On 1/06/2017 10:52 pm, Bruno Marchal wrote:
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.
The epistemological understanding does not have a non-real wave in
space-time interfering in real space-time. It is merely a calculational
device to get the QM probabilities, it doesn't interfere with anything.
In MWI there is a real problem -- how can a complex wave existing in
N-dimensional configuration space influence anything in 3-dimensional
space-time?
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.
Your attempt to draw a parallel between person duplication in a
laboratory and MWI is simply causing confusion. They are in no way
similar. There is no such thing as 'a sort of non quantum
superposition'. That is just an abuse of language. Classical
probabilities are completely unlike quantum superpositions.
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".
No it is not. You fail to understand QM or the MWI.
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.
No, it does not. You have to make the transition from the pure to the
mixed state in QM, no such transition is required in the classical case.
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.
That does not explain the fact that measurements give single definite
results. If the superposition doesn't 'disappear' and the system remains
in a pure state, nothing ever actually happens -- no measurement is ever
completed to give a result. That is not what we actually observe. The
mathematics do not allow this to be interpreted as merely an illusion
from the first person pov. The problem that you encounter is that of the
selection of a basis in which to interpret the wave function. If there
is no decoherence and transition to a mixed state, all bases are
equivalent, so there is no reason (other than magic) for any result in
the observed basis ever to appear.
I suggest you do some reading around the questions of non-locality,
decoherence, and the basis problem. I can suggest the papers by
Schlosshauer (arxiv: quant-ph/0312059; RMP 76 (2004) 1267-1305) and
Brunner et al., (arxiv: 1303.2849; RMP 86(2014) 419-478) and also the
monograph by Schlosshauer, 'Decoherence and the Quantum-to-Classical
Transition' (Springer, 2007).
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.
Are you suggesting that decoherence is *not* a physical phenomenon?
Decoherence and the pure-to-mixed transition do indeed say that MWI is
strictly false -- you need non-unitary additions.
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.
So your non-collapse theory is immediately falsified by every quantum
experiment ever performed.
Bruce
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