Is this a valid Wigner's friend experiment?

[Joao]

TL;DR Summary

I describe an experiment and asks if it makes sense

1. Wigner's friend is inside a lab, and has a pair of entangled photons.
2. He measures one in the vertical axis.
3. He sends the other photon to Wigner.
4. Wigner can: a) measure it in the vertical axis (if so, he will get the same result as Wigner's friend).
5. Or Wigner can: b) send the photon to a double slit experiment (if so, he will see a wave pattern).
6. If Wigner chooses to do b), then he will have contradictory results with his friend (for the friend, the photon has a definite polarization, for wigner, is in a superposition).

So, is this a valid Wigner's friend experiment?
Thanks!

 

[PeterDonis]

5. Or Wigner can: b) send the photon to a double slit experiment (if so, he will see a wave pattern).

No, he won't, because the photons are entangled, and an entangled photon won't show interference in a double slit experiment.

 

[Joao]

No, he won't, because the photons are entangled, and an entangled photon won't show interference in a double slit experiment.

Thanks a lot for the answer!

 

[Joao]

I will try to describe the Frauchiger and Renner thought experiment:
1. Participants: Wigner1, friend1, Wigner2, friend2.
2. Friend1 is in lab1, friend2 is in lab2.
3. Friend1 tosses a coin that has 1/3 of chance of being heads and 2/3 of chance of being tails.
4. If the coin is heads, he makes a particle that has spin down and sends to friend2, if its tails, he makes a particle that's spin right and send to friend2.
5. Friend2 measures the spin in the horizontal axis, and has spin up or spin down as a result.
6. Now I stopped following the experiment. It says that Wigner1 can measure Lab

 

[PeterDonis]

I will try to describe the Frauchiger and Renner thought experiment

We have plenty of PF threads already on this experiment, which you can find using PF's search function. I would suggest taking some time to read them before you try to formulate a question about the experiment.

 

[Joao]

We have plenty of PF threads already on this experiment, which you can find using PF's search function. I would suggest taking some time to read them before you try to formulate a question about the experiment.

Hi Peter! Thanks again for the reply! My original message was cut, somehow my browser has issues when posting something here... I'll try to be more careful!

What was next was: but how can Wigner measure a Lab? I can understand how someone measures a particle, but not how someone measures a lab...

I tried to read the messages that already was posted in other threads, but unfortunately I couldn't find what I was looking for, so I guess I didnt made a good job searching, hehehehehe!

I'm really sorry if I'm polluting the forum, that's really not my intention, please fell free to close the topic, or delete it, if you think it's for the best of the community! =)

I was just hoping that would be something that experts would have in the top of their minds, and would be like a one line reply, like: "in the delayed choice quantum eraser the strange thing is that when we compare the places the signal photon hit in the detector, it makes a wave pattern if we just pick the photons that were entangled with the detector 1", or "the funny thing about the hardy's paradox is that the interference pattern suggests that an electron and a positron did interacted somehow, but didnt annihilated each other"...

I was hoping for an answer like: wigner measures his friends lab because the friend sends an entangled photon and wigner measures it in the horizontal axis or something.

Anyway, thanks again for your reply! At least I found out that the way I thought I understood the experiment was wrong! That's something I could never figure out by myself, hehehehe!
Again, fell free to close or delete the topic! =)

 

[PeterDonis]

how can Wigner measure a Lab?

With very, very, great difficulty--so much so that nobody knows how to do it now, nor do we anticipate that anybody will know how to do it in the foreseeable future.

This is one of the key difficulties with thought experiments of the Wigner's Friend type--they simply assume that Wigner can make measurements and perform quantum operations, including operations similar to "quantum erasure", on the entire lab that contains his friend, the same way that physicists can make measurements and perform quantum operations, including operations like "quantum erasure", on simple systems like qubits today.

 

[Joao]

With very, very, great difficulty--so much so that nobody knows how to do it now, nor do we anticipate that anybody will know how to do it in the foreseeable future.

This is one of the key difficulties with thought experiments of the Wigner's Friend type--they simply assume that Wigner can make measurements and perform quantum operations, including operations similar to "quantum erasure", on the entire lab that contains his friend, the same way that physicists can make measurements and perform quantum operations, including operations like "quantum erasure", on simple systems like qubits today.

Thanks a lot for the reply, once again, Peter! Now I have a clearer vision the experiment! It's very kind of you to take your time to answer my threads! Have a great weekend, and thanks for your amazing efforts moderating this forum! =)

 

[PeterDonis]

Thanks a lot for the reply, once again, Peter!

You're welcome!

thanks for your amazing efforts moderating this forum! =)

Thanks for the kudos!

 

[Secan]

Just to clarify. Is it not Wigner friend invalidates the wave function as real bec of incompatible views. And its possible only in Many Worlds? But all interpretations have same result. So how can it disprove wave function in Copenhagen as real yet possible in Many Worlds?

 

[PeterDonis]

Is it not Wigner friend invalidates the wave function as real bec of incompatible views.

No. What, if anything, the Wigner's friend thought experiment "proves" depends on which interpretation of QM you adopt.

 

[Secan]

No. What, if anything, the Wigner's friend thought experiment "proves" depends on which interpretation of QM you adopt.

If i adopt wave function collapse as real in Copenhagen where the wave function really collapses based on born rule. The Wigner friend falsifies the idea?

 

[PeterDonis]

If i adopt wave function collapse as real in Copenhagen where the wave function really collapses based on born rule. The Wigner friend falsifies the idea?

You have already agreed that all interpretations of QM make the same predictions. That means you cannot falsify any interpretation of QM by comparing predictions with experimental results.

 

[Secan]

You have already agreed that all interpretations of QM make the same predictions. That means you cannot falsify any interpretation of QM by comparing predictions with experimental results.

So how does copenhagen with objective wave function collapse address Wigner friend dilemma? With one view still having superposition. One view having collapse.

 

[PeterDonis]

how does copenhagen with objective wave function collapse address Wigner friend dilemma?

The most obvious way to do so in Copenhagen would be to observe that, if it is possible for Wigner to make quantum measurements on his friend that can detect quantum interference between his friend observing different "measurement results", no collapse of the wave function can have taken place according to Copenhagen, because no actual "measurement" can have occurred; in Copenhagen, a measurement cannot occur, and the Born rule should not even be applied, unless it is impossible for the alternative "results" to interfere.

 

[physika]

.

But all interpretations have same result.

.
Which specific result ?

...concisely please.

.

 

[Secan]

The most obvious way to do so in Copenhagen would be to observe that, if it is possible for Wigner to make quantum measurements on his friend that can detect quantum interference between his friend observing different "measurement results", no collapse of the wave function can have taken place according to Copenhagen, because no actual "measurement" can have occurred; in Copenhagen, a measurement cannot occur, and the Born rule should not even be applied, unless it is impossible for the alternative "results" to interfere.

In other words. Wigner can perceive superpositions and it is many worlds because Wigner friend different results can still be accessed?

Convensionally. When decoherence occurred. Branches are forever split. But how can Wigner still got accessed to both branches of his friend? Maybe Wigner can navigate the Hilbert space at will?

 

[PeterDonis]

Wigner can perceive superpositions and it is many worlds because Wigner friend different results can still be accessed?

No. If no measurement has occurred then there is no "many worlds"; there is just a quantum system that hasn't been measured yet.

When decoherence occurred. Branches are forever split. But how can Wigner still got accessed to both branches of his friend?

If decoherence has occurred, Wigner can't do this.

Before you even try to figure out what particular interpretations say about the Wigner's friend experiment, you first need to figure out what the actual experimental prediction is that QM makes. You do that using the math of QM, which is the same for all interpretations (which is why all interpretations make the same experimental predictions). It is not possible for the math of QM to say both that a measurement has occurred, and that it hasn't, or that both decoherence has occurred, and that it hasn't. The math of QM forces you to say just one or the other.

 

[Secan]

No. If no measurement has occurred then there is no "many worlds"; there is just a quantum system that hasn't been measured yet.
If decoherence has occurred, Wigner can't do this.

Before you even try to figure out what particular interpretations say about the Wigner's friend experiment, you first need to figure out what the actual experimental prediction is that QM makes. You do that using the math of QM, which is the same for all interpretations (which is why all interpretations make the same experimental predictions). It is not possible for the math of QM to say both that a measurement has occurred, and that it hasn't, or that both decoherence has occurred, and that it hasn't. The math of QM forces you to say just one or the other.

Lets take the case if the double slit experiment analogy. When you get a hit then measurement has taken place and it is decoherered. What setup can you still see the superposition? Maybe when you don't know if it passes left or right? But the mere fact you can see the screens mean it is decohered. Because you said "it is possible for Wigner to make quantum measurements on his friend that can detect quantum interference between his friend observing different "measurement results", no collapse of the wave function can have taken place according to Copenhagen". In the case of double slit. If no collapse occurs. There should be no hits because no born rule applied, is it not.

 

[PeterDonis]

When you get a hit then measurement has taken place and it is decoherered.

A hit where? On the detector screen?

What setup can you still see the superposition?

What superposition? Do you mean an interference pattern on the detector screen?

You seem to be confused about the basics of QM; you should not even be trying to understand the Wigner's friend experiment if you haven't yet got a good understanding of the basics of QM.

 

[PeterDonis]

In the case of double slit. If no collapse occurs. There should be no hits because no born rule applied, is it not.

Obviously you have to apply the Born rule in the double slit experiment if you are going to explain how hits appear on the detector screen at all. But you cannot apply the Born rule any place before the detector screen if you want to explain why an interference pattern shows up on the screen after many hits.

 

[Secan]

A hit where? On the detector screen?
What superposition? Do you mean an interference pattern on the detector screen?

You seem to be confused about the basics of QM; you should not even be trying to understand the Wigner's friend experiment if you haven't yet got a good understanding of the basics of QM.

This was because of this confusing statement in msg 15.

The most obvious way to do so in Copenhagen would be to observe that, if it is possible for Wigner to make quantum measurements on his friend that can detect quantum interference between his friend observing different "measurement results", no collapse of the wave function can have taken place according to Copenhagen, because no actual "measurement" can have occurred;

If Wigner can make quantum measurements on his friend that can detect quantum interference between his friend observing different "measurement results (akin to observing hits at the double slit detector). That means there is born rule applied and there is collapse already. Yet you said no collapse of the wave function can have taken place according to Copenhagen, because no actual "measurement" can have occurred. What did you actually mean here? If no born rule and no collapse in the double slits. I know it means no hits on screen in double slit yet so how can Wigner observe his friend interferences if no collapse occured? What were you implying in that sentence?

 

[PeterDonis]

If Wigner can make quantum measurements on his friend that can detect quantum interference between his friend observing different "measurement results (akin to observing hits at the double slit detector). That means there is born rule applied and there is collapse already.

It means the Born rule is applied to Wigner's measurement. But it also means that, if Wigner detects interference, the Born rule cannot be applied before Wigner's measurement--which means it can't be applied to the friend's "measurement" if Wigner detects interference. Just as with the double slit, if you detect interference at the detector screen, you can't tell which slit the particle went through, so you can't apply the Born rule at the slits.

In other words, in the Wigner's friend experiment, having the friend observe a particular result for his measurement is analogous to detecting which slit the particle went through in the double slit experiment--it destroys interference. So if the friend observes a definite result, Wigner can't observe any interference.