Given decoherence, do we still need random quantum jumps and

[Giulio Prisco]

This thread was posted and discussed on Physics Overflow, I am re-posting it here to hear other opinions.
http://www.physicsoverflow.org/36063/given-decoherence-still-random-quantum-jumps-interpretations?

Environmental decoherence explains how the wavefunction of a quantum system q, as a result of the inevitable interactions and entanglement with the environment, appears to collapse upon measurement (Von Neumann's Process 1).

As far as I understand, decoherence works without needing a "real" collapse. The wavefunction of the total system (q plus environment and measurement equipment) continues to evolve according to the deterministic equations of quantum mechanics (Von Neumann's Process 2) and the total information is conserved, but q alone appears to undergo a quantum jump and lose information. The evolution of q taken alone appears as effectively random, but "q taken alone" is an approximation, and the evolution of the total entangled system q+environment is not random.

Decoherence explains why the world seems classical instead of quantum (with the exception of very carefully prepared cases difficult to realize). Decoherence explains why Scroedinger's cat is always either alive or dead, never in a combination of alive and dead states, without requiring ad-hoc additions to the equations of quantum mechanics (such as Von Neumann's Process 1). There is no Process 1, only Process 2.

Questions: Given the success and popularity of decoherence theories, are there reasons to stick to Von Neumann's notion of random collapse? Do we still need an interpretation of quantum physics?

The authors of "Decoherence and the Appearance of a Classical World in Quantum Theory" (which I believe is the "standard" text) do not seem to entirely agree on these points. What do you guys think?

 

[Demystifier]

Decoherence explains why the world seems classical instead of quantum

No exactly. Decoherence explains why probabilistic description of a large ensemble seems classical instead of quantum. But it does not explain why individual events seem classical instead of quantum.

 

[Giulio Prisco]

No exactly. Decoherence explains why probabilistic description of a large ensemble seems classical instead of quantum. But it does not explain why individual events seem classical instead of quantum.

Thanks Demystifier. I think you are referring to the fact that the outcome of an individual event is not predicted. In fact, decoherence effects have been shown to turn amplitudes to probabilities, and a pure state into a mixture of alternative outcomes, one of which is realized, but decoherence theory doesn't predict which one of the alternative outcomes is realized.

However, it seems to be that, for consistency in the decoherence approach, we should think of the outcome as somehow determined by the environment.

 

[Demystifier]

I think you are referring to the fact that the outcome of an individual event is not predicted.

Yes.

However, it seems to be that, for consistency in the decoherence approach, we should think of the outcome as somehow determined by the environment.

I don't see how the environment in the decoherence approach could determine the outcome.

 

[Demystifier]

Do we still need an interpretation of quantum physics?

Absolutely.

 

[Giulio Prisco]

Yes. I don't see how the environment in the decoherence approach could determine the outcome.

Neither do I, but isn't that required for the consistency of the approach? If the system (the observed subsystem plus the measurement apparatus plus the environment) evolves deterministically according to the equations of quantum mechanics, shouldn't the outcome be determined by some aspects of the environment (in-principle, though not in-practice).

 

[Giulio Prisco]

Absolutely.

Why? What's wrong with quantum mechanics if decoherence can explain the only _experimental fact_ that wasn't previously explained (the fact that we don't see macroscopic superpositions)? I mean, yes there are other "weird" things like non-local entanglement, but couldn't we just live with that ("it''s just what nature does") if the theory always agrees with experimental evidence?

 

[bhobba]

If you want to understand what decoherence does and does not do here is the book to get:
https://www.amazon.com/dp/3642071422/?tag=pfamazon01-20

Technically the issue is how does an improper mixed state become a proper one. Informally its why do we get any outcomes at all.

Thanks
Bill

 

[Giulio Prisco]

If you want to understand what decoherence does and does not do here is the book to get:
https://www.amazon.com/dp/3642071422/?tag=pfamazon01-20

Technically the issue is how does an improper mixed state become a proper one. Informally its why do we get any outcomes at all.

Thanks
Bill

Thanks Bill, I have Schlosshauer's book and have read it until the end of Chapter 2. It's a great book.

 

[bhobba]

Why?

Because QM interpretations are often (not always of course, but often) simply arguments about the meaning of probability, which is a minefield of philosophical 'speculation' without any actual resolution:
http://math.ucr.edu/home/baez/bayes.html

At a deep level many things in applied math is a bit of a morass more suited to philosophy than math or physics, which is why you often only find discussions about it in the philosophy literature, and part of the reason by forum rules we don't really discuss it here.

For example in actuarial studies often a decision theory approach like that used in MW is how its viewed, but in queuing theory a frequentest approach seems the most intuitive. Neither is right or wrong, and you can do it either way, but depending on application one seems more natural than the other.

Thanks
Bill

 

[Nugatory]

If the system (the observed subsystem plus the measurement apparatus plus the environment) evolves deterministically according to the equations of quantum mechanics, shouldn't the outcome be determined by some aspects of the environment (in-principle, though not in-practice).

No. The system may evolve deterministically according to the equations of quantum mechanics, but we also need the initial conditions to determine the outcome, and these aren't available. The situation is somewhat different than in classical mechanics where, as Laplace suggested, if we just knew the exact position and momentum of every particle in the universe at some moment, the future evolution of the universe would be determined by Newton's laws (in principle, not in practice). In quantum mechanics it is not possible, even in principle, to completely specify the position and momentum of every particle in the universe.

 

[Demystifier]

Why? What's wrong with quantum mechanics if decoherence can explain the only _experimental fact_ that wasn't previously explained (the fact that we don't see macroscopic superpositions)? I mean, yes there are other "weird" things like non-local entanglement, but couldn't we just live with that ("it''s just what nature does") if the theory always agrees with experimental evidence?

Yes, we can live with that. But it doesn't mean that we don't need an interpretation. Even a minimal interpretation such as "the only purpose of QM is to predict probabilities of measurement outcomes" - is an interpretation.

 

[Giulio Prisco]

No. The system may evolve deterministically according to the equations of quantum mechanics, but we also need the initial conditions to determine the outcome, and these aren't available. The situation is somewhat different than in classical mechanics where, as Laplace suggested, if we just knew the exact position and momentum of every particle in the universe at some moment, the future evolution of the universe would be determined by Newton's laws (in principle, not in practice). In quantum mechanics it is not possible, even in principle, to completely specify the position and momentum of every particle in the universe.

OK but you are assuming that the initial conditions needed to establish the outcome (of "collapse" in the decoherence approach) are conjugate variables like position and momenta. Couldn't the needed initial condition be such that we could (in-principle) specify them up to any accuracy?

 

[Giulio Prisco]

Yes, we can live with that. But it doesn't mean that we don't need an interpretation. Even a minimal interpretation such as "the only purpose of QM is to predict probabilities of measurement outcomes" - is an interpretation.

And a quite sensible one I believe. Yet, it's difficult to resist the temptation to speculate about ultimate reality...

 

[bhobba]

OK but you are assuming that the initial conditions needed to establish the outcome (of "collapse" in the decoherence approach) are conjugate variables like position and momenta. Couldn't the needed initial condition be such that we could (in-principle) specify them up to any accuracy?

What he is saying is that we have realistic deterministic interpretations on QM like CM. In such interpretations probabilities occur because we can't know the initial conditions to predict outcomes. Its fully deterministic ,common sense etc etc but due to its quirky nature it looks probabilistic because you can't in principle know enough.

It not only illuminates QM foundations, but probability applications, where we see another way probabilities may be all than can be predicted rather than something simply being random in mature.

Thanks
Bill

 

[bhobba]

And a quite sensible one I believe. Yet, it's difficult to resist the temptation to speculate about ultimate reality...

Plus fun and educational as long as you know that's what you are doing and its purpose.

Thanks
Bill