Quantum mechanics, many worlds, and its implications -- I have questions

[PeterDonis]

Not the case.

"collapse is something that happens in our description of the system, not to the system itself".[1

/Fuchs and Peres

https://pubs.aip.org/physicstoday/article/53/3/70/411209/Quantum-Theory-Needs-No-Interpretation

Although the paper claims that QM needs no interpretation, the position it is taking is an interpretation, one with which not all other interpretations would agree.

 

[Username34]

Although the paper claims that QM needs no interpretation, the position it is taking is an interpretation, one with which not all other interpretations would agree.

There is no mainstream interpretation (Copenhagen, Many worlds, Bohm, etc) that suggest a collapse in the system. Obscure ones may but those aren't what textbooks are based on

 

[PeterDonis]

There is no mainstream interpretation (Copenhagen, Many worlds, Bohm, etc) that suggest a collapse in the system.

Yes, there is. In Copenhagen, other branches of the wave function besides the one corresponding to the observed result are said to not exist after the measurement, in contrast to Many Worlds, in which those branches are said to exist after the measurement. The former claim is a claim that collapse, or something that produces the same result (only one branch surviving) is a real thing, not just a feature of our description. Copenhagen just refuses to make any claims about how that happens--what is going on "behind the scenes" that makes it the case that only one branch of the wave function survives after a measurement.

 

[Username34]

Yes, there is. In Copenhagen, other branches of the wave function besides the one corresponding to the observed result are said to not exist after the measurement, in contrast to Many Worlds, in which those branches are said to exist after the measurement. The former claim is a claim that collapse, or something that produces the same result (only one branch surviving) is a real thing, not just a feature of our description. Copenhagen just refuses to make any claims about how that happens--what is going on "behind the scenes" that makes it the case that only one branch of the wave function survives after a measurement.

"Generally, Copenhagen-type interpretations deny that the wave function provides a directly apprehensible image of an ordinary material body or a discernible component of some such,[43][44] or anything more than a theoretical concept".

https://en.m.wikipedia.org/wiki/Copenhagen_interpretation

Thus, if the wave function is viewed as nothing more than a theoretical tool, and we are in line with mainstream science, then no collapse of the system can be said to happen. In other words, there is no reason to think it does collapse based on everything else we know about physics. It would not be a mainstream view to propose that it does.

 

[Username34]

Say you have a casino machine that is 100% reliable/continous, but your description of the processes inside that machine, for future states, is only capable of 50% accuracy. Then your description breaks down but not the machine.

But there is more to it than that.. Quantum Mechanics imposes fundamental problems of measurement, even in theory, due to the Heisenberg Uncertainty principle. That is is not the case with macroscopic physics, hence why QM is unique.

So even if you had everything at your disposal, you could still not precisely predict future states of the machine due to a fundamental feature of the universe.

 

[PeterDonis]

In other words, there is no reason to think it does collapse based on everything else we know about physics.

That's not the case, because we only observe single results of measurements, but before a measurement, our best current understanding of QM--not just theory or interpretation but experiments showing quantum interference effects--indicates that before measurement, multiple possibilities are physically present. The transition between multiple possibilities and just one result is "collapse".

I agree that we do not have any interpretation of QM that describes a specific process by which collapse happens. We have some alternate theories, which modify the math of QM and make predictions that can, in principle, be different from standard QM, but AFAIK all such models so far have been ruled out.

 

[PeterDonis]

Quantum Mechanics imposes fundamental problems of measurement, even in theory, due to the Heisenberg Uncertainty principle. That is is not the case with macroscopic physics, hence why QM is unique.

So even if you had everything at your disposal, you could still not precisely predict future states of the machine due to a fundamental feature of the universe.

This is a common viewpoint, but not all interpretations of QM agree with it.

Many worlds says that the wave function is the state of the system (and of the universe as a whole, if we look at its wave function), and it evolves deterministically. There is no uncertainty anywhere.

Bohmian does not have any uncertainty either because the precise state of the system is contained in the particle positions, which are exact; the wave function is just part of the machinery that guides the particles. The particle positions are stated to be unknown to us, and in practice unknowable, but if we could know them exactly, and if we knew the wave function exactly, we could precisely predict future states and results of measurements.

 

[Username34]

This is a common viewpoint, but not all interpretations of QM agree with it.

Many worlds says that the wave function is the state of the system (and of the universe as a whole, if we look at its wave function), and it evolves deterministically. There is no uncertainty anywhere.

Bohmian does not have any uncertainty either because the precise state of the system is contained in the particle positions, which are exact; the wave function is just part of the machinery that guides the particles. The particle positions are stated to be unknown to us, and in practice unknowable, but if we could know them exactly, and if we knew the wave function exactly, we could precisely predict future states and results of measurements.

The Heisenberger Uncertainty Principle would still hold true for MWI in that you can never know which world you end up in when performing a measurement.

 

[PeterDonis]

The Heisenberger Uncertainty Principle would still hold true for MWI in that you can never know which world you end up in when performing a measurement.

No, that's not correct, because in the MWI, there is no "which world you end up in when performing a measurement". That concept doesn't even make sense in the MWI. You end up in all the worlds. All the possible results of the measurement (i.e., all the ones with a nonzero amplitude in the pre-measurement wave function) happen.

 

[Username34]

No, that's not correct, because in the MWI, there is no "which world you end up in when performing a measurement". That concept doesn't even make sense in the MWI. You end up in all the worlds. All the possible results of the measurement (i.e., all the ones with a nonzero amplitude in the pre-measurement wave function) happen.

You can't see those outcomes hence why it's doubtful to advocate Many Worlds as science in reference to Heisenberger Uncertainty Principle, which is viewed as science.

 

[PeterDonis]

You can't see those outcomes hence why it's doubtful to advocate Many Worlds as science in reference to Heisenberger Uncertainty Principle, which is viewed as science.

You yourself listed Many worlds as a "mainstream" interpretation of QM.

 

[Nugatory]

The Heisenberger Uncertainty Principle would still hold true for MWI in that you can never know which world you end up in when performing a measurement.

The uncertainty principle is a constraint on state preparation and has nothing to do with which world you end up in (MWI interpretation) or the certainty of your measurement results (collapse, Copenhagen, Bohm, …).

 

[Username34]

You yourself listed Many worlds as a "mainstream" interpretation of QM.

Yeah, due to popularity. At any event, MWI advocate David Deutch considers it possible to speak in terms of "which world you end up in" without contradicting yourself. It is not a pleasent dicussion to be had though.

 

[PeterDonis]

MWI advocate David Deutch considers it possible to speak in terms of "which world you end up in" without contradicting yourself.

Please give a reference.

 

[Username34]

The uncertainty principle is a constraint on state preparation and has nothing to do with which world you end up in (MWI interpretation) or the certainty of your measurement results (collapse, Copenhagen, Bohm, …).

The Uncertainty Principle accounts for why you can't know the result of a measurement in advance.

 

[Username34]

That's not the case, because we only observe single results of measurements, but before a measurement, our best current understanding of QM--not just theory or interpretation but experiments showing quantum interference effects--indicates that before measurement, multiple possibilities are physically present. The transition between multiple possibilities and just one result is "collapse".

The point is that Einsteins relativity is deterministic. Unless you show that theory to be incorrect, there's no reason to adopt an interpretation of QM where there is a system collapse. I am not ready to dispatch of special relativity anytime soon.

 

[Username34]

Let's not forget the fact that the history of Quantum Mechanics and the Copenhagen interpretation(s) predate the Schrodinger equation, when views on collapse started to soften. Prior to the Schrodinger equation there was a universally held belief that there is a system collapse among the Bohr group. And this is the story that is passed on in popular science articles.

 

[berkeman]

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