- #1

allisrelative

- 26

- 3

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 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).

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

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.