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Gravity is unlikely to be the cause of quantum collapse, experiment suggests

sciencemag.org

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Re: Gravity is unlikely to be the cause of quantum collapse, experiment suggests

#31

Post by gmkiv »

btilly wrote: post #23
macspoofing wrote: post #22

Earlier quoted context omitted.

>Collapse is only mysterious to people who never learned thermofield theory, or non-equilibrium quantum dynamics. Oh yeah? Then what is the physical explanation for collapse?

What is the physical evidence that collapse actually happens? Flash news. Nobody has ever produced any. To the contrast we have lots of lines of evidence that an observer described by quantum mechanics should, upon observing a quantum experiment, be thrown into a superposition of observers. Each of which appears to have observed collapse. The notion is utterly repugnant to our biases so many reject the idea out of ha…

I happen to agree with you, the observer is a quantum system must get entangled with the quantum system, but that still doesn't explain probabilities. If you prepare a system - say sqrt(1/3) spin-down + sqrt(2/3) spin-up, and then observe it, repeatedly, your subjective experience is that you saw spin-down 1/3 of the time, and spin-up 2/3 of the time. I don't understand how purely unitary evolution can explain this. Does it?

Re: Gravity is unlikely to be the cause of quantum collapse, experiment suggests

#32

Post by LittleTester »

btilly wrote: post #23
macspoofing wrote: post #22

Earlier quoted context omitted.

>Collapse is only mysterious to people who never learned thermofield theory, or non-equilibrium quantum dynamics. Oh yeah? Then what is the physical explanation for collapse?

What is the physical evidence that collapse actually happens? Flash news. Nobody has ever produced any. To the contrast we have lots of lines of evidence that an observer described by quantum mechanics should, upon observing a quantum experiment, be thrown into a superposition of observers. Each of which appears to have observed collapse. The notion is utterly repugnant to our biases so many reject the idea out of ha…

Well in the most common family of interpretations "collapse" isn't an actual physical process, just Bayesian updating. So you wouldn't expect to find physical evidence of it in that sense.

It's true that from the perspective of an external superobserver the quantum state evolves to contain terms for each observer observation state. However since all interference observables turn out to be non-physical for macroscopic systems we get a superselection rule and so the probabilities for different macrostates are classical probabilities and thus reflect simple ignorance of the observer's post measurement state.

There's very little motivation for reading the quantum state "ontically" in the way you are doing.

Re: Gravity is unlikely to be the cause of quantum collapse, experiment suggests

#33

Post by LittleTester »

gmkiv wrote: post #31
btilly wrote: post #23

Earlier quoted context omitted.

What is the physical evidence that collapse actually happens? Flash news. Nobody has ever produced any. To the contrast we have lots of lines of evidence that an observer described by quantum mechanics should, upon observing a quantum experiment, be thrown into a superposition of observers. Each of which appears to have observed collapse. The notion is utterly repugnant to our biases so many reject the idea out of ha…

I happen to agree with you, the observer is a quantum system must get entangled with the quantum system, but that still doesn't explain probabilities. If you prepare a system - say sqrt(1/3) spin-down + sqrt(2/3) spin-up, and then observe it, repeatedly, your subjective experience is that you saw spin-down 1/3 of the time, and spin-up 2/3 of the time. I don't understand how purely unitary evolution can explain this.…

No it can't. There have been many attempts and they don't work. The Born rule is independent of unitary evolution. The closest one can get is to declare that the quantum state is fundamentally a statistical object (i.e. the only information in it is observation probabilities) and then with certain assumptions about the size of the state space you can show that the Born rule is the only possible rule for connecting the state to statistics consistent with the unitary dynamics.

So under the assumption that the state encodes probabilities, state space assumptions and consistency with unitary evolution you get the Born rule. However this is not the same as the Born rule arising dynamically from unitary evolution alone.

Re: Gravity is unlikely to be the cause of quantum collapse, experiment suggests

#34

Post by GuB-42 »

alpineidyll3 wrote: post #15
akvadrako wrote: post #4

Gravity was never a leading explanation for collapse; it was always a fringe idea. Gravity is local and apparent collapse is non-local; it doesn’t even pass the smell test.

Collapse is only mysterious to people who never learned thermofield theory, or non-equilibrium quantum dynamics. Unfortunately this is also a sizable fraction of all physicists. Thus we have articles like this :/ Basically all the work in getting large scale quantum computers to work lies in these fields. They will continue to grow.

I am not a physicist but isn't it an instance of "if you think you understand quantum mechanics, you don't understand quantum mechanics"?

The measurement problem, which deals with collapse is still unsolved. It is also the reason why there are so many weird interpretations of quantum mechanics and that even top physicists can't agree on one. In fact the same physicists tend to sweep the problem under the rug and instead focus on the equations that, to be fair, did a lot more to science and technology than trying to solve the measurement problem.

Re: Gravity is unlikely to be the cause of quantum collapse, experiment suggests

#36

Post by Enginerrrd »

tsimionescu wrote: post #27
btilly wrote: post #23

Earlier quoted context omitted.

What is the physical evidence that collapse actually happens? Flash news. Nobody has ever produced any. To the contrast we have lots of lines of evidence that an observer described by quantum mechanics should, upon observing a quantum experiment, be thrown into a superposition of observers. Each of which appears to have observed collapse. The notion is utterly repugnant to our biases so many reject the idea out of ha…

But this doesn't answer the question. If you claim that all of these possible observers 'exist', how does this have a physical meaning? This is what I never understood about MWI, in what physical sense can the many worlds be said to exist? Where are they in our universe? What direction would we have to travel to find them? Do they exert gravity on us? If not, then how can we claim that they exist in a physical sense?

No you're thinking of MWI all wrong. Your conception of the universe you exist in as being non-quantum is fundamentally flawed. The universe with the superposition of all the possible observers exists more purely in hilbert space. Sean Carroll has even started to put together a model for how spacetime could emerge from that hilbert space.

So the universes all exist in the same place, since they are the same universe. Your idea of what an observation is, is just an eigenvalue of that corresponding operator.

Re: Gravity is unlikely to be the cause of quantum collapse, experiment suggests

#37

Post by Enginerrrd »

gmkiv wrote: post #31
btilly wrote: post #23

Earlier quoted context omitted.

What is the physical evidence that collapse actually happens? Flash news. Nobody has ever produced any. To the contrast we have lots of lines of evidence that an observer described by quantum mechanics should, upon observing a quantum experiment, be thrown into a superposition of observers. Each of which appears to have observed collapse. The notion is utterly repugnant to our biases so many reject the idea out of ha…

I happen to agree with you, the observer is a quantum system must get entangled with the quantum system, but that still doesn't explain probabilities. If you prepare a system - say sqrt(1/3) spin-down + sqrt(2/3) spin-up, and then observe it, repeatedly, your subjective experience is that you saw spin-down 1/3 of the time, and spin-up 2/3 of the time. I don't understand how purely unitary evolution can explain this.…

Isn't your subjective experience just one probabilistic eigenvalue of a particular combination of operators corresponding to your observation? How does unitary evolution break down here?

Re: Gravity is unlikely to be the cause of quantum collapse, experiment suggests

#38

Post by LittleTester »

Enginerrrd wrote: post #37
gmkiv wrote: post #31

Earlier quoted context omitted.

I happen to agree with you, the observer is a quantum system must get entangled with the quantum system, but that still doesn't explain probabilities. If you prepare a system - say sqrt(1/3) spin-down + sqrt(2/3) spin-up, and then observe it, repeatedly, your subjective experience is that you saw spin-down 1/3 of the time, and spin-up 2/3 of the time. I don't understand how purely unitary evolution can explain this.…

Isn't your subjective experience just one probabilistic eigenvalue of a particular combination of operators corresponding to your observation? How does unitary evolution break down here?

It's not unitary evolution breaking down, just that the Born rule isn't a consequence of unitary evolution. They're separate independent hypotheses. In most derivations of QM from an axiomatic basis they're consequences of separate combinations of axioms.

Re: Gravity is unlikely to be the cause of quantum collapse, experiment suggests

#40

Post by kanzenryu2 »

GuB-42 wrote: post #34
alpineidyll3 wrote: post #15

Earlier quoted context omitted.

Collapse is only mysterious to people who never learned thermofield theory, or non-equilibrium quantum dynamics. Unfortunately this is also a sizable fraction of all physicists. Thus we have articles like this :/ Basically all the work in getting large scale quantum computers to work lies in these fields. They will continue to grow.

I am not a physicist but isn't it an instance of "if you think you understand quantum mechanics, you don't understand quantum mechanics"? The measurement problem, which deals with collapse is still unsolved. It is also the reason why there are so many weird interpretations of quantum mechanics and that even top physicists can't agree on one. In fact the same physicists tend to sweep the problem under the rug and inst…

The first quote dates to the 1960s before decoherence was understood.
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