On 1/9/2025 1:15 PM, Jesse Mazer wrote:
On Thu, Jan 9, 2025 at 5:38 AM Quentin Anciaux <allco...@gmail.com> wrote:
Brent,
The core disagreement here seems to rest on the role and status of
possibilities. In a single-world framework, the unrealized
possibilities you refer to have no actual existence or causal link
to the realized world. They are simply conceptual tools to
calculate probabilities. But this is precisely what strikes me as
incoherent: why invoke these possibilities as part of the
explanation if they play no real role in shaping the outcome?
I don't see a fundamental problem here, you can interpret it in terms
of the notion of "hypothetical frequentism" where you are just talking
about the frequencies that would obtain if an experiment were
(hypothetically) repeated an infinite number of times, even if such
repetitions don't occur in reality (assuming some sort of ontological
difference between possible worlds and the 'real world', a difference
which some views like Tegmark's MUH or David Lewis' modal realism
might deny--personally I'm philosophically inclined to a sort of
monism that denies a distinction between possible and real worlds, as
well as denying a distinction between mathematical forms and the
physical universe, but I don't think the idea of making such
distinctions is incoherent).
To me there are other reasons for seeing it implausible that "collapse
of the wavefunction on measurement" should be treated as real rather
than just a useful approximation, though. One is just that I expect
all physical phenomenon should be described by some unified set of
physical laws, applying to small collections of particles and large
measuring instruments alike; those Copenhagen advocates who treat the
collapse as objective don't have any sort of mathematical model of the
laws governing measurement instrument/quantum system interaction to
determine when a collapse occurs, they just have to put in the notion
by hand in an ad hoc way.
That's their aspiration. And decoherence theory has gotten them part
way there. But it's still hand waving beyond that: exactly when do the
worlds become orthogonal? Does orthogonallity spread out at the speed
of light (or less) or is instantaneous across the universe(s). And why
isn't all those worlds becoming orthogonal to this one the same as
"collapsing" to this one? If you can say when orthoganlity occurs I can
tell you when collapse occurs.
Brent
There are also "objective collapse theories" which do try to give a
theory in terms of some idea like a collapse happening spontaneously
whenever a collection of entangled particles exceeds a certain mass,
but this would actually give predictions different from standard QM
and seems implausible to me, it is an idea worth testing of course.
The other big reason to see collapse as not ultimately real is point
made by von Neumann that it's actually arbitrary where you place the
collapse in a series of interactions, it doesn't matter in terms of
predictions whether it happens when the measuring instrument interacts
with the quantum system or only when the information about that
interaction enters a human observer's brain. See the paper at
https://www.jstor.org/stable/3541837 (readable if you sign up for a
free jstor membership) which talks starting on p. 123 about von
Neumann's principle of "psycho-physical parallelism" and on p. 125
quotes von Neumann that this principle requires us to be able to show
"that the boundary between the observed system and the observer can be
displaced arbitrarily" and that "this boundary can be pushed
arbitrarily into the interior of the body of the actual observer is
the content of the principle of the psycho-physical parallelism" (p.
126 also quotes him giving an example involving the measurement of
temperature).
I believe one could extend this further and imagine a Wigner's friend
style thought-experiment where a human experimenter is making a bunch
of measurements in a box which is perfectly sealed off from
interactions with the outside world (no decoherence between the
contents of the box and the outside environment) from some time t=0
until we open it at a later time t=T. The person in the box could be
doing a series of measurements in the electron-double slit experiment
for example, in some cases putting measuring devices at the slits to
see which one the electron went through, in other cases not, and
recording the outcome of all experiments. If we assumed each such
individual measurement collapsed the wavefunction, we'd get a
prediction about the statistics in cases where the electron was
observed, and how they differed from the statistics when it wasn't
observed. If on the other hand we assumed everything in the box was
evolving according to the Schrodinger equation with no collapse until
we opened the box at t=T, we would get exactly the same prediction
about the statistics seen in the experimenter's records! Except in
this case the different statistics when a measuring device was present
at the slits would be explained in terms of decoherence when the
electron became entangled with the measuring instrument and records,
plus the final collapse of those records at t=T. (it seems to me that
this is a further reason to be dubious of objective collapse
theories--it would make this agreement into just a 'weird coincidence')
From what I understand the only way we might get different predictions
in the "every measurement causes collapse" picture and the "collapse
of records doesn't happen until box is opened at t=T" picture is if
there's some possibility the records of a measurement could be
thoroughly erased, with no possibility of reconstructing it from the
measured state at t=T. This is the type of thought-experiment Deutsch
suggested to test MWI against "consciousness causes collapse"
interpretations, see discussion of "Experiment #3" proposed by Deutch,
involving a quantum artificial intelligence which makes measurements
and then has its memory erased, starting on p. 15 at
http://www.columbia.edu/~jpp2139/IssuesInQuantumComputingFD.pdf
Jesse
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