> There is a sense in which the past is not unique and determined.

> [......] the "single past" model is quite well-supported by science [.....]

It seems that the quantum measurement traces a certain (consistent)
history, takes a part in the very formation of the past (of that history).

John Wheeler devised a gedanken experiment in which a piece of the apparatus
can be altered just _before_ the arrival of a particle, and this "delayed
can, seemingly, alter the quantum state of the particle at a much _earlier_
long before the choice is made (i.e. when the particle was in the outer space).

J. A. Wheeler in Quantum Theory and Measurement, edited by J. A.
Wheeler and W. H. Zurek, Princeton University Press, Princeton, 1983,
page 182.

J. A. Wheeler in Mathematical Foundations of Quantum Theory, edited
by A.R. Marrow, Academic Press, 1978, page 9.

This (below) is a version of the "delayed choice", by Chiao and Kwiat.

D1 ---- F <--------------------------------- S ------------> I ---- D2
                                    photon 1                         photon 2

S = source of entangled photons 1 and 2
D1 = D2 = dectectors (of coincidence) of the entangled photons
F = remote broad/narrow band filter (setting the energy of photon 1)
I = Michelson interferometer (photon 2 goes into)

The visibility, at I, of the photon 2 fringes depend critically on the bandwidth
of the remote filter F. For a narrowband F, this fringe visibility, at I, is
high.  It should be emphasized that (due to Heisenberg's principle) the width
of the photon 2 wavepacket is determined by the remote filter F, through
which this photon 2 has never passed. If, however, a broadband remote
filter F is used instead, such that the optical path length difference of the
Michelson interferometer I is much greater than the coherence length of
the wavepacket of photon 2, then the fringes disappear. The arbitrary "choice"
of whether the _remote_ filter F should be broadband or narrowband
could then be "delayed" until well after clicks, at D2, had already
been irreversibly registered. In this way, we can be sure that the photon 2
wavepacket on the near side of the apparatus could not have known in
advance the experimenter's "delayed choice" of  F on the remote side of
the apparatus.

Another very good paper, about the "delayed choice" issue, and
its philosophical implications, is at

> [......] The experiment itself is subject to quantum indeterminacy,
> and therefore your ability to know the past remains forever fuzzy.

Heisenberg showed, long time ago, that his "principle" was not effective
in the past. In example it is possible to know perfectly well past position
and past momentum of a particle. (The same must be true for the time -
energy relation case, but this relation is meaningless, because time, in QM,
is not an observable, ... but we must invent some kind of  "proper" time ...)


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