On Tuesday, October 8, 2019 at 6:24:28 PM UTC-5, Brent wrote:
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>
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> On 10/8/2019 2:59 PM, Philip Thrift wrote:
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> On Tuesday, October 8, 2019 at 2:40:33 PM UTC-5, Brent wrote:
>
> That MWI entails other, unobservable "worlds" is neither a bug or a
>> feature, it's just one answer to the measurement problem. If you have a
>> better answer, feel free to state it.
>>
>>
>> Brent
>>
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>
> MWI, according to Sabine Hossenfelder, is not an answer - in the final
> analysis - to the measurement problem
>
> http://backreaction.blogspot.com/2019/09/the-trouble-with-many-worlds.html
>
>
> The many world interpretation, now, supposedly does away with the problem
> of the quantum measurement and it does this by just saying there isn’t such
> a thing as wavefunction collapse. Instead, many worlds people say, every
> time you make a measurement, the universe splits into several parallel
> worlds, one for each possible measurement outcome. This universe splitting
> is also sometimes called branching.
>
> Some people have a problem with the branching because it’s not clear just
> exactly when or where it should take place, but I do not think this is a
> serious problem, it’s just a matter of definition. No, the real problem is
> that after throwing out the measurement postulate, the many worlds
> interpretation needs another assumption, that brings the measurement
> problem back.
>
> The reason is this. In the many worlds interpretation, if you set up a
> detector for a measurement, then the detector will also split into several
> universes. Therefore, if you just ask “what will the detector measure”,
> then the answer is “The detector will measure anything that’s possible with
> probability 1.”
>
> This, of course, is not what we observe. We observe only one measurement
> outcome.
>
>
> The implication is that the above two sentences are contrasting. But
> nobody asks "what will the detector measure". The question asked by the
> experimenter is "which measurement outcome will the detector detect", which
> is perfectly consistent with "we observe only one measurement outcome"
>
> The many worlds people explain this as follows. Of course you are not
> supposed to calculate the probability for each branch of the detector.
> Because when we say detector, we don’t mean all detector branches together.
> You should only evaluate the probability relative to the detector in one
> specific branch at a time.
>
>
> I can't even parse that. You are supposed to calculate the probability of
> each possible measurement outcome and those characterize the branch. It is
> NOT calculating "each branch of the detector" unless you are defining those
> "branches" by what the measurement outcome is.
>
>
> That sounds reasonable. Indeed, it is reasonable. It is just as reasonable
> as the measurement postulate. In fact, it is logically entirely equivalent
> to the measurement postulate.
>
>
> It's not clear here what "logically" equivalent means. It is
> instrumentally equivalent...which is why it's an interpretation and not a
> different theory (as GRW is). It's different from the measurement
> postulate in that the measurement postulate says the wave function
> instantaneously changes to match the observed measured value. MWI says
> those other measured values obtain in other orthogonal subspaces of the
> Hilbert space and you are only observing one. Those are not "logically"
> the same.
>
> The measurement postulate says: Update probability at measurement to 100%.
> The detector definition in many worlds says: The “Detector” is by
> definition only the thing in one branch.
>
>
> What does "only the thing in one branch mean". In MWI there are
> projections of the detector in subspaces which differ only by the value
> detected.
>
> Now evaluate probabilities relative to this, which gives you 100% in each
> branch. Same thing.
>
> And because it’s the same thing you already know that you cannot derive
> this detector definition from the Schrödinger equation.
>
>
> ?? You can't derive the definition of any physical object from the
> Schroedinger equation. You put in the Hamiltonian of the object and
> whatever it interacts with and the initial ray in Hilbert space and the
> Schroedinger equation tells you how it evolves
>
> It’s not possible. What the many worlds people are now trying instead is
> to derive this postulate from rational choice theory. But of course that
> brings back in macroscopic terms, like actors who make decisions and so on.
> In other words, this reference to knowledge is equally in conflict with
> reductionism as is the Copenhagen interpretation.
>
>
> I agree with that point. But once you suppose a probabilistic
> interpretation of the Hilbert space, then Gleason's theorem implies the
> Born rule. That still leaves a small gap in saying why it has
> probabilistic interpretation at all. Whether "self-locating uncertainty"
> is an adequate answer seems to me to require more analysis of human
> thought; although showing the brain is a quasi-classical information
> processor goes a long way.
>
> Brent
>
>
> *And that’s why the many worlds interpretation does not solve the
> measurement problem* and therefore it is equally troubled as all other
> interpretations of quantum mechanics. What’s the trouble with the other
> interpretations? We will talk about this some other time. So stay tuned.
>
> @philipthrift
>
>
>
Sabine later goes on in a comment to say that
"to correctly sum up the total energy, you have to weigh the energy
in each branch
with the probability of that branch"
In the end, I think Sabine's application of probability is a mess.
And to put "self-locating uncertainty" into the mix (now QM is human-brain
dependent) makes things worse.
I posted a course notes of a pedagogical approach of applying probability
theory to the conventional Hilbert space QM here:
Quantum Probability Theory (by Jan Swart)
https://groups.google.com/d/msg/everything-list/8_RCIBNbOis/0Qnlt_GyBQAJ
So QPT and QMT (Quantum Measure Theory, by Rafael Sorkin) both take
probability seriously in a mathematically pedagogical way, but in Many
Worlds (Interpretation) it just looks like a Mega Waste.
@philipthrift
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