[Serious] Is the Past Real?

[HappySkeptic]

i think to put it even more fundamentally, knowable for event A means event A can in principle influence some event B such that progress of event B depends on the specifics of event A

And what if A is in a superposition state?

What is required for your scenario to happen is that the state of A decay *irreversibly* and then affect B. But what happens if the decay isn't irreversible over the long run? Then B also goes into a superposition state and all we can say about A is probabilities, not anything definite.

You can tie yourself in knots with this.  I agree.  This argument applies to irreversible events.

Whether an event is reversible depends on a number of factors. 

It requires the information about the event to be sufficiently reified in the environment.  Usually this requires the information to be both replicated and entangled with an environment (see decoherence theory and pointer states).

Then, there is the problem of what constitutes the environment?  For instance, Schrodinger's cat is a valid thought experiment if it the box were magical, not allowing information (no matter how scrambled) about the cat's state to exit.  If that were possible, the cat, despite being an observer of his own demise, and therefore "collapsing" the wavefunction, is still part of the "system under test", and not the environment from the point-of-view of the person outside.  Therefore, the cat would still be in a superposition to the outside observer (unless some form of objective collapse theory is true for macroscopic systems).

This seeming contradiction is part of the problem of QM, and we have no good way to get data to find what we are missing.  According to QM, the universe is in a superposition, and there is no single past or future, because the wavefunction of the universe cannot collapse itself (and there is no outside observer).  Since this doesn't seem to be the case, we don't understand QM (despite the claims of Many Worlds proponents, that have no explanation for why "you" follow one path and not another).