A A Realization of a Basic Wigner's Friend Type Experiment

[DarMM]

To the person who emailed regarding this:

A follow up paper which may be of interest: https://arxiv.org/abs/1907.05607

It's quite a good paper and no surprise really, it's a more rigorous version of Brukner's argument. Obviously Bohmian Mechanics would violate locality and Many-Worlds and retrocausal views take place outside the framework of the theorem. So the only interesting case is Copenhagen style views which seem to be required to reject "objectivity of facts".

Indeed they do, but it's not in a very surprising or strong way. The theorem essentially says that we have Alice and Bob (the superobservers) and Charlie and Diana (the observers). We have the outcomes $c$ and $d$ from Charlie and Diana performing observations from an entangled pair. Then we have outcomes $a$ and $b$ from Alice and Bob performing superobservations upon the entire atomic structure of Charlie and Diana's labs.

The theorem is basically that $a,b,c,d$ do not occur in a common sample space. Or put another way after Charlie and Diana's measurements there is no "fact of the matter" for how Alice and Bob's measurements will pan out.

This is no surprise for Charlie and Diana's observations are only on the entangled system and don't necessarily imply values for superobservables concerning every single atom in their lab.

What's interesting is that the "Local Friendliness" correlations are strictly wider than Bell's. Meaning there are theories which are fundamentally random and with no hidden variables, but where observations do imply the values of superobservations. Performing a measurement on an atomic system does restrict quite strongly the outcomes of some measurements involving every atom in your lab. This theorem shows QM is not such a theory.

Of course one can just reject the existence of superobservers.

 

[kimbyd]

One thing that irked me about that paper is that in this discussion they made a play at talking about how this wasn't really testable because observers (i.e. humans) tend to have highly-collapsed wavefunctions, and that this could be worked around with "strong AI" that has negative ethical implications.

I think that's pretty absurd. The concept of a quantum observer does not in any way require any sort of intelligence. All that is required is an experimental apparatus that reports a result. So you don't need anything complicated at all: all you need is a microscopic apparatus that emits some signal if it encounters some state or another. Then the experimental apparatus itself is the "observer" and the person running the experiment is the "super-observer". Intelligence isn't at all required to report or record the result of a measurement.

 

[StevieTNZ]

One thing that irked me about that paper is that in this discussion they made a play at talking about how this wasn't really testable because observers (i.e. humans) tend to have highly-collapsed wavefunctions, and that this could be worked around with "strong AI" that has negative ethical implications.

I think that's pretty absurd. The concept of a quantum observer does not in any way require any sort of intelligence. All that is required is an experimental apparatus that reports a result. So you don't need anything complicated at all: all you need is a microscopic apparatus that emits some signal if it encounters some state or another. Then the experimental apparatus itself is the "observer" and the person running the experiment is the "super-observer". Intelligence isn't at all required to report or record the result of a measurement.

That's one interpretation. One interpretation is consciousness causes wave function collapse - as far as I'm aware that hasn't been experimentally refuted.

 

[charters]

I think that's pretty absurd. The concept of a quantum observer does not in any way require any sort of intelligence

The ultimate issue here is quantum time evolution is unitary but collapse is non-unitary. This means that in an experiment with multiple putative observers acting in sequence, such as Wigner's Friend, which measuring system you arbitrarily call "observer" changes experimental predictions/statistics. On its face, this is bad news for any measurement induced collapse interpretation. But, if only one such measuring system is a conscious being, there's an easy way to resolve the tension - simply say that only the conscious being is really a collapsing observer. The reason to introduce multiple conscious beings is to close this loophole.

But note as well that if the Friend is not a person, just a machine, if you model this as a collapsing measurement, then the superobserver isn't able to use unitary quantum theory to make general predictions (i.e., the superobserver cannot unitarily restore the state of closed off lab system), undermining the universality of QT.

 

[charters]

So the only interesting case is Copenhagen style views which seem to be required to reject "objectivity of facts".

Indeed they do, but it's not in a very surprising or strong way

If Charlie and Diana believe themselves to belong to a special class of classical observers, who are not described by a quantum state (only a classical one) and can effect non-unitary changes on quantum states, they would be pretty surprised by the idea that Alice and Bob can act on them/their lab with unitary quantum operations. There's a pretty widespread account of Copenhagen that treats humans exactly this way.

 

[DarMM]

If Charlie and Diana believe themselves to belong to a special class of classical observers, who are not described by a quantum state (only a classical one) and can effect non-unitary changes on quantum states, they would be pretty surprised by the idea that Alice and Bob can act on them/their lab with unitary quantum operations. There's a pretty widespread account of Copenhagen that treats humans exactly this way.

Not all though. I've attempted a more detailed classification of Copenhagen views over in the QBism thread, but it's difficult to repeat something like this when one wants to use the shorthand "Copenhagen". There are forms of Copenhagen that reject superobservers like Peres. I assume that's what you are referring to.

 

[charters]

Not all though. I've attempted a more detailed classification of Copenhagen views over in the QBism thread, but it's difficult to repeat something like this when one wants to use the shorthand "Copenhagen". There are forms of Copenhagen that reject superobservers like Peres. I assume that's what you are referring to.

I am just saying that most people who've had some basic but not too deep exposure to QM and the idea of "collapse" would in fact be surprised to discover that a superobserver could in principle act on an internal observer unitarily, and put them in interfering superpositions, as if they were a quantum state. They would be surprised to learn that what one person sees as collapse, another sees as just entanglement - that the where and when of collapse is not an objective fact.

 

[kimbyd]

That's one interpretation. One interpretation is consciousness causes wave function collapse - as far as I'm aware that hasn't been experimentally refuted.

Pretty sure it's refuted by the measured fact that collapse measurably occurs for systems far less complex than humans, without results being ever observed by humans.

If some want to claim that that's not "real" collapse and "real" collapse only occurs due to consciousness, then that's a pretty extraordinary claim requiring extraordinary evidence.

 

[kimbyd]

For why I say that collapse has been experimentally measured to occur for less complex systems, here's an example:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.77.4887

They manually collapse the wavefunction by increasing the intensity of the light source which interacts with the effective dual-slit system they're using. The light is never observed, or even measured. But it causes the wavefunction to collapse nevertheless.

 

[charters]

For why I say that collapse has been experimentally measured to occur for less complex systems, here's an example:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.77.4887

They manually collapse the wavefunction by increasing the intensity of the light source which interacts with the effective dual-slit system they're using. The light is never observed, or even measured. But it causes the wavefunction to collapse nevertheless.

This only shows interference is increasingly suppressed as the atom-photon entanglement gets stronger. It does not tell us anything about if or when a nonunitary collapse occurs. Loss of interference (explained by decoherence) is not the same thing as collapse to a definite outcome/eigenstate (not explained by decoherence).

 

[DarMM]

I am just saying that most people who've had some basic but not too deep exposure to QM

Ah yes, agreed. Funny in a sense since even Bohr wouldn't have held to this sort of "absolute collapse at all scales".

Chapter 11 of Richard Healey's book "The Quantum Revolution in Philosophy" has a nice account of how Copenhagen approaches Wigner's friend. Similar expositions can be found in the papers of Jeffrey Bub and this paper https://arxiv.org/abs/1901.11274

 

[kimbyd]

This only shows interference is increasingly suppressed as the atom-photon entanglement gets stronger. It does not tell us anything about if or when a nonunitary collapse occurs. Loss of interference (explained by decoherence) is not the same thing as collapse to a definite outcome/eigenstate (not explained by decoherence).

What it does say is that non-unitary collapse is fundamentally unmeasurable. Decoherence occurs at a very small scale, and it should be possible to construct an experiment which demonstrates the stated inequality on a small scale, and that the inequality disappears as you increase the complexity of the system being measured (as in this paper). Measuring that collapse without any measurement interpreted by a conscious observer disproves the notion that consciousness is required for collapse.

Sure, you can imagine all you like that there's a second collapse that happens sometime later after consciousness gets involved, but then you're engaging in speculation that not only has no evidence behind it, but the problem itself is defined in such a way that it is fundamentally impossible to obtain any supporting evidence.

But yes, loss of interference is exactly collapse to a definite outcome. With the a two-slit experiment (or analog, in this case), the "definite outcome" is which slit the particle traveled through.

 

[charters]

But yes, loss of interference is exactly collapse to a definite outcome. With the a two-slit experiment (or analog, in this case), the "definite outcome" is which slit the particle traveled through.

This isn't right. Loss of interference (due to entanglement with an apparatus) yields the improper mixture of passing through one or the other slit, which is not a definite outcome state. Accounting for definite outcomes requires additional interpretive work.

This paper, especially section 1.2.3 is a very clear and concise explanation of this: http://philsci-archive.pitt.edu/5439/

 

[kimbyd]

This isn't right. Loss of interference (due to entanglement with an apparatus) yields the improper mixture of passing through one or the other slit, which is not a definite outcome state. Accounting for definite outcomes requires additional interpretive work.

This paper, especially section 1.2.3 is a very clear and concise explanation of this: http://philsci-archive.pitt.edu/5439/

I don't think that's a valid critique. Interference is caused by superposition, specifically superposition in a basis determined by the experimental apparatus. The elimination of interference implies the system is no longer oscillating (in space or time) in a coherent fashion, i.e. that its components are evolving largely independently of one another.

This section just lays out mathematically what kinds of superpositions interfere and which don't. The pure state is in a different basis from the state determined by the system that would observe interference. The other states, the ones that are not pure states, can be interpreted as the superposition being about our ignorance of the measurement result. In these systems, the particles are definitively in the $|\uparrow\rangle$ or $|\downarrow\rangle$ states before the measurement occurred, and the measurement only let's us know about a collapse that already happened, rather than causing a collapse as in the pure state.

The mathematics of this is quite confusing mostly because a "pure state" depends upon what basis you are talking about. Measurements to observe interference generally depend upon their particles being in a pure state in one basis, but having the experimental apparatus cause components of the wavefunction to evolve differently in time or space in another basis.

 

[charters]

In these systems, the particles are definitively in the |↑⟩|↑⟩|\uparrow\rangle or |downarrow⟩|downarrow⟩|downarrow\rangle states before the measurement occurred, and the measurement only let's us know about a collapse that already happened, rather than causing a collapse as in the pure state.

This is an objective collapse interpretation. It is not equivalent to standard quantum theory, as it denies the possibility (in principle) of unitary reversals despite prior interactions with/measurements by macroscopic devices or people. The purpose of Wigner's friend is to consider exactly quantum theories, which do allow such reversals, as discussed on pg 3 of the topic paper.

 

[kimbyd]

This is an objective collapse interpretation. It is not equivalent to standard quantum theory, as it denies the possibility (in principle) of unitary reversals despite prior interactions with/measurements by macroscopic devices or people. The purpose of Wigner's friend is to consider exactly quantum theories, which do allow such reversals, as discussed on pg 3 of the topic paper.

No, I'm just pointing out that the ontological interpretation of a wavefunction which no longer interferes due to interactions is that collapse has effectively occurred.

 

[charters]

No, I'm just pointing out that the ontological interpretation of a wavefunction which no longer interferes due to interactions is that collapse has effectively occurred.

Effectively, sure, or for practical purposes. But "effectively" is not a meaningful concept in Wigner's Friend discussions, which are predicated on unitarily reversing such interactions. Either the Friend's (whether a human, AI, or machine) measurement can be unitarily reversed, in which case Friend did not cause a collapse from Wigner's perspective. Or Friend can't be reversed, in which case Wigner can't rely on unitary QT. One must make a choice here.

 

[PeterDonis]

Interference is caused by superposition, specifically superposition in a basis determined by the experimental apparatus.

This doesn't seem right, since whether or not a state is a superposition is basis dependent, but interference is an observed phenomenon so it can't be basis dependent.

a "pure state" depends upon what basis you are talking about

This doesn't seem right either. Whether or not a state is pure is basis independent.

Ordinary language is often not suitable for such discussions; it might be helpful to write down explicit math for what you mean.

 

[StevieTNZ]

Pretty sure it's refuted by the measured fact that collapse measurably occurs for systems far less complex than humans, without results being ever observed by humans.

If some want to claim that that's not "real" collapse and "real" collapse only occurs due to consciousness, then that's a pretty extraordinary claim requiring extraordinary evidence.

news to me. measurement problem resolved then?

Perhaps re-read the paper I posted and see the various claims made in there that conflict with what you posted (and I've quoted in this post).

 

[charters]

Similar expositions can be found in the papers of Jeffrey Bub

By coincidence or superdeterminism, Bub just dropped a new paper on Wigner's friend tonight (which I think is wrong in its critique of Everett): https://arxiv.org/abs/1907.06240

 

[kimbyd]

This doesn't seem right, since whether or not a state is a superposition is basis dependent, but interference is an observed phenomenon so it can't be basis dependent.

You're right, my use of language is a little bit sloppy. The difficulty is in correlating the quantum mechanics notion of a "pure state" and the plain language of a "definite state".

After collapse, the measured state of a given quantum system doesn't change in the basis which was measured (note: the state will evolve with time, but won't behave as if it had any components that weren't the value measured).

So the question is: how do we represent a system where a measurement has occurred, but we don't know what the result of that measurement is? The answer is typically an uncorrelated mixed state. Which is also the state you get after complete decoherence. Such an uncorrelated mixed state is the equivalent, therefore, of saying, "This system is definitely in one state or the other, but I don't know which."

Going back to the Wigner's friend idea, the quantum effects there should be limited by the sizes of the systems. Attempting to unitarily reverse the decoherence can only work up to a point. Any such attempt will naturally be pushing up against thermodynamics, and thus will be of limited success as the system gets more complex.

 

[PeterDonis]

After collapse, the measured state of a given quantum system doesn't change in the basis which was measured

I don't understand what this means. Collapse is a change of state--it's a change from an entangled state with multiple terms, each one a product of a particular state of the measured system and a corresponding state of the measuring apparatus ("corresponding" mean "the state of the apparatus that means the particular state of the measured system was measured"), to a state that is just one of those terms--all the other ones disappear. This change obviously has to change the state of the measured system.

how do we represent a system where a measurement has occurred, but we don't know what the result of that measurement is? The answer is typically an uncorrelated mixed state. Which is also the state you get after complete decoherence. Such an uncorrelated mixed state is the equivalent, therefore, of saying, "This system is definitely in one state or the other, but I don't know which."

Assuming that collapse occurs, yes. But this uncorrelated mixed state is a mixture of product states--states that are each just one product of a measured state of the quantum system and the corresponding state of the measuring apparatus. It is not a mixture of entangled states.

the quantum effects there should be limited by the sizes of the systems. Attempting to unitarily reverse the decoherence can only work up to a point. Any such attempt will naturally be pushing up against thermodynamics, and thus will be of limited success as the system gets more complex.

In a practical sense, of course this is true. But there seem to be many physicists who are not willing to extend "in a practical sense" to "even in principle". Only by being so unwilling can they talk about Wigner's friend type experiments in which humans are treated as quantum systems that can be unitarily transformed at will just like qubits can.

 

[kimbyd]

I don't understand what this means. Collapse is a change of state--it's a change from an entangled state with multiple terms, each one a product of a particular state of the measured system and a corresponding state of the measuring apparatus ("corresponding" mean "the state of the apparatus that means the particular state of the measured system was measured"), to a state that is just one of those terms--all the other ones disappear. This change obviously has to change the state of the measured system.

That definition of collapse only works if you only talk about collapse when you have observed the result of the experiment. How do you represent the state of a system that has been measured, but you don't have any access to the result of said measurement?

The paper charters links above talks about this in section 1.2.3:
http://philsci-archive.pitt.edu/5439/

In a practical sense, of course this is true. But there seem to be many physicists who are not willing to extend "in a practical sense" to "even in principle". Only by being so unwilling can they talk about Wigner's friend type experiments in which humans are treated as quantum systems that can be unitarily transformed at will just like qubits can.

Right. My point is that the Wigner's friend type experiment is likely to not show any quantum effects if humans are the observers closest to the observed system. Decoherence will almost certainly eliminate any such quantum effects. But it shouldn't be hard to design a Wigner's friend-type experiment at a much smaller scale with an experimental apparatus playing the part of an "observer" even though it has nothing like consciousness. And it should also be possible to cause the quantum effects to disappear by making the low-level "observer" further from the quantum regime (possibilities include increasing its temperature or increasing its interactions with its local environment, for example).

I'm quite sure that humans are quantum systems, and are subject to this kind of experiment in principle. But it's highly unlikely to be measurable in practice because decoherence will be so complete by the time the experimental system is entangled with a human.

 

[PeterDonis]

How do you represent the state of a system that has been measured, but you don't have any access to the result of said measurement?

Just like you said--with an uncorrelated mixed state. And using such a state implicitly assumes that a collapse occurred when the measurement took place, so the only uncertainty left is our classical uncertainty about which result was measured.

If you don't assume that collapse occurs when a measurement happens, then you are using an interpretation like the MWI, and you would represent the state of the system as an entangled pure state--the entanglement is between the measured system and the measurement apparatus. If you don't have any access to the results of the measurement, that just means you haven't yet interacted with the measured system and/or measurement apparatus, so the overall pure state is a product of your state and the entangled system-apparatus state. Then your gaining access to the measurement result is a further interaction that entangles you with the system-apparatus.

 

[PeterDonis]

I'm quite sure that humans are quantum systems, and are subject to this kind of experiment in principle. But it's highly unlikely to be measurable in practice because decoherence will be so complete by the time the experimental system is entangled with a human.

In other words, you're quite sure that in principle humans could have unitary "quantum eraser" operators applied to them, even though you also think it's highly unlikely to be possible in practice.

To me, this viewpoint shows a huge overconfidence in exact unitary quantum mechanics. If you had just made your statement conditional--if exact unitary QM is true, then in principle humans could be quantum erased--that would be one thing. But you say you're "quite sure" that exact unitary QM is true, even though you admit there is no experimental evidence that it's true for humans (or even objects much smaller and less complex than humans--very small rocks, say) and it is highly unlikely that there ever will be such evidence.

I understand that this viewpoint is not just yours--many physicists seem to hold it. I still think it's huge overconfidence. In Bayesian terms, I see no reason at present to assign basically all of the probability mass to the hypothesis that exact unitary QM is true. I think any prudent Bayesian would retain a significant probability mass for the hypothesis that there is other physics involved that we don't yet understand that keeps exact unitary QM from scaling up all the way to rocks and humans.

 

[kimbyd]

Just like you said--with an uncorrelated mixed state. And using such a state implicitly assumes that a collapse occurred when the measurement took place, so the only uncertainty left is our classical uncertainty about which result was measured.

If you don't assume that collapse occurs when a measurement happens, then you are using an interpretation like the MWI, and you would represent the state of the system as an entangled pure state--the entanglement is between the measured system and the measurement apparatus. If you don't have any access to the results of the measurement, that just means you haven't yet interacted with the measured system and/or measurement apparatus, so the overall pure state is a product of your state and the entangled system-apparatus state. Then your gaining access to the measurement result is a further interaction that entangles you with the system-apparatus.

Right, but the thing is that decoherence will still cause the appearance of collapse in any interpretation. Decoherence is not a phenomenon that is restricted only to MWI. It's a physical process that is an inevitable consequence of the wavefunction dynamics and is independent of any interpretation. Decoherence is a process which requires no assumption beyond quantum wavefunctions evolving over time according to the relevant equations.

If you really thought that the Copenhagen interpretation was correct, and did your math carefully, you'd have to conclude that the wavefunction of the primary observer in a Wigner's Friend-type experiment would have the appearance of having collapsed long before the secondary observer checked on them, assuming the observers are humans or as complex as humans. That's why in order to test this sort of thing, you'd have to use small-scale "observers" who are sufficiently isolated from the environments so as to avoid decoherence that renders the quantum nature of the system unmeasurable.

In principle it's possible for an alternative interpretation to cause collapse other than that caused by decoherence, so it could in principle result in collapse before that predicted by MWI. But it could never cause measurable collapse that occurs later, because decoherence renders any such collapse unmeasurable.

So, yeah, if anybody could successfully develop a sentient computer that was tiny enough to avoid decoherence when measuring a system, then we'd have a direct test of whether consciousness causes collapse.

I just don't see the point of that because I find the idea that consciousness matters one whit when it comes to the behavior of the universe to be ludicrous.

 

[PeterDonis]

Decoherence is not a phenomenon that is restricted only to MWI.

Yes, agreed.

If you really thought that the Copenhagen interpretation was correct, and did your math carefully, you'd have to conclude that the wavefunction of the primary observer in a Wigner's Friend-type experiment would have the appearance of having collapsed long before the secondary observer checked on them, assuming the observers are humans or as complex as humans.

Agreed. But also, if you really thought the MWI was correct, you'd still have to conclude that the unitary "eraser" operations described in the Wigner's Friend-type experiment as being performed on human observers are impossible, because you can only perform them on systems that haven't decohered yet.

 

[kimbyd]

In other words, you're quite sure that in principle humans could have unitary "quantum eraser" operators applied to them, even though you also think it's highly unlikely to be possible in practice.

To me, this viewpoint shows a huge overconfidence in exact unitary quantum mechanics. If you had just made your statement conditional--if exact unitary QM is true, then in principle humans could be quantum erased--that would be one thing. But you say you're "quite sure" that exact unitary QM is true, even though you admit there is no experimental evidence that it's true for humans (or even objects much smaller and less complex than humans--very small rocks, say) and it is highly unlikely that there ever will be such evidence.

I understand that this viewpoint is not just yours--many physicists seem to hold it. I still think it's huge overconfidence. In Bayesian terms, I see no reason at present to assign basically all of the probability mass to the hypothesis that exact unitary QM is true. I think any prudent Bayesian would retain a significant probability mass for the hypothesis that there is other physics involved that we don't yet understand that keeps exact unitary QM from scaling up all the way to rocks and humans.

When you're talking about systems that are so far separated from the quantum realm that it's effectively impossible to ever measure their quantum behavior, what is the point in asserting that something different is happening?

Quantum mechanics predicts Newtonian behavior in the macroscopic world we inhabit. Just as General Relativity predicts Newtonian behavior for most Solar System observations.

I see no reason to explicitly state that I'm failing to assume that some fundamentally unmeasurable process happens between the quantum realm and the world we are more familiar with when the equations predict the same outcome in either event, just as I see no reason to explicitly state that I don't think General Relativity suddenly stops describing gravity near the surface of the Earth.

 

[kimbyd]

Agreed. But also, if you really thought the MWI was correct, you'd still have to conclude that the unitary "eraser" operations described in the Wigner's Friend-type experiment as being performed on human observers are impossible, because you can only perform them on systems that haven't decohered yet.

That conclusion is independent of interpretation. It's not restricted to MWI.

 

[PeterDonis]

When you're talking about systems that are so far separated from the quantum realm that it's effectively impossible to ever measure their quantum behavior, what is the point in asserting that something different is happening?

Because something different does happen. You can quantum erase qubits. You can't quantum erase humans.

Quantum mechanics predicts Newtonian behavior in the macroscopic world we inhabit.

More precisely, QM in a particular classical approximation that basically ignores all quantum effects predicts Newtonian behavior in the macroscopic world.

Just as General Relativity predicts Newtonian behavior for most Solar System observations.

Yes, but the approximation used here is very different from the QM one. GR predicts that the effects that depart from Newtonian behavior are too small to measure for most solar system observations. But GR is still a classical deterministic theory that predicts one result for all observations, just as Newtonian mechanics was.

The approximation that gets "Newtonian behavior" out of QM, however, has to ignore the fact that QM has a measurement problem. "Newtonian behavior" means a single deterministic trajectory. The unitary math of QM does not predict a single deterministic trajectory. It predicts a huge entangled mess of things that don't even amount to trajectories at all in ordinary 3-dimensional space: the only deterministic trajectory is in the configuration space of the system, which for a macroscopic object has something like $10^{25}$ degrees of freedom. You have to either assume that collapse occurs (Copenhagen) or assume that it makes sense to talk about a particular branch of a horribly messy entangled state as a "single trajectory" (MWI) to get Newtonian behavior.

The usual answer to the latter problem is decoherence--all the branches are decohered when we're talking about macroscopic objects, and a single trajectory is what is measured in each branch. I think this still sweeps a lot of issues under the rug, but in any case the point is that we don't need to go through any of this to get Newtonian behavior from GR.

In short, you're basically asking why you shouldn't consider QM to be a theory of everything (which is what you are doing when you ask what the point is in saying "something different is happening"). My answer is to ask why I should consider QM to be a theory of everything when it has such obvious foundational issues and I have perfectly good classical theories for things that behave classically. The viewpoint that both our best current classical theory, GR, and QM are both approximations to some other more fundamental theory that we don't have yet seems to me to be much more reasonable than the viewpoint that, well, QM just has to be the theory of everything so why not make the best of it.

 

[kimbyd]

The approximation that gets "Newtonian behavior" out of QM, however, has to ignore the fact that QM has a measurement problem. "Newtonian behavior" means a single deterministic trajectory. The unitary math of QM does not predict a single deterministic trajectory. It predicts a huge entangled mess of things that don't even amount to trajectories at all in ordinary 3-dimensional space: the only deterministic trajectory is in the configuration space of the system, which for a macroscopic object has something like $10^{25}$ degrees of freedom. You have to either assume that collapse occurs (Copenhagen) or assume that it makes sense to talk about a particular branch of a horribly messy entangled state as a "single trajectory" (MWI) to get Newtonian behavior.

This is fundamentally incorrect. You don't need a single deterministic trajectory to match observations to Newtonian mechanics. You only need the appearance of one.

And the unitary evolution of the wavefunction in QM predicts the appearance of a single deterministic trajectory in the classical limit. This fact is apparent no matter what. You can layer some assumptions about wavefunction collapse on top of that, but those assumptions are fundamentally untestable unless they result in a collapse that happens before the effective collapse that happens in any interpretation due to decoherence.

In short, you're basically asking why you shouldn't consider QM to be a theory of everything (which is what you are doing when you ask what the point is in saying "something different is happening"). My answer is to ask why I should consider QM to be a theory of everything when it has such obvious foundational issues and I have perfectly good classical theories for things that behave classically. The viewpoint that both our best current classical theory, GR, and QM are both approximations to some other more fundamental theory that we don't have yet seems to me to be much more reasonable than the viewpoint that, well, QM just has to be the theory of everything so why not make the best of it.

I don't honestly see why you have a problem with the simple fact that unitary evolution of the wavefunction leads to the appearance of wavefunction collapse.

The fact that this happens is a really trivial analysis: entanglement of a system with a complex system extends the interference time dramatically. You don't need much complexity before the interference time becomes longer than the age of our universe. And the extension of that interference time to absurdly long timescales means that thereafter, the wavefunction branches will evolve as if they were independent.

I see no way in which this simple analysis can be effectively argued against. The only question that needs to be answered on top of that is whether the apparent wavefunction collapse has the same probability properties as the Born rule. And due to the work of Davids Deutsch and Wallace (and others) over a decade ago, we now know that the Born rule drops out of the theory naturally given some very basic assumptions about observers (see here: https://arxiv.org/abs/0906.2718).

 

[PeterDonis]

You don't need a single deterministic trajectory to match observations to Newtonian mechanics. You only need the appearance of one.

If you are willing to accept the MWI, yes, I suppose this is true.

the Born rule drops out of the theory naturally given some very basic assumptions about observers

I'm personally not convinced by these arguments, but that's a subject for a different discussion (and not one that can really be had here).

 

[kimbyd]

If you are willing to accept the MWI, yes, I suppose this is true.

You don't have to accept MWI, though. Do you really think that decoherence doesn't happen if MWI isn't correct?

 

[DarMM]

You don't have to accept MWI, though. Do you really think that decoherence doesn't happen if MWI isn't correct?

Decoherence doesn't give all of collapse. It shows the event space of the device pointer states become a Boolean lattice. However it doesn't select which event occurs.

 

[kimbyd]

Decoherence doesn't give all of collapse. It shows the event space of the device pointer states become a Boolean lattice. However it doesn't select which event occurs.

Right. But if the observer is also a quantum object, then after decoherence it will look like they observed collapse, regardless of which outcome you look at.

 

[DarMM]

Right. But if the observer is also a quantum object, then after decoherence it will look like they observed collapse, regardless of which outcome you look at.

Collapse means reducing the state down to one outcome. You don't get that from decoherence. You still have all pointer states.

 

[PeterDonis]

Do you really think that decoherence doesn't happen if MWI isn't correct?

Of course not. But the whole idea of "we only need the appearance of a single trajectory" only makes sense if you believe the MWI is true.

 

[kimbyd]

Collapse means reducing the state down to one outcome. You don't get that from decoherence. You still have all pointer states.

But an observer will only observe one outcome after decoherence, even if the wavefunction still describes multiple.

I really don't understand what the confusion is here. I'm making two points:
1) Decoherence is a feature of the wavefunction dynamics of QM, and is therefore independent of interpretation.
2) After sufficient decoherence, any observer described by QM will observe what looks like collapse.

Point (2) means that if you are trying to measure something like this Wigner's friend effect, it is necessary to use very small, isolated pseudo-observers which don't have issues with decoherence. The Wigner's friend experiment might be able to unroll the effects of decoherence somewhat, but it's not likely to be by all that much.

 

[kimbyd]

Of course not. But the whole idea of "we only need the appearance of a single trajectory" only makes sense if you believe the MWI is true.

Again, this is an observational statement. It is observationally impossible to distinguish between "real" collapse and the appearance of collapse.

 

[charters]

Point (2) means that if you are trying to measure something like this Wigner's friend effect, it is necessary to use very small, isolated pseudo-observers which don't have issues with decoherence. The Wigner's friend experiment might be able to unroll the effects of decoherence somewhat, but it's not likely to be by all that much.

But this is just rejecting the premise of the thought experiment, which is that Wigner has complete control over all degrees of freedom in the closed friend/lab system, so Wigner can reverse decoherence at his leisure. Saying this is not feasible in practice is besides the point, as we are trying to assess the logical consistency of the theory under maximally extreme circumstances. Your argument is similar to saying since nobody can live inside a black hole, it doesn't matter if GR breaks down at the singularity.

 

[PeterDonis]

It is observationally impossible to distinguish between "real" collapse and the appearance of collapse.

Yes, so which position one takes on this issue depends on whether one thinks of QM as the theory of everything and "Newtonian" classical behavior as an approximation that one is free to interpret however one likes (as your arguments in this thread do), or whether one thinks of QM as a theory that has been shown to work well for microscopic objects but which we have no evidence for when macroscopic objects are concerned (nobody has ever observed quantum interference, erasure, etc. with rocks), so one should be skeptical of claims like "we only need the appearance of collapse" that only make sense on the assumption that exact unitary QM applies to everything.

 

[DarMM]

I really don't understand what the confusion is here. I'm making two points:
1) Decoherence is a feature of the wavefunction dynamics of QM, and is therefore independent of interpretation.
2) After sufficient decoherence, any observer described by QM will observe what looks like collapse.

Decoherence shows (with certain caveats) that it is consistent for observer A with device A measuring observer B with device B to consider the latter as having reduced their wavefunction. In other words it shows the consistency of the kinematic constraints on the statistics on one level with a dynamical account on a higher one.

However we still lack collapse for observer A. Somebody will always lack collapse.

 

[kimbyd]

But this is just rejecting the premise of the thought experiment, which is that Wigner has complete control over all degrees of freedom in the closed friend/lab system, so Wigner can reverse decoherence at his leisure. Saying this is not feasible in practice is besides the point, as we are trying to assess the logical consistency of the theory under maximally extreme circumstances. Your argument is similar to saying since nobody can live inside a black hole, it doesn't matter if GR breaks down at the singularity.

I don't think that's an accurate characterization. Here is the abstract:

The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics, the objectivity of observations is not so clear, most dramatically exposed in Eugene Wigner's eponymous thought experiment where two observers can experience fundamentally different realities. While observer-independence has long remained inaccessible to empirical investigation, recent no-go-theorems construct an extended Wigner's friend scenario with four entangled observers that allows us to put it to the test. In a state-of-the-art 6-photon experiment, we here realize this extended Wigner's friend scenario, experimentally violating the associated Bell-type inequality by 5 standard deviations. This result lends considerable strength to interpretations of quantum theory already set in an observer-dependent framework and demands for revision of those which are not.

Emphasis mine. This isn't supposed to just be a paper about thought experiments, but something that could actually be tested in practice.

Yes, so which position one takes on this issue depends on whether one thinks of QM as the theory of everything and "Newtonian" classical behavior as an approximation that one is free to interpret however one likes (as your arguments in this thread do), or whether one thinks of QM as a theory that has been shown to work well for microscopic objects but which we have no evidence for when macroscopic objects are concerned (nobody has ever observed quantum interference, erasure, etc. with rocks), so one should be skeptical of claims like "we only need the appearance of collapse" that only make sense on the assumption that exact unitary QM applies to everything.

Why do you think that QM should differ substantially at the macro scale, when it predicts macro-scale behavior correctly?

I'm not assuming that exact unitary QM applies to everything. All that I'm saying is that the specific features that people try to tack on to wavefunction dynamics are both unnecessary and observationally impossible to verify. It is certainly conceivable for the universe to not be entirely unitary, but stating that it's non-unitary in a specific, unmeasurable way is making unwarranted assumptions.

 

[PeterDonis]

Why do you think that QM should differ substantially at the macro scale, when it predicts macro-scale behavior correctly?

You're framing the question backwards. I'm not starting from the theory and asking what should happen to it at the macro scale. I'm starting from the observation that we see all sorts of quantum phenomena at the micro scale, but not at the macro scale. It's not that "QM should differ", it's that the observed phenomena do differ.

One possible way to deal with this is to try to modify QM so that its predictions change as you go from the micro to the macro scale. But it's not the only possible way. Another obvious way is to look for a different theory that has QM as one approximation, at the micro scale, and the appropriate macro-scale theory as another approximation at the macro scale. This is one way of viewing the current search for a theory of quantum gravity.

I'm not assuming that exact unitary QM applies to everything.

Then whatever interpretation of QM you are using, it isn't the MWI, which requires exactly that assumption.

stating that it's non-unitary in a specific, unmeasurable way is making unwarranted assumptions.

Only if you are taking "exactly unitary at all scales" as the null hypothesis, the one we should accept in the absence of evidence to the contrary. But how did that particular hypothesis, which comes from a particular theory that only has experimental validation at the micro scale, somehow get itself to be the null hypothesis?

 

[kimbyd]

Decoherence shows (with certain caveats) that it is consistent for observer A with device A measuring observer B with device B to consider the latter as having reduced their wavefunction. In other words it shows the consistency of the kinematic constraints on the statistics on one level with a dynamical account on a higher one.

However we still lack collapse for observer A. Somebody will always lack collapse.

There's no actual collapse anywhere in MWI. There's only the appearance of collapse.

Decoherence causes interactions between different components of the wavefunction to be suppressed. Once those interactions are suppressed enough, an observer who is described by one branch of that wavefunction will not have any access to information in the other branches. That observer's experimental results look like only one outcome occurred.

That fact is the case whether you're observer A or observer B. The point of the Wigner's Friend thought experiment is to show that observer A will (ideally) still be able to measure some quantum effects for observer B, even if observer B doesn't see them.

The entire point I'm trying to get across is that decoherence places limits on the complexity of observer B in order for observer A to be able to measure anything. What I would expect to see in an experiment where the amount of decoherence in observer B is tunable is that there will be a regime where observer A sees the quantum effects, but as the decoherence is turned up (e.g. by increasing the intensity of a light source), those quantum effects should disappear.

Ideally, if this observation is to be useful at all, there will be a regime where observer B reports no visible quantum effects, but observer A still sees them.

 

[charters]

This isn't supposed to just be a paper about thought experiments, but something that could actually be tested in practice

But outside of the authors and their press releases, most do not really agree this recent optics experiment was a valid investigation of the Wigner's friend problem. The full fledged humanity of the internal observer is essential to avoiding loopholes that distract from the point of the thought experiment.

 

[kimbyd]

But outside of the authors and their press releases, most do not really agree this recent optics experiment was a valid investigation of the Wigner's friend problem. The full fledged humanity of the internal observer is essential to avoiding loopholes that distract from the point of the thought experiment.

But why should that be the case? All that you'd need to show is that the quantum effects disappear as decoherence is dialed up. If the observer isn't human, and the quantum effects disappear once the decoherence is turned up sufficiently, then that proves that you'll never get anything but a null result if the observer is human (assuming that we can't effectively isolate the human from the outside observer, which is probably always going to be the case).

 

[DarMM]

There's no actual collapse anywhere in MWI. There's only the appearance of collapse.

Of course hence @PeterDonis 's statement that this is fine in MWI, but it doesn't resolve it in other views like Copenhagen. Thus it's not interpretation neutral.

That observer's experimental results look like only one outcome occurred.

It shows that the higher observer's statistics for the lower observer, prior to measuring the lower observer, are consistent with the assumption that an outcome has occured. This is not actual collapse though. The higher observer retains all terms, they have not reduced to one term. When they observe the lower observer they then reduce using an axiom separate to the unitary dynamics.

 

[charters]

But how did that particular hypothesis, which comes from a particular theory that only has experimental validation at the micro scale, somehow get itself to be the null hypothesis?

For me, Occam's razor and the non existence of an objective collapse theory that can reproduce the Standard Model and exactly explain the collapse triggers.

 

[charters]

But why should that be the case? All that you'd need to show is that the quantum effects disappear as decoherence is dialed up. If the observer isn't human, and the quantum effects disappear once the decoherence is turned up sufficiently, then that proves that you'll never get anything but a null result if the observer is human (assuming that we can't effectively isolate the human from the outside observer, which is probably always going to be the case).

Again, a proper a Wigner's friend scenario assumes the decoherence can always be reversed in principle, ignoring the practically insurmountable realistic challenges. The most philosophically interesting problem (for assessing the consistency of the theory) is when we imagine a decohered human being gets unitarily reversed.