From: *Jason Resch* <[email protected] <mailto:[email protected]>>
On Sun, Jun 17, 2018 at 6:24 PM, Bruce Kellett
<[email protected] <mailto:[email protected]>> wrote:
Maybe it just means that we don't yet fully understand the
collapse. There are plenty of possibilities that don't resort to
magic.
I agree.
But what facts about our observations of collapse are not already
fully explained by Decoherence?
Decoherence does not explain the transition from FAPP orthogonality to
full orthogonality of the branch states. In other words, decoherence is
unitary, so cannot explain the non-unitary trace over unobserved
environmental entanglements inherent in the projection of actual
experimental results.
In other words, what is left to solve about it? I thought Decoherence
solved this (back in 1952 with Bohm).
Decoherence was not introduced by Bohm. The idea originated with Dieter
Zeh in around 1974, if I remember correctly.
When you say non-local what type of non-locality do you mean? It
is a local theory in the sense that physical objects interact
only with other physical objects in their proximity, and carry
information only at luminal or subluminal speeds. See Q12 on
http://www.anthropic-principle.com/preprints/
<http://principle.com/preprints/>manyworlds.html
Price's argument here has been shown to be invalid -- he
surreptitiously relies on non-locality.
Care to explain this non-locality and where it appears in a MWI
explanation of the EPR paradox, for example? I've provided
explanations on this list before of how EPR/Bell operates under MWI
without FTL influences. So if you think they are required I would be
interested to know where you think they appear and are necessary.
I have pointed out the flaw in Price's account previously on the list.
Tipler makes the same mistake, as do several others. But rather that
going through the argument here, I will postpone it to my discussion of
your attempted local account. You make essentially the same mistake, so
we can look at it then.
John Clark often says MWI is non local because the branches are not
local to each other, but I think this is a redefinition of the common
sense use of the term locality in physics. Is this what you mean by
MWI being non local?
Not really, but John does have a point.
How do you explain the finite computational resources of a table-top
quantum computer factoring a prime number in seconds when it would
take a classical computer the size of the solar system 10^100 years to
do the same calculation?
David Deutsch notes that quantum computers present a strong challenge
to defenders of single-universe interpretations, saying “When a
quantum computer delivers the output of such a computation, we shall
know that those intermediate results must have been computed
somewhere, because they were needed to produce the right answer. So I
issue this challenge to those who still cling to a single-universe
world view: if the universe we see around us is all there is, where
are quantum computations performed? I have yet to receive a plausible
reply.”
That might be Deutsch's opinion, but plenty of others think
differently. Quantum computers can easily be understood in a
single world account.
But it can't be explained in non-realist views of the wave function.
For example, those that say it is nothing but a convenient tool for
computing probabilities.
Why can't that account for quantum computing?
The reason is, here this "convenient tool" is computing results for us
that we have no hope of ever computing ourselves. How is something
which isn't real, and isn't really there, yielding results of
computations?
Quantum mechanics is weird!
You say others think differently, but don't allude to who those other
thinkers are or what their thoughts are. Do you have an explanation
for quantum computers that works with the assumption the wave function
is not real?
Yes. The particular person I was thinking of here is Scott Aaronson. He
is no fan of Deutsh's approach to quantum computing and many worlds. He
points out that quantum computers rely on interference between the
components, and that is possible only in a single world.
What would you say about a conscious AI implemented on a quantum
computer? Would it or would it not be capable of existing in and
experiencing "many simulations"?
A quantum computer would be no different from a classical computer in so
far as implementing AI is concerned. Why would you think it would be
different?
[snip]
Bell had an implicit assumption in his reasoning, which is that only
a single definite result is obtained by all parties for any given
measurement (including those not performed). This is not true under
MWI so Bell's reasoning that there must be non-locality fails for
MWI, as many-worlds lacks contra-factual definiteness.
Again from:
http://www.anthropic-principle.com/preprints/manyworlds.html
<http://www.anthropic-principle.com/preprints/manyworlds.html>
*To recap. Many-worlds is local and deterministic. Local measurements
split local systems (including observers) in a subjectively random
fashion; distant systems are only split when the causally transmitted
effects of the local interactions reach them. We have not assumed any
non-local FTL effects, yet we have reproduced the standard predictions
of QM.*
*So where did Bell and Eberhard go wrong? They thought that all theories
that reproduced the standard predictions must be non-local. It has been
pointed out by both Albert [A] and Cramer [C] (who both support
different interpretations of QM) that Bell and Eberhard had implicity
assumed that every possible measurement - even if not performed - would
have yielded a *single* definite result. This assumption is called
contra-factual definiteness or CFD [S]. What Bell and Eberhard really
proved was that every quantum theory must either violate locality *or*
CFD. Many-worlds with its multiplicity of results in different worlds
violates CFD, of course, and thus can be local.*
As I said above, Price has an invalid argument. If Bell's theorem
does not apply to MWI, why is it that no one has been able to give
a simple, clear, local account of the violations of the Bell
inequalities?
Let me try:
In the EPR experiment, a pair of photons is created. Each photon is
in a super position of every possible polarization, and because it is
created as a pair, it's dual in the superposed state always has
exactly the opposite polarization (rotated 180 degrees).
OK.
When you perform a measurement of your left-traveling photon on Earth,
you become entangled (correlated) with it, and all the possible states
of that photon, when measured, leak into the room, starting with the
measuring device, then your eyes, then your brain, then your notebook,
etc. until now everything is in the room, and soon Earth is now in
many states which contagiously spread from that photon.
OK. Your result (and you) become entangled with your environment.
Also, because the photon you measured was entangled (correlated) with
its pair in the superposition, whatever result you measure for the
photon's polarization tells you immediately what the polarization of
its pair is (in your branch at least). So any future communication
you get from me on Pluto will necessarily align with the result you
measured.
This is where the mistake creeps in. My measurement tells me the
polarization of the entangled photon in the branch in which my
measurement was made. When you come to measure your entangled photon on
Pluto, how do you know what branch my measurement was made in? You are
at a spacelike separation from me, and completely independent. So I ask
again, how come you assume that your measurement will be in the same
branch as mine was?
This is effectively Price's mistake, except he made it even more obvious
by writing out the equations.
Effectively, "single" hidden variables don't work under Bell's
inequality, but many hidden variables do.
Maudlin concludes his discussion of many worlds by questioning
whether sense can be made of correlations if all outcomes occur at
both ends of a Bell-type experiment. He concludes: "If some sense
can be made of the existence of correlations, we have to
understand how. In particular, if appeal is made to the
wave-function to explicate the sens e in which, say, the "passed"
outcome on the right is paired with the "absorbed" outcome on the
left to form a single "world", then we have to recognize that
this is not a /local/ account of the correlations since the
wave-function is not a local object. (Quantum Non-Locality &
Relativity, 3rd Edition.)
To see why it is local you need to trace the event back to the
creation of the paired photons. Each photon, in a sense, has already
measured each other. If the photons are always created in opposite
aligned pairs, knowing one you already know the other. You know you
have ended up in the branch of the wave function where my photon has
the opposite angle of yours the moment you measure your photon,
because both photons were created in a way that has such a property.
That is what the entangled pair means.
Also, both of these photons, which already measured each other,
traveled at no more than the speed of light to reach you and me. When
we measured them, the branching of the wave function was entirely
local. Starting with the polarization screen, then the photon
detector, then the light flash, then your retina, the nerves along
your optic nerve, then your brain. Nothing happened instantaneously
across space or time, and nothing exceeded the speed of light in this
process.
Sure, the local measurements are local. But you have no local way of
knowing which branch contains my measured result.
Regarding preferred bases, both of the papers I provided which
began this thread address that question:
https://arxiv.org/pdf/1104.2324.pdf
<https://arxiv.org/pdf/1104.2324.pdf>
* Note that quantum interferences between different terms in
Eq. (39) are extremely small, since overlaps between
macroscopically different configurations, such as and
, are suppressed by the huge dimensionality of the
corresponding Hilbert space. In fact, for any observables
constructed out of local operators, matrix elements between
macroscopically distinct states are highly suppressed, This,
therefore, provides preferred bases for any macroscopic systems.*
and
https://arxiv.org/pdf/1105.3796.pdf
<https://arxiv.org/pdf/1105.3796.pdf>
*Decoherence Decoherence1 explains why observers do not
experience superpositions of macroscopically distinct quantum
states, such as a superposition of an alive and a dead cat.
The key insight is that macroscopic objects tend to quickly
become entangled with a large number of “environmental”
degrees of freedom, E, such as thermal photons. In practice
these degrees of freedom cannot be monitored by the observer.
Whenever a subsystem E is not monitored, all expectation
values behave as if the remaining system is in a density
matrix obtained by a partial trace over the Hilbert space of
E. The density matrix will be diagonal in a preferred basis
determined by the nature of the interaction with the environment*
*This preferred basis is picked out by the apparatus
configurations that scatter the environment into orthogonal
states. Because interactions are usually local in space, ρSA
will be diagonal with respect to a basis consisting of
approximate position space eigenstates. This explains why we
perceive apparatus states |0iA (pointer up) or |1iA (pointer
down), but never the equally valid basis states |±iA ≡ 2 −1/2
(|0iA±|1iA), which would correspond to superpositions of
different pointer positions.*
That is just the explanation that Schlosshauer gives. And as I
have pointed out, it is circular. One could justify any Quantum
operator at all as the position operator on this basis -- physics
would be different. But then, what is required is an account of
why physics is as it is.
Could you explain to me what is the expected observational difference
between QM and MWI, under the "preferred basis problem"?
Why should there be a difference? MWI and CI are just alternative
interpretations -- both necessarily give the same results for any
experimental observation. But you are just avoiding the preferred basis
problem.
What does eliminating collapse of the wave function cause that you see
as leading to MWI being ruled out by either any experiment or
observation that has been performed?
Why do you think I have said that MWI is ruled out? In Scott Aaronson's
terms, I am a "bullet-dodger": I do not stake everything on thinking
that I know the answer to everything! Since all conventional quantum
experiments give the same results under any interpretation of the QM
framework, no experiment can decide between them. Of course, unless, and
until, some collapse mechanism is actually observed. The collapse model
is directly falsifiable, and MWI would be falsified were collapse
observed. Such as in Penrose's gravitation induced collapse, or some
GRW-type flash impulse collapse mechanism.
Bruce
--
You received this message because you are subscribed to the Google Groups
"Everything List" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To post to this group, send email to [email protected].
Visit this group at https://groups.google.com/group/everything-list.
For more options, visit https://groups.google.com/d/optout.
