Finally, there’s an actual testable prediction: make a reversible device to measure electron spin. Measure one axis to prepare the electron. Measure an orthogonal axis, then reverse that measurement. Finally measure again on the first axis. You’ve lost your recording of the 2nd measurement, but in Copenhagen the 1st and 3rd should agree 50% of the time by random chance, because there was an intermediate collapse, whereas in MWI they agree 100% of the time, because the physical process was fully reversed, bringing the branches back into coherence.
But such device is not physically realizable, as it would involve reversing the thermodynamic arrow of time.
You can reversibly entangle an electron’s spin to the state of some other small quantum system, that’s not questioned by any interpretation of QM, but unless this entanglement propagates to the point of producing a macroscopic effect, it is not considered a measurement.
It’s even worse than that. Zurek’s einselection relies on decoherence to get rid of non-eigenstates, and reversibility is necessarily lost in this (MWI-compatible) model of measurement. There is no size restriction, but the measurement apparatus (including the observer looking at it) must necessarily leak information to the environment to work as a detector. Thus a reversible computation would not be classically detectable.
Which is why the experiment as described in the link I provided requires an artificial intelligence running on a reversible computing substrate to perform the experiment in order to provide the macroscopic effect.
Indeed. Truly reversing the measurement would involve also forgetting what the result of the measurement was, and Copenhagenists would claim this forgotten intermediate result does not count as a “measurement” in the sense of something that (supposedly) collapses the wave function.
But such device is not physically realizable, as it would involve reversing the thermodynamic arrow of time.
? What aspect of measuring an electron’s spin is not reversible? Physics at this scale is entirely reversible.
You can reversibly entangle an electron’s spin to the state of some other small quantum system, that’s not questioned by any interpretation of QM, but unless this entanglement propagates to the point of producing a macroscopic effect, it is not considered a measurement.
It’s even worse than that. Zurek’s einselection relies on decoherence to get rid of non-eigenstates, and reversibility is necessarily lost in this (MWI-compatible) model of measurement. There is no size restriction, but the measurement apparatus (including the observer looking at it) must necessarily leak information to the environment to work as a detector. Thus a reversible computation would not be classically detectable.
Which is why the experiment as described in the link I provided requires an artificial intelligence running on a reversible computing substrate to perform the experiment in order to provide the macroscopic effect.
That is, it would require inverting the thermodynamic arrow of time.
If you define a measurement as an the creation of a (FAPP) irreversible record....then, no.
Indeed. Truly reversing the measurement would involve also forgetting what the result of the measurement was, and Copenhagenists would claim this forgotten intermediate result does not count as a “measurement” in the sense of something that (supposedly) collapses the wave function.