On 8/1/2018 3:51 AM, Bruno Marchal wrote:
On 31 Jul 2018, at 21:46, Brent Meeker <[email protected]
<mailto:[email protected]>> wrote:
On 7/31/2018 9:11 AM, Bruno Marchal wrote:
On 30 Jul 2018, at 22:27, Brent Meeker <[email protected]
<mailto:[email protected]>> wrote:
On 7/30/2018 9:58 AM, John Clark wrote:
>
/Forget collapse./
Many, perhaps most, physicists do exactly that because they
believe in the "Shut Up And Calculate" quantum interpretation and
are only interested in predicting how far to the right a indicator
needle on a meter moves in a particular experiment. But for some
of us that feels unsatisfying and would like to have a deeper
understanding about what's going on at the quantum level and
wonder why there is nothing in the mathematics that says anything
about a wave collapsing.
That's not true. "The mathematics" originally included the Born
rule as part of the axiomatic structure of QM.
In the usual QM, yes. But this use a vague notion of observer, and a
seemingly forbidden process, a projection (a Kestrel!), I mean
forbidden if we apply the wave to the couple observer-particle.
Most of all they want to know what exactly is a "measurement" and
why it so mysterious.
The problem with the Born rule was that its application was ambiguous:
Ah! Exactly.
Where was the Heisenberg cut? Why was "the needle basis"
preferred? But decoherence theory has given answers (at least
partially) to those questions. Given those answers, one can just
replace "collapse" with "discard", i.e. discard all the predicted
possible results except the one observed. Is there really any
difference between saying those other predictions of the wf are in
orthogonal, inaccessible "worlds" and saying they just didn't
happen. That seems to be Omnes approach. He writes, "Quantum
mechanics is a probabilistic theory, so it only predicts
probabilities.”
OK, but the honest, and perhaps naive inquirer would like to have an
idea about what are those probabilities about, and where they come
from.
That was the source of resistance to Born's paper. Physicists assumed
that probability could only arise from ignorance of an ensemble.
Since there was no ensemble in Heisenberg's (or Schroedinger's) QM
they resisted the idea. Lots of attempts were made to reintroduce
ensembles, or at least virtual ensembles, so that they could feel
comfortable with having a probabilistic theory. Omnes' is just
saying "Get over it!"; probabilities are fundamental.
Yes, but he said all this after defending Everett (or its own better
version of Everett). Then, this introduces a notion of ensemble (the
set of all consistent histories), and, at least in some book, just ask
us to be irrational and to dismiss the ensemble at make probability
fundamental, only to make the “other worlds” disappear. In one book he
lakes clear that such a decision is irrational, and that he makes it
because he dislike of find shocking the idea that all quantum possible
outcome are realised. It is a bit like a christian who understand the
evolution theory, but add that it makes just God having invented
evolution instead of Adam.
There's nothing irrational about discarding that which is not observed
and keeping that which is observed. That's what probability means:
somethings happen and some don't. The idea that all the possibilities
happen is what has made MWI incoherent. Gleason's theorem supports the
use of the Born rule to define a probability measure; but the problem is
the metaphysical one of whether there is any meaning to "probability"
when everything happens.
Everett's MWI is appealing to the same intuition...that probabilities
must refer to ensembles.
Wich in my opinion is the only way to make sense of any notion of
probabilities. You need a space or set of events too which the
probabilities applies.
But it must be an ensemble from which somethings happen and some don't.
So the ensemble will be multiple-worlds. But that didn't really work
because Schroedinger's equation didn't predict multiple worlds with
the right ratios, it just gave real number probabilities. So people
like Bohm and Bruno invented infinite ensembles to explain the
probability numbers. Which is OK, but one should recognize that they
are /*not */just explicating Schroedinger's equation.
There is no probabilities at all in the schroedinger equation. But
then that equation describes also a vast set of relative state
describing indexical probabilities.
No, it doesn't describe "indexical probabilities". That's why Born had
to come up with an interpretative rule in order that there be a relation
between the wf and observations.
It is really similar to the WM-duplication. From the 3p perspective,
there is no probabilities at all, but the duplication (and mechanism)
explains entirely why all first person concerned (having done the
self-duplication) encounter probabilities. Somehow, Shannon entropy,
or Botzmann, use something similar.
Or QBism.
Brent
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