Doesn't Wigner's Friend Experiment solve the measurement problem?

  • #1
If you look at the recent Wigner's Friend experiment, it seems to support Carlo Rovelli's Relational Interpretation which says there's no real measurement.

Wiger's Friend carries out a polarization measurement. Before he does, the quantum system is in a superposition of horizontal/vertical polarization. He carries out a measurement and gets horizontal. He records that outcome. The record and the quantum system are sent to Wigner outside of the lab. Wigner checks for interference and sees it. He concludes that his friend in the lab hasn't carried out a measurement. Here's more from the Relational Interpretation.

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 possess 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

What this says is that wavefunction collapse and what’s called self measurement doesn’t occur. What we call measurement isn’t a problem. It’s just some observer gaining information about a quantum system. The problem occurs because people assume a measurement must cause “collapse” even though this isn’t anywhere to be found in Quantum Mechanics or Quantum Field Theory.

So an observer gains knowledge about the quantum system and it’s wavefunction just expands to include the observer's obtaining knowledge about the quantum state. So an observer that’s entangled with the wavefunction of the quantum system can’t measure interference because he or she is a part of the entire system described by the wave function. So Schrodinger’s cat is alive and dead.

Now, an observer O’ that’s external to the S+O system in the lab in the case of Wigner’s friend, is a quantum system and Wigner can do an interference measurement and see his friend, the system and the lab in a superposition of both states.

So, you start with a quantum system in superposition, when you learn the state of the system you become part of the system. There's no collapse or measurements, you just become part of an O+S system and there's a version of you that sees vertical polarization and a version of you that see horizontal polarization. You can't see interference because you're part of the O+S system but an observer O' outside of the O+S system can measure interference until O' becomes part of the quantum system.
 

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  • #2
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O itself cannot say anything with respect to its own "state", because its own "state" is defined only relative to another observer, O'.
What do you mean by "say"? What does it mean that an electron can "say" something?
 
  • #3
What do you mean by "say"? What does it mean that an electron can "say" something?

It's not saying the electron "says" anything.

In this case, the electron would be S and the one who says would be O or the Observer in an S+O system. So O couldn't say anything about it's own state when it becomes part of an S+O system. Another Observer, O' can say something about the S+O system.

Say Wigner's Friend(O) is going into the lab to measure the spin of an electron(S). Before the measurement, the spin is in a superposition of up/down. Wigner's Friend(O) becomes part of the S+O system when he observes the state of the electron.

The S+O system now contains spin up/spin down, an observer observing both spin up and spin down, measuring apparatus that measures both spin up and spin down and a lab where spin up is measured and a lab where spin down is measured.

O can only interact with one part of the system therefore he thinks he observes collapse. He can't measure interference because he's part of the S+O system and doesn't know it's own Hamiltonian or interaction Hamiltonian for the measurement.

Now O' or Wigner(Observer) outside of the lab can do an interference measurement on the entire S+O system and he sees interference. He can conclude that his friend(O) in the lab never carried out a measurement even though in the lab he did carry out a measurement and recorded the results.

So a measurement or collapse never occurs. It's just O has a lack of information about the S+O system and sees what he thinks is a classical outcome but O' outside of the S+O system can see that there wasn't any collapse or measurement.

So measurement is relative to the observer because the observer has a lack of information when he becomes apart of the S+O system when he interacts with the electron or photon which would be S in the S+O system.
 
  • #4
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In this case, the electron would be S and the one who says would be O or the Observer in an S+O system.
Can RQM be formulated without reference to observers? I think Rovelli says that it can, but from your formulation it seems that it can't.
 
  • #5
Can RQM be formulated without reference to observers? I think Rovelli says that it can, but from your formulation it seems that it can't.

I don't see how RQM can be formulated without reference to observers. It's the observer's reference frame that's paramount in RQM. Here's an article on RQM from Scientific American.

One of the weirdest theoretical implications of quantum mechanics is that different observers can give different—though equally valid—accounts of the same sequence of events. As highlighted by physicist Carlo Rovelli in his relational quantum mechanics (RQM), this means that there should be no absolute, observer-independent physical quantities. All physical quantities—the whole physical universe—must be relative to the observer. The notion that we all share the same physical environment must, therefore, be an illusion.

Such a counterintuitive prediction—which seems to flirt dangerously with solipsism—has been clamoring for experimental verification for decades. But only recently has technology advanced far enough to allow for it. So now, at last, Massimiliano Proietti and collaborators at Heriot-Watt University, in the U.K., seem to have confirmed RQM; as predicted by quantum mechanics, there may well be no objective physical world.


https://blogs.scientificamerican.com/observations/the-universe-as-cosmic-dashboard/

Rovelli says a conscious observer is no different than a non conscious observer. I disagree with him.

An observer can measure the state of a quantum system and record that state in it's memory. A non conscious observer can record the state of a system in it's memory and that's just stored information. Something from the outside has to extract that information.

A conscious observer can measure the state of a quantum system and can extract that information from within. Conscious observers can write books about it, publish papers or build technologies around the information.

So as far I know RQM can't be formulated without observers.
 
  • #6
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Massimiliano Proietti and collaborators at Heriot-Watt University, in the U.K., seem to have confirmed RQM;

This is, unfortunately, the kind of overblown claim that even Scientific American now routinely peddles.

For multiple previous PF discussions of the experiment described in that paper and what it actually does or does not show, see this search:

https://www.physicsforums.com/search/1935924/?q=1902.05080&o=relevance\

The short version: the results of this experiment match the predictions of standard QM, and all QM interpretations agree on those predictions, so no such experiment can possibly confirm any one interpretation over the others.
 
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  • #7
This is, unfortunately, the kind of overblown claim that even Scientific American now routinely peddles.

For multiple previous PF discussions of the experiment described in that paper and what it actually does or does not show, see this search:

https://www.physicsforums.com/search/1935924/?q=1902.05080&o=relevance\

The short version: the results of this experiment match the predictions of standard QM, and all QM interpretations agree on those predictions, so no such experiment can possibly confirm any one interpretation over the others.

I disagree.

I don't see the experiment supporting many worlds because you still need an interpretation of the evidence. You have to say you think that Wigner and his Friend are measuring the same global physical wave function. This experiment doesn't support a global physical wave function in any way.

With RQM, the evidence supports it. It doesn't postulate a global, physical wave function. It says what we call collapse of the wave function isn't really collapse it's just these measurements are relative to the observer.

So Wigner's Friend can measure vertical polarization and this is only "real" in the lab. Wigner outside of the lab can still measure interference and conclude his friend hasn't carried out the experiment in the lab.

This is RQM. There's no global, physical wave function and there isn't any evidence of a global, physical wave function in this experiment.
 
  • #8
PeterDonis
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I don't see the experiment supporting many worlds

As I said, no experiment can "support" or "disprove" any QM interpretation, because they all make the same predictions for the outcomes of all experiments.

you still need an interpretation of the evidence

Interpretation is different from prediction. Prediction is the actual numbers from the theory that get compared with the numbers from the data. "Interpretation" is a story you tell about "what happened", but doesn't change any of the numbers.

With RQM, the evidence supports it.

The evidence supports all QM interpretations, because they all make the same predictions for what evidence to expect.
 
  • #10
Good to know!

It's a very important distinction in this debate. I've watched a few of Rovelli's lectures on RQM and on time and he comes right up to the line in these areas.

I think RQM illuminates many things when it comes to how we should define consciousness apart from awareness. In order to do this, you first need to go back to the origins of Wigner's thought experiment.

It started with John Von Neumann. He basically reasoned that everything physical is made of atoms and so everything physical is described by a wavefunction. Therefore nothing physical can collapse the wavefunction. He then said something non physical must collapse the wavefunction and he said this must be consciousness.

Wigner agreed at the time and came up with the Wigner's Friend thought experiment.

The thought experiment was designed to show consciousness collapses the wave function. So his friend in the lab carries out a measurement and records the results in the lab. His consciousness has collapsed the wavefunction in the lab but outside of the lab, the wave function didn't collapse because Wigner isn't conscious of his friend's results.

Fast forward to today, and I think Wigner's Friend actually supports primordial quantum consciousness and awareness.

Consciousness needs to be redefined. A measuring apparatus would be conscious because it contains knowledge in it's memory of the quantum state. This is different from awareness of the knowledge that's recorded in the memory of an observer.

This would suggest the universe is conscious. It simply contains recorded knowledge about the quantum state. This would be stored knowledge about the system. It would take something outside of the system to extract that information. It also points to a primordial awareness.

If Wigner's Friend calls him up and says, hey Wigner I measured vertical polarization then Wigner can no longer see interference. The wave function has "collapsed" for Wigner so to speak.

The question has to be asked. How did the quantum system know that the friend called Wigner and told him about the state the system is in? Did the system eavesdrop on the phone call or is knowledge of the quantum state also a quantum system?

If so, then it's easy to see quantum consciousness.

With MWI, it's a different story. Everett added the global physical wavefunction in an ad hoc way. He must have realized his initial formalism still centered on the observer. In fact, RQM is partly built on Everett's initial relative state formulation which didn't include a physical global wavefunction.

So when O' doesn't have knowledge that O recorded about S then O' can still measure interference between the S+O system. When O' learns about the recorded state of S then O' can no longer measure interference. We can conclude that S must be aware of O's knowledge about its state.

How could you reach any other conclusion based on the experiment?
 
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  • #11
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It started with John Von Neumann. He basically reasoned that everything physical is made of atoms and so everything physical is described by a wavefunction. Therefore nothing physical can collapse the wavefunction. He then said something non physical must collapse the wavefunction and he said this must be consciousness.

That was not his reasoning. He proved the quantum-classical cut can be placed anywhere. The only place that was 'different' was the consciousness of the observer. Wigner also held that view except towards the end after seeing early work on decoherence by Zeth (I think). He then realised there was a special place - where decoherence occured and changed his mind.

Secondly QM is a mathematical model that describes the physics - what it is describing, because it is outside our direct experience, is anyone's guess. Our physical intuition is developed here in the classical world, and by the classical world. Taking that intuition and applying it to a realm removed from that leaves just one route - a mathematical description, rather than anything in terms of our everyday experience. You can try this idea, that idea - I have tried far too many myself - but in the end without an experiment to distinguish one idea from the other, you really have not achieved that much. Sure you get a deeper appreciation of the issues involved, but you get no closer to what Einstein wanted: 'What really interests me is whether God could have created the world any differently; in other words, whether the requirement of logical simplicity admits a margin of freedom.'

Personally I think the question should be reversed - why has mathematical beauty (ie logical simplicity) proved such a good approach:
https://www.ted.com/talks/murray_gell_mann_beauty_truth_and_physics/transcript?language=en#t-5678

Thanks
Bill
 
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  • #12
That was not his reasoning. He proved the quantum-classical cut can be placed anywhere. The only place that was 'different' was the consciousness of the observer. Wigner also held that view except towards the end after seeing early work on decoherence by Zeth (I think). He then realised there was a special place - where decoherence occured and changed his mind.

Secondly QM is a mathematical model that describes the physics - what it is describing, because it is outside our direct experience, is anyone's guess. Our physical intuition is developed here in the classical world, and by the classical world. Taking that intuition and applying it to a realm removed from that leaves just one route - a mathematical description, rather than anything in terms of our everyday experience. You can try this idea, that idea - I have tried far too many myself - but in the end without an experiment to distinguish one idea from the other, you really have not achieved that much. Sure you get a deeper appreciation of the issues involved, but you get no closer to what Einstein wanted: 'What really interests me is whether God could have created the world any differently; in other words, whether the requirement of logical simplicity admits a margin of freedom.'

Personally I think the question should be reversed - why has mathematical beauty (ie logical simplicity) proved such a good approach:
https://www.ted.com/talks/murray_gell_mann_beauty_truth_and_physics/transcript?language=en#t-5678

Thanks
Bill

I think a lot of this comes down to how we define consciousness. Definitions are just the way we describe some underlying reality. We call a cup a cup but if we started off calling a cup a shoe it will just be a different name of the same thing.

I think the first cause of confusion with consciousness is the way we define it. We define consciousness and awareness of consciousness as basically the same thing and I think they're different.

If you simply describe consciousness as a system that can measure and record information in it's memory about the quantum state or it's environment then it's conscious.

By this definition, the universe would be conscious and science would be intelligent humans discovering information stored in the memory of the universe. There's been a few recent studies in this area.

Gravity’s Residue
An unusual approach to unifying the laws of physics could solve Hawking’s black-hole information paradox—and its predicted gravitational "memory effect" could be picked up by LIGO.


https://fqxi.org/community/articles/display/233

You then look at recent studies on entanglement in time.

Another example of temporal entanglement comes from a team led by Stephen Brierley, a mathematical physicist at the University of Cambridge. In a paper last year, Brierley and his collaborators explored the bizarre intersection of entanglement, information and time. If Alice and Bob choose from just two polarizer orientations, the correlations they see are readily explained by a particle carrying a single bit. But if they choose among eight possible directions and they measure and remeasure the particle 16 times, they see correlations that a single bit of memory can’t explain. “What we have proven rigorously is that, if you propagate in time the number of bits that corresponds to this Holevo bound, then you definitely cannot explain what quantum mechanics predicts,” said Tomasz Paterek, a physicist at Nanyang Technological University in Singapore, and one of Brierley’s co-authors. In short, what Alice does to the particle at the beginning of the experiment is correlated with what Bob sees at the end in a way that’s too strong to be easily explained. You might call this “supermemory,” except that the category of “memory” doesn’t seem to capture what’s going on.

https://www.quantamagazine.org/time-entanglement-raises-quantum-mysteries-20160119/

Here's the paper:

Non-classicality of temporal correlations

https://arxiv.org/abs/1501.03505

So if the universe can record information in it's memory than it's conscious. It's an observer.

There's a difference between conscious and awareness of information stored in conscious memory. Living organisms have awareness of consciousness and I think this is quantum in nature.

Animals have awareness of consciousness but it's not as robust as humans awareness of consciousness which allows us to build civilizations.

I'm working on a paper now that will show that there's Quantum Awareness and we're either dealing with a weak Quantum Awareness or what I call the (QA) postulate or a stronger more robust (QA) postulate.
 
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  • #13
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Wiger's Friend carries out a polarization measurement. Before he does, the quantum system is in a superposition of horizontal/vertical polarization. He carries out a measurement and gets horizontal. He records that outcome. The record and the quantum system are sent to Wigner outside of the lab. Wigner checks for interference and sees it. He concludes that his friend in the lab hasn't carried out a measurement.
No, Wigner (an RQM guy) concludes that his friend in the lab is in superposition of having measured vertical and having measured horizontal. He does not conclude the measurement has not been taken. He can know it has been done if the friend does it at a time agreed upon beforehand.
 

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