Tim May writes:
> This arises with quantum measurements of course. Once a measurement is 
> made--path of a photon, for example--all honest observers will report 
> exactly the same thing. There simply is no basis for disputing the 
> past, for Alice to say "I saw the photon travel through the left slit" 
> but for Bob to say "I saw it travel through the right slit."

That's an interesting example, because usually the point of two-slit
experiments is that there is no "fact of the matter" about which slit
the particle went through.  That's why you get interference from
the double-slit.  What would you say about the past in that case?
Are there two pasts, one where the particle went through each slit,
which have now recombined to form the present?  Or just one past, where
the particle managed to go through both slits at once?

Hal Finney

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