# Insights The Quantum Mystery of Wigner's Friend - Comments

#### RUTA

Perhaps, but I would say that such approaches do not take the Schrodinger equation seriously. If the wave function describes only a subjective knowledge of an agent, then all the stuff about which the agent knows nothing (and there is certainly a lot of such stuff) is not described by the Schrodinger equation. In fact it is not described by anything, because in subjective approaches the agent cannot describe something that he does not have a knowledge about. The wave function of the Universe, which makes sense in approaches (like BM and MWI) in which the Schrodinger equation is taken seriously, does not make any sense in subjective approaches.

Is there a wave function of the Moon when the agent knows nothing about the Moon? Subjective approaches say no, other approaches say yes.
By "take the SE seriously," you mean wave function realism, right (so-called "psi-ontologists")? The wave function and therefore the SE is still very meaningful for psi-epistemologists (like me), it's just not considered part of objective reality.

#### DarMM

Gold Member
Perhaps, but I would say that such approaches do not take the Schrodinger equation seriously
Usually it's taken to be the way to update one's probabilities in the absence of observations.

If you take the Hilbert space and its geometry as epistemic, the Schrodinger equation is quite natural as the evolution of your probabilities as encoded in the wave-function via something like Stone's theorem. Of course the Hamiltonian isn't specified. I've often wondered if QBism considers the form of the Hamiltonian to not be purely epistemic.

#### Demystifier

2018 Award
By "take the SE seriously," you mean wave function realism, right (so-called "psi-ontologists")?
Perhaps the realism is not the right word. In BM, for instance, the wave function is something analogous to the Hamiltonian in classical mechanics. Would you say that classical Hamiltonian is real?

The wave function and therefore the SE is still very meaningful for psi-epistemologists (like me), it's just not considered part of objective reality.
But you assume that there is some objective reality, right? Is there anything more specific you can say about that reality? Do you agree that the Bell theorem implies that this reality obeys non-local laws? How do you avoid the conclusion of the PBR theorem that wave function is, in a certain sense, real?

#### Demystifier

2018 Award
I've often wondered if QBism considers the form of the Hamiltonian to not be purely epistemic.
That's a good question.

#### DarMM

Gold Member
But you assume that there is some objective reality, right? Is there anything more specific you can say about that reality? Do you agree that the Bell theorem implies that this reality obeys non-local laws? How do you avoid the conclusion of the PBR theorem that wave function is, in a certain sense, real?
Roughly speaking there are two types of $\psi$-epistemic theories. Adán Cabello calls them type I and type II $\psi$-epistemic. There is also the names $\psi$-statistical and $\psi$-epistemic, which I'll use because I think they're more distinctive, but just note that many use $\psi$-epistemic to mean both.

In the former the wavefunction is statistical in the sense of classical statistical mechanics, it is in essence a probability distribution over more fundamental degrees of freedom, hidden variables. In $\psi$-epistemic theories however QM is not the statistical mechanics of some hidden variables. Rather it is just a general theory of inference for classical observables and you can't really recover the underlying reality directly from it, this is because all properties of the wavefunction just reflect generalised inference rules or normative expectations agents should hold. They're not related to "underlying/more fundamental" degrees of freedom.

As a direct contrast $\psi$-statistical views would say $(\psi,\phi) \neq 0$ means that the two distributions $\psi$ and $\phi$ have some ontic states in common, where as in $\psi$-epistemic views it just means that an agent who prepared $\psi$ should expect some chance to have a click on a $\phi$ measuring device.

Without going into much detail the PBR theorem essentially eliminates $\psi$-statistical explanations that don't allow retrocausality or acausality. It says nothing at all about $\psi$-epistemic views. Similarly retro/acausal $\psi$-statistical and $\psi$-epistemic views can escape the nonlocality conclusions of Bell's theorem.
The real force of the Frauchiger-Renner theorem and why it is causing interest in the Foundations community is because it seems to be the first result to say something about $\psi$-epistemic views.

In the case of $\psi$-statistical views something very definitive is being said of reality, it would just depend on the particular theory's hidden variables as to what that is.

$\psi$-epistemic views say that you can recover little about the underlying reality as so much of the QM formalism is simply "Agent-Reasoning" based. For example QBism says that the dimension of the Hilbert space (e.g. you need $d = 3$ for spin-$1$) reflects something as it seems to be agent independent. However they'd all basically say you can't really recover reality from QM, a new and very very different theory would be needed. QM will not turn out to be about ignorance of hidden more fundamental degrees of freedom. The most extreme view along these lines would be Bohr and Heisenberg style Copenhagen where the underlying reality has no hope of being recovered.

#### RUTA

Roughly speaking there are two types of $\psi$-epistemic theories. Adán Cabello calls them type I and type II $\psi$-epistemic. There is also the names $\psi$-statistical and $\psi$-epistemic, which I'll use because I think they're more distinctive, but just note that many use $\psi$-epistemic to mean both.

In the former the wavefunction is statistical in the sense of classical statistical mechanics, it is in essence a probability distribution over more fundamental degrees of freedom, hidden variables. In $\psi$-epistemic theories however QM is not the statistical mechanics of some hidden variables. Rather it is just a general theory of inference for classical observables and you can't really recover the underlying reality directly from it, this is because all properties of the wavefunction just reflect generalised inference rules or normative expectations agents should hold. They're not related to "underlying/more fundamental" degrees of freedom.

As a direct contrast $\psi$-statistical views would say $(\psi,\phi) \neq 0$ means that the two distributions $\psi$ and $\phi$ have some ontic states in common, where as in $\psi$-epistemic views it just means that an agent who prepared $\psi$ should expect some chance to have a click on a $\phi$ measuring device.

Without going into much detail the PBR theorem essentially eliminates $\psi$-statistical explanations that don't allow retrocausality or acausality. It says nothing at all about $\psi$-epistemic views. Similarly retro/acausal $\psi$-statistical and $\psi$-epistemic views can escape the nonlocality conclusions of Bell's theorem.
The real force of the Frauchiger-Renner theorem and why it is causing interest in the Foundations community is because it seems to be the first result to say something about $\psi$-epistemic views.

In the case of $\psi$-statistical views something very definitive is being said of reality, it would just depend on the particular theory's hidden variables as to what that is.

$\psi$-epistemic views say that you can recover little about the underlying reality as so much of the QM formalism is simply "Agent-Reasoning" based. For example QBism says that the dimension of the Hilbert space (e.g. you need $d = 3$ for spin-$1$) reflects something as it seems to be agent independent. However they'd all basically say you can't really recover reality from QM, a new and very very different theory would be needed. QM will not turn out to be about ignorance of hidden more fundamental degrees of freedom. The most extreme view along these lines would be Bohr and Heisenberg style Copenhagen where the underlying reality has no hope of being recovered.
I will assume this answers Demystifier's questions in post #28.

As for my view personally, as I pointed out in many of my Insights and our book, "Beyond the Dynamical Universe," I'm in the psi-statistical camp where QM provides the distributions of momentum-energy transfer between classical objects in spacetime via adynamical global (4D, spatiotemporal) constraints without causal mechanisms or hidden variables. By analogy, it would be like having Fermat's principle of least time for a light ray without any consensus dynamical counterpart (Snell's law). For example, conservation of angular momentum on average supplies a compelling 4D constraint with no consensus dynamical counterpart. So we're simply saying the 4D constraint is fundamental and without controversy while any dynamical counterpart is a matter of personal preference.

#### idea2000

There seems to be a new experiment done recently that confirm's the wigner's friend hypothesis, published in Februrary 2019. Could anyone provide a simple explanation of what was done in that experiment?

#### PeterDonis

Mentor
There seems to be a new experiment done recently that confirm's the wigner's friend hypothesis, published in Februrary 2019.
As a PF search would tell you, we have a thread open on this:

#### RUTA

There seems to be a new experiment done recently that confirm's the wigner's friend hypothesis, published in Februrary 2019. Could anyone provide a simple explanation of what was done in that experiment?
You should read the thread Peter supplies in post #33. I have a student working on the calculations now and I will post an overview once the analysis is complete (probably late in the semester, as things are very busy now). As you will see in that thread, consensus is forming that their experiment is not Wigner's friend.

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