be unplausible in some situations where such a reversal is possible):
20 Mar 2017 TK: historical defence of Copenhagen

But I disagree that the quoted passage is technical. If he adheres to this passage, then Heisenberg cannot claim that Schrödinger’s cat would be both alive and dead, or that the moon would not be there if nobody watches.

Others, like Christopher A. Fuchs and Asher Peres in "Quantum Theory Needs No Interpretation", are apparently less sure whether (neo-Copenhagen) quantum theory is so clear about that fact. Hence they try to weasel out by claiming: “If Erwin has performed no observation, then there is no reason he cannot reverse Cathy’s digestion and memories. Of course, for that he would need complete control of all the microscopic degrees of freedom of Cathy and her laboratory, but that is a practical problem, not a fundamental one.”

This is non-sense, because the description of the experiment given previously was complete enough to rule out any possibility for Erwin to reverse the situation. Note the relevance of “… a consistent interpretation of QM as applied to what we do in a physical laboratory and how practitioners experience QM in that context.” If Erwin had access to a time machine enabling him to realistically reverse the situation, then it might turn out that Cathy and Erwin indeed lived multiple times through both situations (and experienced real macroscopic superpositions), as depicted in movies like “Back to the Future”.

 

[Sambuco]

Since AoE is true FAPP, decoherence cannot be reversed in standard QM

When you say this, you are referring to the following post

If decoherence can be reversed, then it is impossible for AoE to be true.

As I explained in post #17 and #18, that's not true, because the term AOE does not have the meaning Peter was giving it.

In addition, you seem to claim that AoE would remain true in Bohmian mechanics, even if decoherence could be reversed. Here I am unsure. This seem to depend on how you interpret the de Broglie-Bohm theory. Or maybe AoE is true in that theory, indepedent of how you interpret it. I am currently not convinced, but open for good arguments.

The fact that AOE is true in Bohmian mechanics is mentioned in the original paper by Cavalcanti's group (https://www.nature.com/articles/s41567-020-0990-x). They explicitly say: "Bohmian mechanics violates L but not the other assumptions." The other assumptions are AOE and no-superdeterminism.

We have to take into account that this kind of Extended Wigner's Friend Scenario (EWFS) assumes that quantum mechanics is "universal," meaning that any observer can model their system under observation as a quantum mechanical system, regardless of its composition. This is, let's say, the zeroth assumption.

Lucas.

 

[Demystifier]

Let me remind you that this is a discussion about AoE and it relation to decoherence:

Since AoE is true FAPP, decoherence cannot be reversed in standard QM, FAPP. And depending on how you expand AOE, it may simply be true, not just true FAPP:



In addition, you seem to claim that AoE would remain true in Bohmian mechanics, even if decoherence could be reversed. Here I am unsure. This seem to depend on how you interpret the de Broglie-Bohm theory. Or maybe AoE is true in that theory, indepedent of how you interpret it. I am currently not convinced, but open for good arguments.

I suspect we have different understandings of what AoE means. Can you explain it on some examples unrelated to quantum physics?

 

[gentzen]

I suspect we have different understandings of what AoE means. Can you explain it on some examples unrelated to quantum physics?

Let me repeat my old example cited in my reply to Sambuco:

If Erwin had access to a time machine enabling him to realistically reverse the situation, then it might turn out that Cathy and Erwin indeed lived multiple times through both situations (and experienced real macroscopic superpositions), as depicted in movies like “Back to the Future”.

If you have a time machine, then "observed events" are not necessarily "absolute". They may have happened for one person, but nevertheless not have happened for another person, even if those two persons should happen to talk with each other.

 

[PeterDonis]

In other interpretations of QM the results of all experiments are random, not determined by anything.

No, that's not correct. It's correct for collapse interpretations, but not for, e.g., the MWI.

By not controlling and not reversing Bohmian hidden variables, you effectively make the results of all experiments random

And since in actual fact, we can't control the Bohmian hidden variables, the actual results of quantum experiments are random in Bohmian mechanics. The only difference is that in Bohmian mechanics, unlike collapse interpretations, the randomness has the standard ignorance interpretation: we don't know the exact initial conditions. The underlying dynamics is fully deterministic. But since we can never know the exact initial conditions, the fact that the underlying dynamics is fully deterministic doesn't help us at all if we want to do things like run a Wigner's friend experiment with a human friend. We still can't control the Bohmian hidden variables.

 

[PeterDonis]

Clearly, this evolution can be reversed by reversing the sign of $B$.

Not just that, no. Pick a time when the electron has just emerged from your apparatus, and reverse the sign of $B$. Does the electron go back through, reversing its previous path? Of course not.

What will be the case is that if you reverse the sign of $B$ for the next electron, it will go the opposite way. But to call that "reversing the evolution" seems to me to be a gross misuse of language.

 

[Sambuco]

If Erwin had access to a time machine enabling him to realistically reverse the situation, then it might turn out that Cathy and Erwin indeed lived multiple times through both situations (and experienced real macroscopic superpositions), as depicted in movies like “Back to the Future”.

If I understood the example correctly, Erwin plays the role of Wigner, right? In that case, Wigner is on the classical side of the Heisenberg cut, so any reversal of decoherence does not affect him (at least in principle).

(and that macroscopic superpositions might not be unplausible in some situations where such a reversal is possible

That's a good point! I'm not aware of any publications on this issue.

If you have a time machine, then "observed events" are not necessarily "absolute". They may have happened for one person, but nevertheless not have happened for another person, even if those two persons should happen to talk with each other.

But that's not the AOE assumption in local friendliness theorem. AOE means that a probability can be assigned to the ocurrence of an experiment by Wigner's friend (see eq. (3) in the Methods section in Cavalcanti's paper). In other words, if AOE is false, it means that Wigner cannot assume that an event occurred inside his friend's lab.

Lucas.

 

[Demystifier]

Not just that, no. Pick a time when the electron has just emerged from your apparatus, and reverse the sign of $B$. Does the electron go back through, reversing its previous path? Of course not.

What do you mean by electron path? It's not even defined, except in the Bohmian interpretation. Since the thought experiment is defined in terms which do not depend on the choice of interpretation, the reversing can only refer to the wave function, not to the electron path. The Bohmian interpretation comes only afterwards; if the wave function is manipulated in such and such way, the Bohmian interpretation can say what happens with the electron path. And you are right, the path will not be reversed even if the wave function will. But that doesn't mean that the experiment cannot be performed. It only means that the experiment is devised so that only the wave function is reversed, nothing more and nothing less.

 

[PeterDonis]

What do you mean by electron path?

I didn't use the word "path", nor does my argument depend on the electron having a particular path. My point was simply that there is a time when the electron has left the region of space where the magnetic field is present--and reversing the sign of $B$ does not magically cause the electron to go back through the field after it has already emerged. So reversing the sign of $B$, by itself, does not reverse the evolution. Something else also has to be reversed. What that something is, will depend on which interpretation of QM you adopt; but there will always have to be something.

the reversing can only refer to the wave function, not to the electron path

No, you didn't say the wave function was reversed. You said the sign of $B$ was reversed. My point is that that alone is not enough to reverse the evolution. Indeed, "reverse the evolution" implies that you are somehow reversing the entire experiment--starting with the end state of the original experiment, and arranging things so the system returns to the initial state of the original experiment. Reversing the sign of $B$ in itself certainly doesn't do that.