On 1/27/2021 2:28 PM, Bruce Kellett wrote:
On Thu, Jan 28, 2021 at 3:08 AM Bruno Marchal <marc...@ulb.ac.be
<mailto:marc...@ulb.ac.be>> wrote:
Also, worlds interfere statistically, by do not interact at all. A
term in a superposition cannot interact with any other terms, but
we can make them interfering, like with the two slits.
Your grasp of the relevant physics is rather tenuous, I'm afraid
Bruno. The idea of "worlds interfering statistically without
interacting" is just a nonsense. There can only be interference if
there is an interaction. And there certainly is an interaction between
the photons on the two possible paths in the two slit experiment. The
two paths arrive at the screen with different amplitudes and phases --
if the signs are the same, they add. But if the signs are different
they cancel -- partially or completely depending on the relative
amplitudes.
The trouble is that David Deutsch has really screwed up the
understanding of "worlds" for a lot of people. He has talked as though
each path in the two slit case is a separate "world", and then has to
resort to magic to reproduce the interference. The Everett concept of
a "world" is a "relative state", in which an "observer" sees a
definite result. This idea was made more precise with the
introduction of the idea of decoherence, and generalized entanglement
with the environment. If "worlds" are defined as the result of
decoherent histories, then Deutsch's confusion should not arise. A
"world" is the result of (FAPP irreversible) decoherence. There is no
decoherence at the slits in the two slit experiment, so no separate
"worlds" are formed. If you induce decoherence by measuring at the
slits, then the interference pattern disappears -- you have
certainly created a separate "world" for each path, but these can no
longer interfere. That is part of the definition of the "worlds" that
are created by irreversible decoherence.
That's where I think there is still a gap in the theory. We know that
in the C60 double slit experiment the interference is wiped out because
sufficiently short wavelength IR photons from the buckyballs record
their position in the environment, presumably when they are absorbed in
the laboratory walls. But what would happen if they weren't registered
any where. What if the experiment were in outer space and the IR
photons just went off into infinity in a spherically symmetric
wavefunction that never "collapsed"?
Brent
So the concept of "world" is, indeed, well-defined in physics. It
might not be defined in logic or metaphysics, but this is of no
concern to the working physicist -- we know perfectly well what we
mean by "a world". And we can readily tell when someone is talking
nonsense by claiming that "worlds interfere statistically without
interacting". The superposition of the paths in the two slit case
extends right to the screen: that is what produces the interference --
superposition means that the two components are added together with
their intrinsic phases intact. If you destroy the superposition at any
point, such as by interacting with the paths at the slits, there is no
more interference -- you have produced separate "worlds" that can no
longer interact so there is no interference. As Scott Aaronson is fond
of saying: quantum computers work by interference, so the computations
must all occur in one "world". As Scott recently posted: "BREAKING:
President Biden signs executive order banning people from saying
"Quantum computers solve problems by just trying all possible
solutions in parallel"."
Bruce
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