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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

#51

Post by fsh »

mgraczyk wrote: post #48

What percentage of practicing physicists actually think there is something "missing" in the quantum formalism as it relates to "wavefunction collapse"? I'm not a physicist, but from the few years of QM I took in college my take is that there is nothing special about "measurement", it's just a label we apply to certain states becoming entnagled. As long as you don't believe there is anything magical about humans or ot…

In the lab we do measure things and we do observe wavefunction collapse. There is no satisfying explanation why "us becoming entangled with the quantum system" or whatever explanation one chooses looks like collapse to us.

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

#53

Post by mgraczyk »

fsh wrote: post #51
mgraczyk wrote: post #48

What percentage of practicing physicists actually think there is something "missing" in the quantum formalism as it relates to "wavefunction collapse"? I'm not a physicist, but from the few years of QM I took in college my take is that there is nothing special about "measurement", it's just a label we apply to certain states becoming entnagled. As long as you don't believe there is anything magical about humans or ot…

In the lab we do measure things and we do observe wavefunction collapse. There is no satisfying explanation why "us becoming entangled with the quantum system" or whatever explanation one chooses looks like collapse to us.

I've always been content to know that after measurement, the quantum system that includes both me and the device has a certain quantum state which is a superposition of all the measurement eigenstates, but by construction that state couldn't be measured in such a way that I would have ever experienced an inconsistency.

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

#54

Post by akvadrako »

alpineidyll3 wrote: post #45
LittleTester wrote: post #41

Earlier quoted context omitted.

Decoherence doesn't really solve these issues. It gives you an approximately diagonal density matrix for the macroscopic degrees of freedom, but: (a) Not exactly diagonal (b) There isn't a unique decomposition of the macroscopic density matrix even after decoherence thus it cannot be taken as simply ignorance of some set of macrostates. You need something stronger, namely superselection or irreversibility.

That's correct, But there are numerous good methods to propogate irreversible dynamics in qm. Many of these are exact in the limit of a noninteracting bath of bilinearly coupled oscillators which is sufficient to describe a measurement collapse. There's no mystery or inexact proscription to such a simulation of a collapse process. It's just complicated.

You are really missing the point - the details of how a measurement happens with specific instruments is not what the measurement problem is about.

The issue is linear evolution means the measurement of a superposition leads to a superposition of measurement devices. If the quantum state is real that gives you many worlds.

If you are suggesting there is nonlinear evolution, well a) it must be non-local and b) the theoretical research suggests it would be inconsistent. QM is a very rigid theory - “an island in theory space”. It isn’t easy to slightly modify.

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

#55

Post by akvadrako »

ClumsyPilot wrote: post #42
akvadrako wrote: post #30

Earlier quoted context omitted.

Existence isn't based on something affecting our world, obviously - that's just absurdly self-centered. But anyway, the other worlds do effect our world - that's why we get interference patterns in double slit experiments.

> "Existence isn't based on something affecting our world, obviously - that's just absurdly self-centered." This is very unfair. This is a niche field with contested interpretations, don't make people feel stupid for asking fair questions. It's obvious what the other person meant: what does 'our world' and 'other worlds' mean, and how do you know it's not just a figment of your imagination, as a scientific theory mus…

I wasn’t making fun of anyone but your “obvious“ reading seems wrong.

I’m pretty sure he suggested that something only exists physically if it has some measurable effect on us.

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

#56

Post by lmm »

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.…

> I don't understand how purely unitary evolution can explain this. Does it?

What's the alternative? Assuming unitary evolution and some fairly common-sense axioms about how we'd expect subjective experience to behave (things like: we never experience being in a branch that has amplitude zero; if we experience being in a given branch then we continue to be in that branch), the Born probabilities are the only model anyone's ever come up with for how our subjective experience should go. So what's there to explain?

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

#57

Post by AgentME »

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?

An object that moves far enough away from us is said to leave the observable universe, because with the continual expansion of space, it or anything it interacts with would have to travel faster than light back toward us in order to have any effect on us. Should we say that objects that leave the observable universe continue to exist? Should we amend our theories to include a new fall-off effect separate from gravity that says things stop existing when they exit our observable universe?

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

#58

Post by Gene_Parmesan »

brianberns wrote: post #52
corey_moncure wrote: post #3

Lazy evaluation.

It does seem like some aspects of QM point to reality being simulation that tries to avoid the expense of unnecessary calculations.

QM-as-RNG for a simulated universe has been one of my favorite "for fun" pet theories over the years. Finding whether the electron went through slit A or slit B during the double-slit interference tests just feels a bit too much like running a Monte Carlo sim to me. I know it's just my human brain really trying to find an explanation that makes rational sense but it's a fun thought experiment.

I mean, quantum fluctuations during inflationary expansion are (in my understanding) what's responsible for the tiny differences in mass distribution that led to the eventual formation of gas clouds/stars/galaxies. The negative gravity during expansion worked to keep the matter in an extremely low-entropy (highly ordered) state, but those unfathomably small quantum fluctuations were blown up by the same unfathomably massive proportions as everything else.

Universe needed that RNG.

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

#59

Post by millstone »

lmm wrote: post #56
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.…

> I don't understand how purely unitary evolution can explain this. Does it? What's the alternative? Assuming unitary evolution and some fairly common-sense axioms about how we'd expect subjective experience to behave (things like: we never experience being in a branch that has amplitude zero; if we experience being in a given branch then we continue to be in that branch), the Born probabilities are the only model an…

The alternative is non-MWI theories, which typically introduce the Born rule via new axioms.

Regarding what's to explain, it's quantum randomness (which distills the Born rule objection). Our subjective experience is that we see spin-down 1/3rd of the time, and our theories say the result is otherwise impossible to predict, even in principle. But a deterministic theory cannot produce a random outcome, even a subjective one.

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

#60

Post by lmm »

millstone wrote: post #59
lmm wrote: post #56

Earlier quoted context omitted.

> I don't understand how purely unitary evolution can explain this. Does it? What's the alternative? Assuming unitary evolution and some fairly common-sense axioms about how we'd expect subjective experience to behave (things like: we never experience being in a branch that has amplitude zero; if we experience being in a given branch then we continue to be in that branch), the Born probabilities are the only model an…

The alternative is non-MWI theories, which typically introduce the Born rule via new axioms. Regarding what's to explain, it's quantum randomness (which distills the Born rule objection). Our subjective experience is that we see spin-down 1/3rd of the time, and our theories say the result is otherwise impossible to predict, even in principle. But a deterministic theory cannot produce a random outcome, even a subjecti…

> Our subjective experience is that we see spin-down 1/3rd of the time, and our theories say the result is otherwise impossible to predict, even in principle. But a deterministic theory cannot produce a random outcome, even a subjective one.

Whyever not? What else would you expect the subjective experience of being in a state like 1/sqrt(3)|x> + 2/sqrt(3)|y> to be like?

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