On 29-06-2021 01:46, Bruce Kellett wrote:
On Tue, Jun 29, 2021 at 8:16 AM 'Brent Meeker' via Everything List
<[email protected]> wrote:
On 6/28/2021 9:27 AM, John Clark wrote:
On Mon, Jun 28, 2021 at 11:13 AM Jason Resch <[email protected]>
wrote:
And there is no reason to suppose that such worlds exist.
_> There is: how do quantum computers work unless the wave function,
and all its superpositional states, are "really real"?_
Forget Quantum Computers, I can't even understand how the 2 slit
experiment could work as it does if those other Everett worlds didn't
exist.
The photons can't go thru slits in different worlds and still
interfere is this one.
I think John's trouble here is that he still adheres to David
Deutsch's concept of worlds. Deutch talks as though every component of
a superposition is a separate world. This leaves Deutsch no language
to talk about decohered worlds, pointer states, and all the other
usual apparatus of quantum interpretations. The trouble with taking
every component of a superposition as a separate world is that in
Hilbert space (as in any vector space) you can define an infinite
number of different sets of basis vectors, so any vector in the space
is represented by an infinity of different worlds, and there is no way
to distinguish between these.
I think Bruno has flirted with this idea as well. Deutsch, through his
popular writings, has done an immense amount of harm to the cause of
quantum interpretations.
Bruce
There ids a large body of rigorous work in this field, it's not that you
have just a handful of advocates who are defending the MWI based on
dodgy nonrigorous arguments. Of course, you can't just take nay
component of a superposition as a separate world. But given that Worlds
do exist and given that time evolution is given as a linear operator, it
follows that if QM is a fundamental theory that also describes
observers, that you inevitably end up with superpositions of entire
Worlds.
This conclusion does not depend on any assumptions of how observers
should be defined rigorously, how experiments and ultimately
observations arise out of the physics. These issues that are not yet
100% solved, are totally irrelevant provided QM is indeed a fundamental
theory.
It's not any different from someone claiming that conservation of
momentum may not be true. How do we convince this person that it is
true? We can appeal to fundamental laws of physics and argue on the
basis of symmetries, Noether's theorem and then say that this rigorously
establishes conservation of momentum. But the skeptic can then take
issue with the assumption about the validity of the fundamental laws,
he will insist that it's still possible for momentum to get lost. If he
does an experiment involving many particles, then he'll say that unless
you measure the momentum of each particle to infinite accuracy, you
can't really tell that momentum is conserved. He'll then turn the logic
about the fundamental laws upside down by arguing that because you can't
really be sure about momentum conservation, you can't therefore say that
the fundamental laws have been all that well established.
Of course, there is then a lot to argue about this reasoning suggesting
that there is room for momentum nonconservation. But the arguments
against MWI (regardless of whether or not you need to add Born's rule as
a postulate and if so, regardless about any discussions about this then
invalidating the original goals of some MWI advocates), are of the same
nature. Here you have a supposedly fundamental theory, QM and it implies
in a rather straightforward way the existence of parallel Worlds, and
because people don't like that conclusion, you have arguments against it
that can only work if QM is not true as a fundamental theory. The
problem with those arguments is then that it's invoked as a standalone
argument against the MWI. If these arguments were well motivated on
their own merits, then they would form the basis of a lot of physics
research in many different fields ranging from condensed matter,
particle physics etc. But that's not the case.
Saibal
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