[Serious] Is the Past Real?

[Jehanne]

Exactly. It has a truth value that *could* be determined.

Now, for quantum events, that is not the case.

So, suppose that I pick a single uranium atom. ALL we have is probabilities for when it will decay. We do NOT have, even in principle, a way to determine when it will decay. So the statement that it will decay tomorrow at noon is neither true nor false.

But I can go further. Suppose I have isolated a uranium atom last week. I have neither looked at it nor had any detector consider it. Is the statement that it decayed two days ago at noon one that has a truth value?

I would say not, just as the statement that it will decay tomorrow at noon does not.

I think we need to separate what is in principle unknowable to us, as opposed to what is in principle not knowable at all, when we say whether the past is real.    It seems to me If it is in principle knowable to someone somewhere, then it is real.   It is just that we don’t know it and can’t know it.  Chaucer’s last meal is in principle knowable to someone, even if we hypothesize that some quantum event forever obscures it from us and anyone with whom we in principle can communicate. 

It seems to be that the principle of indestructibility of information says all past must be knowable to someone somewhere, so it is real.

What if Chaucer dined alone before his death?

[Anomalocaris]

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

[Anomalocaris]

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

[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.

[polymath257]

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.

[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).

[Angrboda]

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.

If I were to travel to the time of Chaucer's meal, would I deterministically land in his actual past, or only probabilistically?

[Jehanne]

If I were to travel to the time of Chaucer's meal, would I deterministically land in his actual past, or only probabilistically?

What? You didn't see Back to the Future?!

[polymath257]

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.

If I were to travel to the time of Chaucer's meal, would I deterministically land in his actual past, or only probabilistically?

Maybe it depends on how you travel. Any ideas?

And what, precisely, does 'actual past' mean in this context?

[Angrboda]

If I were to travel to the time of Chaucer's meal, would I deterministically land in his actual past, or only probabilistically?

Maybe it depends on how you travel. Any ideas?

And what, precisely, does 'actual past' mean in this context?

I would brace myself against the shoulder of Orion, lasso a black hole and swing it 'round the nebulas until it doubled back on itself and slingshotted me there.