Reversibility

[computably]

@khinsen

The discussion became a lot more productive when people started to look at it pragmatically, in terms of what can be observed in experiments. The most important concept emerging from this shift was decoherence. But physicists never managed to disentangle this point of view from the continuing philosophical debate about the true nature of the wave function.

The main issue in the pragmatic debate is the nature of measurement. Measurement … must transfer sufficient information about the system, and that sets a lower bound to the perturbation implied. But then the agreement ends. Some say we could look at the system plus the instrument as … its combined wave function. Which, unfortunately, nobody has been able to actually do for some real measurement device. In theory, this approach eliminates collapse as something fundamental. At the other extreme, some maintain that measurement requires a conscious observer, and that implies such a huge bunch of atoms that studying its wave function is not even envisageable.

I think we mostly agree on the relevant facts? i.e. There are too many unknowns to make absolute claims like “physical interactions are fundamentally reversible” without caveats.

That said, if I get your point correctly, you question whether the QM picture is accurate or even testable for macroscopic systems. To comment on that point, I thought e.g. superconductors and Bose-Einstein condensates constitute macroscopic quantum phenomena (to be specific given the context, phenomena that are presently best explained by the SM).

Taking a step back, while some interpretations might differ only in purely philosophical ways, isn’t the open question simply how accurate the wave function picture is? I don’t see questions of “are wave functions real?” or “is collapse real?” as categorically different than “is there a coherent theory of quantum gravity?” as long as the one interprets the questions and answers as empirical.

[khinsen]

@computably

if I get your point correctly, you question whether the QM picture is accurate or even testable for macroscopic systems.

Exactly. There some macroscopic quantum effects, such as superconductivity, but those are exceptions and in fact large but very simply structured systems. We cannot solve the QM equations for a drop of water, so we cannot test if QM is applicable to a drop of water.

@computably

isn’t the open question simply how accurate the wave function picture is?

If you ask that question, you already imply that it might not be the Fundamental Truth of the universe. Which I agree with! I’d phrase the question a bit more generally: what are the limits of validity of the wave function picture?

[spenc]

I think reversibility is cool, but a lot easier to achieve than uncovering the mysteries of the universe.

For the parts of your software that are able to be modeled deterministically and you have control over, you can “go backwards” by restoring a snapshot of the previous state. More details are in this old USENIX talk: Eidetic Systems | USENIX

[neauoire]

Watched the talk over coffee this morning, it has nothing to do with reversibility, but it was interesting nonetheless.

[spenc]

In some sense, it would be logically reversible, since it’s not destroying information. So for a given purely deterministic machine every state has a unique predecessor. But in retrospect this seems a silly point to argue

Agreed it has nothing to do with physically reversible computers & their incredible energy savings which are also extremely neat.