Gravity is unlikely to be the cause of quantum collapse, experiment suggests • Hacker News Board

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

#61

Post by im3w1l » Tue, Sep 08, 2020, 8:40 AM UTC

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.…

To paraphrase you, how do we get from probability amplitude to observed frequencies if there is no collapse?

This is were we have to invoke philosophy. Specifically how does consciousness interact with time? The common-sense thinking is that our soul is tied to our body and is traveling forward through time with it. Another way of thinking is that the soul is tied to a given position of the space-time-probability. It does not travel. You today is not the same as you tomorrow or yesterday. The you that observes spin up is not the same you as the one that observes spin down. Your soul is perceiving reality from a randomly chosen vantage point among all the possibilities with which have a compatible body. If we condition on those bodies belonging to experimenters who have observed frequencies, then we get the distribution.

This is one possibility anyway.

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

#62

Post by Joker_vD » Tue, Sep 08, 2020, 8:51 AM UTC

akvadrako wrote: post #54

alpineidyll3 wrote: post #45

Earlier quoted context omitted.

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 the…

> 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.

And that's problematic because? Because it explains away the whole measurement problem, there is nothing to explain, it's an artifact of a macroscopic observer's point of view?

It's like saying that SR/GR with their space-time continuum being real is problematic, so let's keep to the (post)-Newtonian point of view, but oh no, it now has all this weird amendments and additional terms, and when you try to extend it to the whole of the Universe, it breaks down/gives really weird stuff. Well, duh, of course it does, if one tries to pull an owl on a globe, it simply won't fit.

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

#63

Post by Orlanthai » Tue, Sep 08, 2020, 8:51 AM UTC

akvadrako wrote: post #54

alpineidyll3 wrote: post #45

Earlier quoted context omitted.

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 the…

Well you don't need to have nonlinear evolution to get what alpineidyll3 is saying. It's sufficient for the observable algebra of macroobservables to be commutative. This allows the evolution to be linear and have no interferences.

The QM is "an island in theoryspace" idea isn't strictly true either. QM is one among an entire family of probability theories. It's only rigid when considered purely from the point of view of Probability theories based around vectors in Hilbert space. However considered as part of OPTs in general there's nothing that makes it difficult to modify.

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

#64

Post by Orlanthai » Tue, Sep 08, 2020, 8:53 AM UTC

lmm wrote: post #60

millstone wrote: post #59

Earlier quoted context omitted.

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?

There's a continuous infinity of alternate basis expansions. There's no reason to think you'd experience along that particular basis.

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

#65

Post by im3w1l » Tue, Sep 08, 2020, 9:01 AM UTC

Xcelerate wrote: post #49

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…

> then there doesn't seem to be anything to figure out about collapse. Then why is quantum state evolution seemingly continuous and unitary some of the time, and sharply discontinuous at other times?

It seems continuous and unitary when the experimenter takes a lot of care to create a simple and very isolated system. When it's brought into contact with the mess that is the rest of the world it changes character very quickly, and interactions with the rest of the world completely swamp the internal dynamics.

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

#66

Post by Orlanthai » Tue, Sep 08, 2020, 9:01 AM UTC

alpineidyll3 wrote: post #44

GuB-42 wrote: post #34

Earlier quoted context omitted.

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…

There is no measurement problem. It so happens that most measurement apparatus are macroscopic and at a temperature much higher than the quantum gap of the system being probed. Measurements in qm imply a rotation between systems, and mixture with an incoherent thermal state implies a quantum superposition will lose coherence. Full stop. This is all understood in full, grab any text on non equilibrium quantum dynamics…

It depends on what one calls the measurement problem.

This solves the "consistency/small problem", i.e. treating the macroscopic apparatus as boolean is justified.

It doesn't resolve the "outcome problem", i.e. which outcome is selected. Of course if you accept the world is not deterministic this isn't really a problem.

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

#67

Post by thegabriele » Tue, Sep 08, 2020, 9:02 AM UTC

alpineidyll3 wrote: post #44

GuB-42 wrote: post #34

Earlier quoted context omitted.

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…

There is no measurement problem. It so happens that most measurement apparatus are macroscopic and at a temperature much higher than the quantum gap of the system being probed. Measurements in qm imply a rotation between systems, and mixture with an incoherent thermal state implies a quantum superposition will lose coherence. Full stop. This is all understood in full, grab any text on non equilibrium quantum dynamics…

What do i measure of a quantum system if nothing collapse?

Honest question.

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

#68

Post by Orlanthai » Tue, Sep 08, 2020, 9:02 AM UTC

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…

Derivations like that don't work, you've just declared it by fiat but there's no such proof that is known to work.

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

#69

Post by Orlanthai » Tue, Sep 08, 2020, 9:06 AM UTC

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…

Measurement is not just the same as entanglement. If you try that you get paradoxes and you don't match actual observations.

If you treat a measurement as entanglement then by the Kochen-Specker theorem you can't condition on the measurement outcome. However we seem to be able to do this in actual experiments.

Thus measurement is not entanglement alone, but also the elimination of other bases.

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

#70

Post by im3w1l » Tue, Sep 08, 2020, 9:17 AM UTC

corey_moncure wrote: post #3

Lazy evaluation.

Quantum computers are asymptotically more powerful than classical computers. This means that our world is actually harder to simulate (rather than easier as you seem to suggest) than a classical world would be.