Tim May wrote:

On your point about "many pasts are fundamentally caused by quantum uncertainty in memory devices," I strongly disagree. There is only one past for one present, whether RAMs dropped bits in recording them or historians forgot something, etc.

(This is captured by the formalism of observations, as well. Even with Uncertainty, all honest observers will report the same outcome of an experiment. We have not seen a violation of this, nor is one expected. There are various ways to look at this, including the topos-theoretic view of subobject classifiers. But the point is that in our history either an event happened or it did not. This is independent of whether the event was observed, recorded, written about, remembered, etc.)

But this is a topic of great fascination for me, and I hope we can continue to discuss it. I am quite strongly persuaded that "many pasts for a particular present" is not a reality.

Understand that I am not including "current interpretations," as in "Some historians think the Roman Empire fell because of lead in their plumbing" sorts of theories of the past. I am referring to space-time events.

As noted, I also view time and events as a lattice. But lattices have certain properties of importance here. More on this later.

--Tim May
What do you mean by this, exactly? In a deterministic universe with time-symmetric laws, there'd be only one possible past history for a given present state, and only one future history as well, while in a universe with stochastic laws, or deterministic laws in which paths in phase space could converge, there might be multiple past histories that would lead to exactly the same present state. Of course, even in this type of universe, one could advocate a sort of "hidden variables" view in which there is only one *true* past history, even though it is absolutely impossible for us in the present to determine which one it was. Is this what you're suggesting? I think this idea would pretty much preclude the "everything" idea of this list, since it only really makes sense if you believe the "true" history is the only real one that was actually experienced by observers in it, with all the other possible histories condemned to nonexistence.

Of course, in quantum mechanics it's not even clear that we can talk about the "present state of the universe" as if it's a well-defined entity, in which case it may not make sense to ask whether "the" present has multiple pasts or multiple futures. A MWI advocate would say we could talk about the present state of the universal wavefunction, but that's different from the present state of an individual "world"--I believe there's a fair amount of controversy about what people even mean by "worlds" in the MWI. With a hidden variables interpretation of QM you can talk about the universe's present state, but the exact details of the present state would always be unknowable.

--Jesse Mazer

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