On 1/27/2021 3:06 PM, Bruce Kellett wrote:
On Thu, Jan 28, 2021 at 9:51 AM 'Brent Meeker' via Everything List
<[email protected]
<mailto:[email protected]>> wrote:
On 1/27/2021 2:28 PM, Bruce Kellett wrote:
On Thu, Jan 28, 2021 at 3:08 AM Bruno Marchal <[email protected]
<mailto:[email protected]>> 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"?
I thought that was answered in one of Zeilinger's delayed choice
experiments. The idler photons that carry the 'welcher weg'
information do not have to be measured or intercepted. As long as they
exist anywhere in the universe, the interference is destroyed. You
have to actually 'quantum erase' the 'welcher weg' information they
carry in order to restore the interference.
As I recall, the experiment showed that you could erase the welcher weg
information /after /the Young's slits photons were already recorded,
but there was no test of not registering them at all...which is
understandably hard to arrange. That's why I had to postulate an
experiment in outer space. I suppose you could interpret the experiment
as saying welcher weg photons flying off into space will be registered
somewhere sometime and certainly won't have their information erased and
so the experiment shows that the interference pattern would be wiped
out. But it's not actually true that the idler photons will necessarily
be registered somewhere sometime. We see photons from the CMB. So
photons can just get redshifted so far that they no longer carry the
necessary information.
Running the idler photons into the wall is not quantum erasure.....
I think I explicitly said that.
Brent
Bruce
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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