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Mark_Friedenbach comments on If Many-Worlds Had Come First - Less Wrong
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By definition, interpretations don't give testable predictions. Theories give testable predictions.
EDIT: having said that, rQM ontology, where information is in relations, not in relata, predicts a feature of the formalism--that when you combine Hilbert spaces, what you have is a product not a sum. That is important for understanding the advantages of quantum computation.
Definitions can be wrong.
I understand that well-meaning physics professor may have once told you that. However the various quantum mechanics interpretations do in fact pre-suppose different underlying mechanisms, and therefore result in different predictions in obscure corner cases. For example, reversible measurement of quantum phenomenon results in different probabilities on the return path in many-worlds vs the Copenhagen interpretation. (Unfortunately we lack the capability at this time to make fully reversible experimental aparatus at this scale.)
A real testable difference between QM interpretations is a Nobel-worthy Big Deal<tm>. I doubt it will be coming.
There are real testable differences:
http://www.hedweb.com/manworld.htm#unique
That page lists three ways in which MWI differs from the Copenhagen interpretation.
One has to two with further constraints that MWI puts on the grand unified theory: namely that gravity must be quantized. If it turns out that gravity is not quantized, that would be strong evidence against the basic MWI explanation.
The second has to do with testable predictions which could be made if it turns out that linearity is violated. Linearity is highly verified, but perhaps it does break down at high energies, in which case it could be used to communicate between or simply observe other Everett branches.
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.
We just lack the capability to make such a device, unfortunately. But feel free to do so and win that Nobel prize.
But such device is not physically realizable, as it would involve reversing the thermodynamic arrow of time.
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.
? What aspect of measuring an electron's spin is not reversible? Physics at this scale is entirely reversible.
If you define a measurement as an the creation of a (FAPP) irreversible record....then, no.
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.
Actually, Nobel does not begin to cover it, whether it would be awarded or not (even J.S. Bell didn't get one, though he was nominated the year he died). Showing experimentally that, say, there is an objective collapse mechanism of some sort would probably be the biggest deal since the invention of QM.
And even just formally applying all the complexity stuff that is alluded to in the sequences, to the question of QM interpretation, would be a rather notable deed.