On 01 Jun 2017, at 13:02, Bruce Kellett wrote:
On 1/06/2017 8:21 pm, Bruno Marchal wrote:
On 31 May 2017, at 09:15, Bruce Kellett wrote:
On 31/05/2017 4:40 pm, Pierz wrote:
>> On 5/30/2017 7:30 PM, Pierz wrote:
>>> Thanks for these clarifications Bruce. I find your
explanations to be very lucid and helpful - they also confirm my
own understanding. IIRC, you weren't a particular fan of MWI when
I last conversed with you on this list. I wonder if you'd care to
comment on my original argument on this thread - which has of
course now been swamped by the usual brawls. Does not a single
history + the physical insignificance of the notion of a current
moment mean that there is also only a single possible future?
>> I don't see how that follows. In the usual model of forking
in the future direction every current moment has a single
history, but multiple futures.
Multiple futures = MWI, surely.
Not necessarily. If you insist on Schrödinger determinism, then
yes, by reification of the wave function. But if the wave function
is only epistemological, then there is only one probabilistic
future.
How could an epistemological wave interfere all by itself? It seems
simpler to say that the wave is real, but that the local, one
branch, perception is epistemological.
I think you misunderstand this use of the word "epistemological".
When it is said that the wave function is merely epistemological,
what is meant that it is merely a calculational device: the wave
function encapsulates our knowledge of the system of interest, and
tells us how to calculate the probabilities of various outcomes. In
other words, it is not a 'real' wave in space-time.
Yes. That is the problem. How a non real wave in space time can
interfere really in space-time? Of course, the answer is simple with
the MWI, but asks for real infuence at a distance with the Mono-world
assumption.
When I say a single history, I mean that from the Big Bang
forward, the universe only followed one branch.
And that can lead into the future, only one probabilistic future.
>>
>>> And if the future is predetermined in this way, isn't this a
serious issue for single universe models of QM? How can the
outcome of quantum events be both inevitable and random?
>> Having a single future isn't the same as being determined. A
single future in which situation A is sometimes followed by B and
sometimes by C is still random.
It depends on perspective. It's true that from 1p, it looks
random. But from 3p, it is static and always the same, and in
this sense determined.
Not true, the 3p view of the future is still just a single
probabilistic world line. It is only the bird view, from outside
space and time, that gives the appearance that there are multiply
existing futures. Neither we from the 1p perspective, or from
outside ourselves in the 3p perspective, can we ever see any of
the other branches predicted by MWI.
Yes, like in WM-duplication.
No, that is a serious mistake. Unitary evolution of the wave
function is not at all like the person duplication experiments. The
main difference is that the duplicates of the Helsinki man are never
in a pure quantum state.
That was not the point. The comparison is on the reasult measurement,
not on the reason of the parallel states. With mechanism, the reason
are the same (the first person indeterminacy). With collapse, we need
magic.
The teleportation to W or M is of separate classical entities, There
is no superposition principle for duplicated persons.
You can defined for the Helsinki guy. He is in a sort of (non quantum)
superposition state in helsinki, which indeed, like in the MWI, is
just his ignoance on which computations he belongs. I use Y = II, if
that is still needed to say.
The guy in Helsinki can predict that his first person future will
be unique, even if he knows that in the "bird" picture, that
outside the teleportation/duplication boxes, there will be two
versions of him.
That is not the "bird" picture, that is merely the objective 3p
picture of the ordinary classical world. The bird picture is solely
to do with the pure state that is preserved in the unitary evolution
pure state of the wave function.
I do not assume QM in that context. I was just doing an anlogy, at
this stage.
This is where the problems with MWI really show up. When you have a
quantum event with, say, two possible outcomes with equal
probabilities, such as measuring the polarization of an unpolarized
photon, the initial probability is 0.5 for each polarization. But
after the measurement, the probability for the observed result
(horizontal or transverse as observed) is unity -- because the
result has been observed, it is now certain. So how did the
probability suddenly change from 0.5 to 1.0?
Well, with the MWI, it is the same as the dropping from the Helsinki
1/2, to the W(M) "local certainty".
In the classical case, such as the duplicates of the H-man going to
W or W, there is no problem. The system is already in a mixed state,
so the change in probability is simply the result of getting
additional information.
That remains the acse in the MWI.
Just as in the toss of the fair coin, heads and tails are equally
likely, but the jump of the probability for 0.5 to 1.0 on observing
the result is purely a classical epistemological effect. This is not
true for the quantum pure state. In order for the change in
probability to be understood epistemologically, the pure state has
to be reduced to a mixed state (a process that is not necessary in
the classical examples).
With the MWI, mixed states never occur. A superposition never
disappear, except from the frog which looks at the polarization state,
from its first person pov. But the wave describes this only by
"splitting" the frog along the measurement obtained. Things get mixed
only in apparence for each person pov.
In quantum mechanics, this change is brought about by the unitary
processes of decoherence,
As Everett explains well, and suggest already this makes any influence
at a distance only apparent, but never real. Einstein can sleep well.
and the non-unitary trace over the environmental degrees of freedom.
Which reflects only, in the MWI, the contagion of the superposition to
the environment.
This is an essential difference between classical and quantum
physics, and the necessity for this non-unitary reduction of the
pure state to a mixture is ultimately why MWI is actually no better
at explaining quantum measurement than are the collapse models.
Only if you interpret the decoherence as a physical phenomenon, but
then we get non unitary evolution in Nature, and -quantum mechanics-
without collapse is false.
The question is, what determined (from the 3p view) that the
universe followed that particular path and not any of the others?
Why do you reject out of hand that the universe might be
probabilistic? It is possible 'nothing' determined which path
from the possibilities was actually followed. All that is known
are the probabilities for each path. We do not know that the other
paths are followed, either 1p or 3p.
In QM, we do have evidences that many path are taken all together.
if only the two slits.
That is not really a relevant comment. Quantum mechanics is
characterized by the presence of superpositions -- that is what
makes the theory work, and why it is so different from classical
physics. Superpositions generally represent pure states, and these
must be reduced to mixed states by the measurement process.
Without collapse, that never happens.
Bruno
Bruce
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