On Tuesday, January 14, 2003, at 12:35  PM, Hal Finney wrote:

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.
No, _if_ (iff) a measurement of which slit the photon goes through is made, then no interference pattern is observed. This is standard QM at this point.

My point was that Alice and Bob will also agree with what the outcome of the measurement was.

 That's why you get interference from
the double-slit.
The interference pattern is only seen when no measurement of which slit the photon passed through was made.



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?
I would say "nothing" about the past prior to a measurement being made.


--Tim May

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