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Experiments spell doom for physical-collapse explanation of quantum weirdness

quantamagazine.org

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Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#261

Post by tsimionescu »

GoblinSlayer wrote: post #213
tsimionescu wrote: post #122

Earlier quoted context omitted.

MWI is exactly as simple mathematically as CI, so not sure what you mean by "simpler". MWI still postulates the equivalent of wave function collapse, but instead of it happening only for the quantum system being measured, it is happening in the mind of the observer, as each "version" of that mind gets entangled with a single "version" of the outcome. Even if you were to accept that this process is more natural (so no…

>MWI still postulates the equivalent of wave function collapse It's not postulated, but deduced from the Schrodinger equation. MWI is simpler in a sense that it has one fewer axiom. But Occam's razor isn't really applicable here, because it selects from otherwise equal theories, which CI isn't. There are more important criteria to use before Occam's razor.

You can't deduce the Born rule from the Schrodinger equation. MWI can say "look at every basis separately", but you still have to postulate that the probability of finding yourself asking that basis vector (seeing that measurement outcome) is proportional to the square root of the amplitude.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#262

Post by jamiek88 »

pdonis wrote: post #166
ryeights wrote: post #158

Earlier quoted context omitted.

Can you elaborate on this? The Wikipedia page for MWI says >The many-worlds interpretation implies that there are most likely an uncountably infinite number of universes. https://en.wikipedia.org/wiki/Many-worlds_interpretation

One should not be trying to learn physics from Wikipedia, particularly not for something as complicated as QM, even more particularly not for an aspect of QM as contentious as the MWI. The MWI says that there is one single universal wave function, which evolves in time by unitary evolution forever, and that that universal wave function is all that exists. That is one universe, not an infinity of them. It is just one…

Your last paragraph is verbatim gonna be the ‘Lord’s Prayer’ of my space cult.

Wanna join? You can be number 2 thus have second dibs at the buffet.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#263

Post by tsimionescu »

whimsicalism wrote: post #149
tsimionescu wrote: post #122

Earlier quoted context omitted.

MWI is exactly as simple mathematically as CI, so not sure what you mean by "simpler". MWI still postulates the equivalent of wave function collapse, but instead of it happening only for the quantum system being measured, it is happening in the mind of the observer, as each "version" of that mind gets entangled with a single "version" of the outcome. Even if you were to accept that this process is more natural (so no…

> you then need to recover the relationship between the probability of observing a certain outcome and the amplitude of that outcome in the wave-function. I have never understood how that is a strong objection. We've experimentally determined that the state you are more likely to find yourself in is based on the squared amplitude. How is this different from CI but with probability of observing given state - which was…

> How is this different from CI but with probability of observing given state - which was also determined empirically?

It's not different, that's the point. For both interpretations, the relationship between the probability of observing a certain outcome and the amplitude of that outcome in the wave function are an extra postulate, the exact same extra postulate in fact, the Born rule.

> The worlds and their number are equivalent to the states & probability of CI without having to introduce the "measurement apparatus" that is distinct from the quantum system.

The measurement apparatus as a separate thing from the quantum system was actually partially explained by decoherence (ironically discovered by MWI proponents), which needs this separation for the same reasons as CI: we need some way to explain why quantum phenomena don't happen at our scale. Now, MWI makes this concept relative to an observer, whereas in CI it is often assumed to be absolute.

Basically, we can deduce from the Schrodinger equation alone that after the interaction of a quantum system with the environment, coherence is lost, and the different "states" of the wavefunction can no longer interfere with each other.

CI postulates that, as a result of this interaction, only one of the states will remain, with the Born rule probability. This is the most direct way of interpreting our experimental results.

MWI says that nothing changes after this interaction. It postulates though that, if we were to ask how likely we are to be in the same "state" as a particular result, we should expect that to be the Born rule probability. This explanation takes the theoretical description of the wave function to be more real in some sense than the actual observations we make.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#264

Post by eterevsky »

TheOtherHobbes wrote: post #94
dzdt wrote: post #4

There is an often-repeated statement that different quantum mechanical theories are equivalent. This is an over-simplification. Is wave function collapse is a real thing or just an approximation of what happens when a large system is entangled? Some smart people have tried to make theories where wave function collapse is a real thing. These new experiments are working to rule out those ideas. My feeling is that Evere…

I don't see how relabelling "collapse" to "separation" and adding more or less infinitely proliferating universes solves the problem. MWI also contradicts itself. Supposedly the universes are independent, but if their influence doesn't define the wave equation they do nothing to help explain it. It's a very strong and exceptional claim with absolutely no evidence to support it. "Feeling" isn't enough.

MWI doesn't posit that the universes are separated. Different classical states of a single quantum computer are technically in different "universes", but are still interacting with each other.

Separation that you are talking about boils down to different parts of wave function being uncorrelated with each other. If this is the case, then events in one "universe" can't have causal connection with another because that would imply correlation.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#265

Post by eterevsky »

lupire wrote: post #9
dzdt wrote: post #4

There is an often-repeated statement that different quantum mechanical theories are equivalent. This is an over-simplification. Is wave function collapse is a real thing or just an approximation of what happens when a large system is entangled? Some smart people have tried to make theories where wave function collapse is a real thing. These new experiments are working to rule out those ideas. My feeling is that Evere…

Disproving some form of measurable wave function does not provide evidence for MWI, which is unmeasurable and untestable nearly by definition.

MWI in a sense it a null theory regarding the collapse, so it should be assumed true by default if collapse is not observed.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#266

Post by raattgift »

phkahler wrote: post #240
jfengel wrote: post #144

Earlier quoted context omitted.

No. They don't behave at all like dark matter. Among other things, there's no reason for them to be distributed in a halo, nor for some galaxies to lack them.

There's no reason for any dark matter to be distributed in a halo. Maybe it makes some math work better for the observations but nobody offers a dynamical explanation of how it would come to be a halo.

> There's no reason for any dark matter to be distributed in a halo.

This was one of the more eye-catching statements in this thread. To be blunt, you're wrong. A straightforward and almost inevitable dynamical explanation has been available (and explored in ever greater detail) since at least the mid-1990s, driven in part by the Saskatoon experiment and subsequently supported by BOOMERanG's observation of the first acoustic peak, and many others lines of evidence since then (see [1] for a less-heavy explanation than what follows).

You may wish to check this chapter from a forthcoming graduate-level textbook in galactic dynamics from well-known galactic astrophysicist Jo Bovy http://astro.utoronto.ca/~bovy/>, which you can read for free at his site: https://galaxiesbook.org/chapters/IV-01.-Formation-DM-Halos....>.

There are of course other already-completed textbooks. For example, one can find an excellent pedagogic (if advanced) treatment of primordial Gaussian Random Fields[2] as applied to halo formation in chapter 7 of Mo, van den Bosch & White (2010) https://www.cambridge.org/core/books/abs/galaxy-formation-an...> (you can find the chapter on scy hob via its DOI if you don't have institutional or otherwise affordable access).

- --

[1] https://lweb.cfa.harvard.edu/~deisenst/acousticpeak/acoustic... from https://astronomy.fas.harvard.edu/people/daniel-eisenstein>

[2] If you're more visually inclined and can't find the illustrated version of Mo et al., see the diagrams on p.3-5 of this slide deck on GRFs https://www.astro.rug.nl/~weygaert/tim1publication/lss2016/l...> from Rien van de Weijgaert of the Kapteyn Institute at the University of Groningen.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#267

Post by glenstein »

lamontcg wrote: post #219
GoblinSlayer wrote: post #196

Earlier quoted context omitted.

You end up in all branches and your consciousness ends up in all branches, where your mind works. MWI works best with physicalism, yep :) Maybe your copy in another branch even posted the same comment.

But how does bifurcation of consciousness happen and how does it diagonalize the state so that we never see a cat in a complex linear superposition of |alive> and |dead> at the same time? 1/sqrt(2) |alive> + 1/sqrt(2) |dead> should be an equally valid outcome of the experiment via purely unitary evolution.

I feel like this is a pretty reasonable question being met with non answers. And it's a shame bc I think MWI is the frontrunner as of 2022, but this is a pretty significant question with Many Worlds I had not considered before.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#268

Post by guerby »

While reading about the various QM experiments I've always wondered why people use the word "measurement" for what looks more like to me as "particle collision with large enough macroscopic effect to be seen by human".

A few QM experiments:

https://en.wikipedia.org/wiki/Double-slit_experiment

Little dots on the screen are some collision with a macroscopic visible effect.

https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experime...

Silver atoms going one way or another again on a screen and little dots.

https://en.wikipedia.org/wiki/Aspect%27s_experiment

Detectors probably some collision again and counting or imaging apparatus.

And of course a collision is a physically mostly discrete event so no one would question wether there's a need for an "observer" for it.

Would love to be pointed at some QM where there's no collision or similar discrete event for "measurement" as I'm a total noob in QM :).

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#269

Post by plutonorm »

simonh wrote: post #164
plutonorm wrote: post #157

Earlier quoted context omitted.

Broad statements, do you have the time to elaborate?

Dark matter affects some galaxies, but the rotation of others is completely explained by the observable matter. Why would one galaxy be influenced by its many worlds twins, but not another?

Well because that galaxy in that position might be a low probability event - not many universes have that galaxy at that position. So therefore that galaxy appears to have less dark matter.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#270

Post by isitmadeofglass »

dzdt wrote: post #4

There is an often-repeated statement that different quantum mechanical theories are equivalent. This is an over-simplification. Is wave function collapse is a real thing or just an approximation of what happens when a large system is entangled? Some smart people have tried to make theories where wave function collapse is a real thing. These new experiments are working to rule out those ideas. My feeling is that Evere…

The MWI isn’t “no wave function collapse” it’s an infinite number of universes all collapsing with different outcomes.
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