Consciousness and quantum mechanics

[PeterDonis]

How do you define an actual outcome?

An outcome that cannot be reversed.

How are you defining decoherence?

Loss of quantum coherence due to entanglement spreading out among a large number of untrackable degrees of freedom.

You are correct that a single definite outcome, by itself, does not follow from decoherence alone. All decoherence ensures is that the outcomes are definite, i.e., there is no interference between them and each one is irreversible. But decoherence by itself does not rule out interpretations like the MWI where all possible outcomes actually occur, not just one of them. It just says that, even in interpretations like the MWI, the outcomes are irreversible and don't interfere. So if Wigner's friend undergoes decoherence, Wigner can't observe interference between different outcomes for his friend.

 

[PeterDonis]

Does a "Wigner's friend experiment" imply repetitions of some experiment - call it "the basic experiment" - so that statistics from the basic experiment can be used to demonstrate interference?

Not as I understand it. A Wigner's friend experiment requires Wigner to be able to, in a single run of the experiment, subject the friend, along with his entire lab and the observations he makes in it, to something like a Mach-Zehnder interferometer, where Wigner can cause the different "branches" of the friend/lab wave function to recombine, as an MZI does at the second beam splitter, and thus enable interference between the branches.

 

[allisrelative]

An outcome that cannot be reversed.
Loss of quantum coherence due to entanglement spreading out among a large number of untrackable degrees of freedom.

You are correct that a single definite outcome, by itself, does not follow from decoherence alone. All decoherence ensures is that the outcomes are definite, i.e., there is no interference between them and each one is irreversible. But decoherence by itself does not rule out interpretations like the MWI where all possible outcomes actually occur, not just one of them. It just says that, even in interpretations like the MWI, the outcomes are irreversible and don't interfere. So if Wigner's friend undergoes decoherence, Wigner can't observe interference between different outcomes for his friend.

Undergoes decoherence? What does that mean? I know you undergo anesthesia but I never heard of someone that undergoes decoherence.

Let's answer it though using your vernacular.

Wigner's Friend "undergoes" decoherence not Wigner which is illustrated by Rovelli's Relational interpretation. This is only occurring in Wigner's Friend's frame of reference not Wigner's.

All physical interactions are, at bottom, quantum interactions, and must ultimately be governed by the same rules. Thus, an interaction between two particles does not, in RQM, differ fundamentally from an interaction between a particle and some "apparatus". There is no true wave collapse, in the sense in which it occurs in the Copenhagen interpretation.

Because "state" is expressed in RQM as the correlation between two systems, there can be no meaning to "self-measurement". If observer O measures system S, S's "state" is represented as a correlation between O and S. O itself cannot say anything with respect to its own "state", because its own "state" is defined only relative to another observer, O'. If the S+O compound system does not interact with any other systems, then it will possesses a clearly defined state relative to O'. However, because O's measurement of S breaks its unitary evolution with respect to O, O will not be able to give a full description of the S+O system (since it can only speak of the correlation between S and itself, not its own behaviour). A complete description of the (S+O)+O' system can only be given by a further, external observer, and so forth.

Taking the model system discussed above, if O' has full information on the S+O system, it will know the Hamiltonians of both S and O, including the interaction Hamiltonian. Thus, the system will evolve entirely unitarily (without any form of collapse) relative to O', if O measures S. The only reason that O will perceive a "collapse" is because O has incomplete information on the system (specifically, O does not know its own Hamiltonian, and the interaction Hamiltonian for the measurement).

https://en.wikipedia.org/wiki/Relational_quantum_mechanics

So for Wigner's Friend he "undergoes" decoherence and becomes part of an S+O system when he gains knowledge of the state the quantum system is in. What do I mean by gains knowledge?

It means Wigner's Friend gains knowledge of both states of the quantum system. You have a Wigner's Friend that records vertical polarization of the photon in his memory and Wigner's friend who records horizontal polarization in his memory. The interference between these two states decoheres but not because any wave function collapse. It's because both versions of Wigner's Friend has a lack of information about the overall S+O system and they perceive "collapse" and record the outcome.

The wave function has not collapsed so why shouldn't Wigner(O') be able to measure interference of the S+O system as long as he doesn't gain knowledge about his friend's state?

I think confusion comes into play because you're equating loss of intereference with collapse of the wave function.

 

[PeterDonis]

Undergoes decoherence?

If you are not familiar with decoherence theory, I would recommend taking some time to learn about it. It is an important part of our current understanding of QM.

Decoherence is a physical process, and a person is a physical system, so I don't see anything at all wrong with saying that a person undergoes decoherence.

The wave function has not collapsed so why shouldn't Wigner(O') be able to measure interference of the S+O system

Because the S+O system has a huge number of degrees of freedom, most of which cannot be tracked. The process of the friend (O) observing the system (S) spreads entanglement among all of those degrees of freedom. That is the physical process of decoherence.

The effect that that physical process has is to make it impossible for Wigner to, in the terminology of the Wikipedia article, have "full information" about the S+O system, because such information would have to include the information that has been spread by decoherence among the huge number of mostly untrackable degrees of freedom in the S+O system.

If, on the other hand, we stipulate by fiat that, however infeasible it is given the huge number of degrees of freedom in the S+O system, Wigner does have "full information" about that system, then we are stipulating that decoherence has not occurred in that system, and that in turn means the friend has not made any actual observation of anything. The "S" subsystem might have interacted with the "O" subsystem, but that is not the same as an actual observation being made. It is no different from the interaction of an electron with a Stern-Gerlach magnet, which by itself is not a measurement of the electron's spin; it just entangles the electron's spin with its momentum. Similarly, whatever is going on in the S+O system that does not lead to decoherence can't be a measurement of anything; it is just entangling some degrees of freedom in S with some degrees of freedom in O. And since that entanglement is reversible (because we have stipulated, however unrealistically, that decoherence has not occurred), it is not a measurement.

In other words, you can't have it both ways. You can't both say that the friend has actually made an observation, which requires decoherence to occur, and also say that Wigner can measure interference, which requires decoherence to not occur.

I think confusion comes into play because you're equating loss of intereference with collapse of the wave function.

No. See above.

 

[PeterDonis]

I think confusion comes into play because you're equating loss of intereference with collapse of the wave function.

Perhaps it's worth elaborating on my "no" response to this a little more.

In a no collapse interpretation, such as the MWI, decoherence still occurs--more precisely, decoherence is still required for "splitting" into multiple "branches" to occur. If interference between different alternatives is still possible, decoherence has not occurred, and those alternatives are not separate "worlds" or "branches" according to the MWI.

So in a Wigner's friend experiment where we stipulate by fiat (however unrealistically) that Wigner can detect interference, the MWI will agree with interpretations that do have collapse, because both kinds of interpretations will say that no measurement by the friend occurred at all--the friend, once again, is no different from an electron that has gone through a SG magnet but has not yet hit any detector screen.

 

[allisrelative]

Perhaps it's worth elaborating on my "no" response to this a little more.

In a no collapse interpretation, such as the MWI, decoherence still occurs--more precisely, decoherence is still required for "splitting" into multiple "branches" to occur. If interference between different alternatives is still possible, decoherence has not occurred, and those alternatives are not separate "worlds" or "branches" according to the MWI.

So in a Wigner's friend experiment where we stipulate by fiat (however unrealistically) that Wigner can detect interference, the MWI will agree with interpretations that do have collapse, because both kinds of interpretations will say that no measurement by the friend occurred at all--the friend, once again, is no different from an electron that has gone through a SG magnet but has not yet hit any detector screen.

Decoherence is still required for splitting into multiple branches?

Where's the scientific evidence for this? Is this speculation or observed evidence you're talking about?

You said:

If interference between different alternatives is still possible, decoherence has not occurred, and those alternatives are not separate "worlds" or "branches" according to the MWI.

What about many interacting worlds theory? Which speculative theory are you saying is the final word when it comes to decoherence?

Quantum Phenomena Modeled by Interactions between Many Classical Worlds

We investigate whether quantum theory can be understood as the continuum limit of a mechanical theory, in which there is a huge, but finite, number of classical “worlds,” and quantum effects arise solely from a universal interaction between these worlds, without reference to any wave function. Here, a “world” means an entire universe with well-defined properties, determined by the classical configuration of its particles and fields. In our approach, each world evolves deterministically, probabilities arise due to ignorance as to which world a given observer occupies, and we argue that in the limit of infinitely many worlds the wave function can be recovered (as a secondary object) from the motion of these worlds. We introduce a simple model of such a “many interacting worlds” approach and show that it can reproduce some generic quantum phenomena—such as Ehrenfest’s theorem, wave packet spreading, barrier tunneling, and zero-point energy—as a direct consequence of mutual repulsion between worlds. Finally, we perform numerical simulations using our approach. We demonstrate, first, that it can be used to calculate quantum ground states, and second, that it is capable of reproducing, at least qualitatively, the double-slit interference phenomenon.

https://journals.aps.org/prx/abstract/10.1103/PhysRevX.4.041013

Which speculative theory do you want me to go by?

Yes, Wigner can measure interference. That was the whole point of the published paper. If you want to refute it, then write a paper and get it peer reviewed.

Again, you're making the mistake that Wiki doesn't even make by confusing decoherence with collapse of the wave function.

Decoherence has been used to understand the collapse of the wave function in quantum mechanics. Decoherence does not generate actual wave-function collapse. It only provides an explanation for apparent wave-function collapse, as the quantum nature of the system "leaks" into the environment. That is, components of the wave function are decoupled from a coherent system and acquire phases from their immediate surroundings. A total superposition of the global or universal wavefunction still exists (and remains coherent at the global level), but its ultimate fate remains an interpretational issue. Specifically, decoherence does not attempt to explain the measurement problem.

https://en.wikipedia.org/wiki/Quantum_decoherence

So I ask again.

If the wave function hasn't actually collapsed for Wigner's friend and he just perceives collapse then why couldn't Wigner measure interference between the S+O system as long as he doesn't have knowledge of the state his friend observed?

 

[PeterDonis]

Decoherence is still required for splitting into multiple branches?

Yes.

Where's the scientific evidence for this?

There can't be any scientific evidence for one interpretation of QM over another.

The statement I said "yes" to above is a postulate of the MWI. It's not something that can be proved or disproved by evidence. It's part of the interpretation.

Which speculative theory do you want me to go by?

In this thread and this forum, none. In this thread and this forum, we are discussing interpretations of QM. Not interpretations of some other speculative theory.

If you want to talk about some other speculative theory, please start a new thread (with appropriate references) in the appropriate forum, which will probably be the Beyond the Standard Model forum.

Wigner can measure interference. That was the whole point of the published paper.

You can certainly stipulate that Wigner can measure interference in a particular thought experiment. What you can't do is stipulate that and then make use of intuitive arguments that assume that Wigner's friend has made an actual observation, the same way that we would make an actual observation in ordinary life.

If you search these forums, you will find plenty of previous threads on the Wigner's friend experiment, in which references to multiple papers are given that make arguments on both sides of questions like this.

 

[allisrelative]

What? You said:

You can certainly stipulate that Wigner can measure interference in a particular thought experiment. What you can't do is stipulate that and then make use of intuitive arguments that assume that Wigner's friend has made an actual observation, the same way that we would make an actual observation in ordinary life.

First, it wasn't just a thought experiment it was an actual experiment.

Next you said the way we make an observation in ordinary life? Are you talking about local realism? What exactly do you mean? Explain the science of "ordinary life."

Death by experiment for local realism

A fundamental scientific assumption called local realism conflicts with certain predictions of quantum mechanics. Those predictions have now been verified, with none of the loopholes that have compromised earlier tests.

https://www.nature.com/articles/nature15631

So scientifically, what do you mean by "ordinary life.?"

So you used an intepretation that you admit has no evidence to equate decoherence with wave function collapse?

Again, it's not a thought experiment, it's an actual experiment. The paper ended like this:

Because quantum theory does not distinguish between information recorded in a microscopic system (such as our photonic memory) and in a macroscopic system, the conclusions are the same for both: The measurement records are in conflict regardless of the size or complexity of the observer that records them. Implementing the experiment with more complex observers would not necessarily lead to new insights into the specific issue of observer independence in quantum theory. It would, however, serve to show that quantum mechanics still holds at larger scales, ruling out alternative (collapse) models (20). However, this is not the point of a Bell-Wigner test—less demanding experiments could show that.

https://www.researchgate.net/publication/335953720_Experimental_test_of_local_observer_independence

I'm not talking about "ordinary life" whatever that means or speculation about self collapse theories that could be right but at this point there's no evidence to support them. I'm talking about science that has been observed and tested.

Why don't you just admit that you used speculative M.W.I. to confuse decoherence with the collapse of the wave function?

 

[PeterDonis]

it wasn't just a thought experiment it was an actual experiment.

There have not been any actual Wigner's friend experiments involving people. There have been "Wigner's friend" experiments involving qubits. Qubits have only one degree of freedom. People have something like $10^{25}$. Big difference.

 

[PeterDonis]

quantum theory does not distinguish between information recorded in a microscopic system (such as our photonic memory) and in a macroscopic system as long as the system remains isolated and quantum coherence is maintained.

See my bolded addition in the quote above. It makes a big difference.

 

[allisrelative]

There have not been any actual Wigner's friend experiments involving people. There have been "Wigner's friend" experiments involving qubits. Qubits have only one degree of freedom. People have something like $10^{25}$. Big difference.

You're not debating science, you're speculating.

It's not a big difference according to Quantum Theory. Let me quote from the published paper again:

Because quantum theory does not distinguish between information recorded in a microscopic system (such as our photonic memory) and in a macroscopic system, the conclusions are the same for both: The measurement records are in conflict regardless of the size or complexity of the observer that records them. Implementing the experiment with more complex observers would not necessarily lead to new insights into the specific issue of observer independence in quantum theory. It would, however, serve to show that quantum mechanics still holds at larger scales, ruling out alternative (collapse) models (20). However, this is not the point of a Bell-Wigner test—less demanding experiments could show that.

https://www.researchgate.net/publication/335953720_Experimental_test_of_local_observer_independence

Your claim that it makes a big difference is just speculation. Does it make a big difference because of self collapse or M.W.I.?

You're not being very scientific. You use M.W.I. to confuse decoherence with wave function collapse. Now you claim there's a BIG DIFFERNCE without any evidence. Quantum Theory doesn't make a distinction between information recorded in a microscopic or macroscopic system.

Your BIG DIFFERENCE has to come from speculation just like your "Ordinary Life."

 

[PeterDonis]

You're not debating science, you're speculating.

No, I'm making valid points which you are not even responding to, you're just repeating the same claims over and over again.

Let me quote from the published paper again

Repeating the same quote over and over again is pointless.

Thread closed.