Wigners Friend where is the paradox?

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Discussion Overview

The discussion revolves around the Wigner's Friend thought experiment, exploring the implications of quantum measurement, entanglement, and the nature of observation in quantum mechanics. Participants examine the paradoxes associated with the experiment, considering various interpretations of quantum theory and the role of observers.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • One participant questions the notion of quantum entanglement between Wigner's friend and the observed system, suggesting that measurement leads to a definitive collapse of the wave function.
  • Another participant argues that the friend's lab remains in a superposed state, which contradicts the expected outcome of a definite observation, indicating that the convergence to a collapsed state does not occur.
  • It is proposed that the friend's observation is linked to the state of their measuring device, and that Wigner's perspective could be reconciled by considering the decoherence of the device pointer observable.
  • A later reply suggests that the paradox can be understood through classical theories with epistemic limits, asserting that Wigner's superposed state results from tracking the environment rather than being inherently quantum.
  • Some participants assert that the Wigner's Friend scenario is only paradoxical if one is uncertain about which quantum interpretation to adopt, implying that consistency in interpretation resolves the paradox.

Areas of Agreement / Disagreement

Participants express differing views on whether the Wigner's Friend scenario constitutes a genuine paradox. Some believe it can be resolved through consistent interpretation, while others maintain that the implications of measurement and observation remain contested.

Contextual Notes

Participants highlight the dependence on interpretations of quantum mechanics, such as collapse, many worlds, and Bohmian mechanics, which influence their understanding of the paradox. The discussion also reflects on the role of decoherence and the nature of information in quantum systems.

ManyNames
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So i try and try to understand why physics should suggest a quantum entanglement of wigner friend and the observed system, because instantaneous measurement makes a collapse in the system, there should be no question when the wave function is determined.

Suppose we consider two equations:

\zeta_{\delta^{\alpha=i}_{\beta=i}} |\psi (t)>=\sum_n |\psi_n (\alpha_{\delta}, \beta_{\delta*} (t))|^2

and

\zeta_{\delta^{\alpha=j}_{\beta=j}} |\psi (t)> \ne \sum_n |\psi_n (\alpha_{\delta}, \beta_{\delta*} (t))|^2

It's destinctive immediately that eq. 1 has a squared modulus value solution, whereas the equation with the subscripts containing j in eq. 2 shown here as \delta^{\alpha=j}_{\beta=j} do not lead to a real positive value of 1. The Dirac Delta is used trivially to express when both equations can be valid. In these equations, \alpha is the observer and \beta is the observed. Reference of the states of \alpha and \beta is given by the association of the collapse when both \alpha and \beta converge in a collapse.

If the observer and the observed have not collapsed, then the remain in a superpositioning in a joint state with the observed system:

|\psi \alpha_{\delta_{t_1}} ... ... \psi \alpha_{\delta_(t_2)}> + |\psi \beta_{\delta_{t_1}}... ... \psi \beta_{\delta_{t_2}}>

The joint state can be seen as evolving linearly, and being in a superpositining state then by:

\zeta_{\delta^{\alpha=i}_{\beta=j}} |\psi (t)>=|\psi \alpha(t)> + |\psi \beta(t)>

Remember that iff \alpha and \beta have values of i can they collapse -interaction between the scientist and the state happens - no question of when the wave function truely has collapsed as the scientist holds it as an absolute fact of the exeriment.

Let's take \alpha* as the second observer. If |\psi \alpha_{\delta_{t_1}} ... ... \psi \alpha_{\delta_(t_2)}> + |\psi \beta_{\delta_{t_1}}... ... \psi \beta_{\delta_{t_2}}> converges into a collapse (the collapsed states as the observer and the observed) in an earlier time than which the second observer measures the experiment, then \alpha*(t)<\alpha (t). Information on the state of \beta is then assertained in an early period. The description of:

\zeta_{\delta^{\alpha=i}_{\beta=i}} |\psi (t_1)>=\sum_n |\psi (\alpha, \beta(t_1)|^2

states that the wave function of the observed system has collapsed, and time has been expressed in a past time sense t_1. Since one of the questions of Wigners Friend is if the first observer and the system are themselves in a state of quantum superpositioning?

How could it be though, if not vanishingly small, since we can state that the observers wave function and the observed wavefunction coheres, and the observed system has observable attributes; it seems valid to state the wave function collapsed when the meaurement was first taken, not whether or not a second observer is required.
 
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The supposed paradox is that the friend's lab does not seem to settle down into a collapsed state. If you actually model the evolution they remain superposed, despite that the friend should see an outcome. So it seems Wigner's description of his friend contradicts the definite observation of an outcome. The convergence to collapsed state that you describe does not occur.

However it isn't really a paradox for two reasons.

Firstly the friends observation of an outcome is related to the state of his measuring device. If Wigner traced over the environment within the lab, he would see even his superposed state describes decoherence of the device pointer observable and thus it is fine to take an ignorance based view of its statistics (i.e. there is an outcome but I don't know it).

An even deeper reason it isn't a paradox though is that one can arrange the paradox exactly as it is in Classical theories with an epistemic limit like Spekkens toy model and you can see Wigner's superposed state is just a result of him tracking the environment. His maximum information state isn't as "tight" as that of the friend who only tracks the device.

There's nothing Quantum about Wigner's friend as these Classical theories have it.
 
Wigner friend, like Schrödinger cat or delayed choice, is a paradox only if you are not sure which of the "official" quantum interpretations you use (be it collapse, many worlds, Bohmian or whatever). Once you fix the interpretation and stick to it consistently, there is no paradox.
 
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Demystifier said:
Wigner friend, like Schrödinger cat or delayed choice, is a paradox only if you are not sure which of the "official" quantum interpretations you use (be it collapse, many worlds, Bohmian or whatever). Once you fix the interpretation and stick to it consistently, there is no paradox.
And it's also not even a Quantum paradox really.
 

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