[effie]

You are using "real" in a way I don't recognize. In my view, the past that is real is that which corresponds to and is consistent with records of past events. This allows for many different pasts, and many can have some plausibility. But if my diary on a given date says I popped my knee, then any past which is inconsistent with this record is not real.

In orthodox quantum theory, past events, even those that happened in experiments showing quantum effects, such as double-slit experiment with single particle, are determinate in the sense they can sometimes be retrodicted from the present knowledge, even when they could not have been predicted before they happened (e.g. which hole the particle went); only future events are not determined by psi.

[fallingfrog]

No, that’s not correct- there are no hidden variables such that a particle really took only one path to a point and we just don’t know which one it was. It really did go through all of them. All the ones that are consistent with the current state of the system. (So, no I’m not talking about pasts that are not consistent like your popped knee).

And that’s super cool, and something not many people understand! It’s the basis of the schrodingers cat experiment- you’ve heard of it. The cat is really, actually both alive and dead before the system is opened to the world, assuming that no information leaves or enters the box. It’s not that we just don’t know. It’s actually in both states. And although it’s practically speaking impossible to do the experiment, you could throw different versions of the cat through some diffraction grating a zillion times and prove that yes, the live and dead version interfere with each other. They’re both real.

I hope I’m blowing your mind a little bit with this, or if not at least being entertaining :). It’s super counterintuitive!

[effie]

> there are no hidden variables such that a particle really took only one path to a point and we just don’t know which one it was.

We don't know that. It is a statement of your belief in such interpretation of quantum theory.

But I wasn't suggesting hidden variables predict which path will be taken - that is a different idea. I admit my description was somewhat confusing. My point is that retrodiction of trajectory is much easier than prediction of trajectory, because of records of the past. In other words, past is constrained by existing records of the past, while future is not constrained by records of the future, because those do not exist yet.

This retrodicted past is still loaded with uncertainty stemming from the records having uncertainty, and is speculative and not experimentally testable, of course.

> It really did go through all of them. All the ones that are consistent with the current state of the system.

This is also a speculative statement that is experimentally untestable. And I agree it is consistent with quantum theory, if we add "...and consistent with records of the past". So both stances are just an interpretation of what is going on, there is no experimentally testable idea here.