[Serious] Is the Past Real?

[polymath257]

if chaucer dined without human companions then his menu is still in principle knowable to any rats waiting to dine on the scrap. :p

I think in principle knowable doesn’t imply any actual sentient knower.  it only requires the event to be encoded in some form, for example a collection of photons zipping through space, the maps back to the event in a way that is sufficiently unique such that in principle, if a intelligent knower with unlimited access to all physically feasible means is so inclined, he could reconstruct the details of the event

But that would be a rat *at that time*. Could anyone, even in principle, determine *now* what Chaucer ate for his last meal?

Suppose he ate his last meal in a dark, sealed, room with nobody else there. What would, in principle, allow us to determine what he ate?

The best I can come up with is that whatever radiation Chaucer emitted at that point could, in principle, be detected and, maybe, the event reconstructed. But I am far from sure that could be done even in principle.

Your comment on the preservation of information (essentially, the unitarity of solutions of the Schrodinger equation) is relevant, but it seems to me that even that requires the past to be described probabilistically and the *specific event* may not be knowable even in principle.

Maybe we could, in principle, determine Chaucer had fish with a 60% probability and gruel with a 40% probability and that is all that could be known.