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

#81

Post by amai » Tue, Sep 08, 2020, 4:50 PM UTC

Every measurement (gravitational, electronic, magnetic etc. ) causes a collapse of the probabilities. This has nothing to do with quantum mechanics. This is simply statistics/math and has nothing to do with physics.

A simple example: As long as one doesn't look at a coin, the probability that it shows head or tail is 50%. After the measurement it "collapses" to 100% for one of the options.

But the "collapse" is only a mathematical "collapse", not a physical one.

Physics only limits how precise and fast your measurement apparatus can be.

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

#82

Post by saalweachter » Tue, Sep 08, 2020, 5:56 PM UTC

AgentME wrote: post #71

brianberns wrote: post #52

Earlier quoted context omitted.

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

There's no work QM is saving: it doesn't delay work or get to do less until when you physically look at something and then get to skip work for things you don't look at. * Things that are in superposition would require more computation for all their many alternate versions (most of which aren't meaningfully interacted with), not less, and the extra work for managing superpositions continues forever if MWI is right. A…

That time-steps for atomic interactions need to be picoseconds or smaller, but interesting biological reactions take minutes+ [eg,protein folding] is also a PITA.

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

#83

Post by tsimionescu » Tue, Sep 08, 2020, 7:10 PM UTC

akvadrako wrote: post #77

tsimionescu wrote: post #74

Earlier quoted context omitted.

In physical terms, we do generally define existence that way - for example, we say that time and space didn't exist 'before' the big bang, because there was nothing that could have a position or change. I was thinking of the same notion of existence and how it can be applied to MWI - essentially existence in the physical sense must mean that something is measurable, that it has some effect on the world (perhaps in th…

I don't think we do define existence that way. Say you and your friend both go to opposite ends of the visible universe; due to inflation you'll never be able to communicate again. I suspect most people would say their friend continues to exist. This is very analogous to the many worlds situation.

Thinking about the extreme distances and time spans that entails makes it difficult, and of course relativity has its own "unreasonable" results. Still, they do exist in your past, and they also can assign coordinates in space-time to your current position, even though they are outside your light-cone. On the other hand, you can't meaningfully speak of them existing "now" in relativity, as there is no consistent definition of what "now" means for observers that are space-like separated.

I guess the best answers about MWI is that the other versions of these particles continue to exist at different coordinates in Hilbert space, and that they do interact with each other in observable ways, such as the interference patterns in double-slit experiments.

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

#84

Post by tsimionescu » Tue, Sep 08, 2020, 7:11 PM UTC

ben_w wrote: post #29

tsimionescu wrote: post #27

Earlier quoted context omitted.

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?

Speaking as a barely informed enthusiast, we can say they exist in the Occam’s Razor sense that the maths is much less complicated when we assume they do. I think there’s also an experimental setup, whose name I forget, but which is essentially nested Schrödinger's cat setups: Alice is in a box, Bob is in a box which contains Alice’s box, Carol is outside; Alice goes into superposition of |Alice+> and |Alice->, Bob o…

The maths is the same whether we interpret it as many worlds, wave function collapse, and others.

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

#85

Post by mrwnmonm » Tue, Sep 08, 2020, 7:12 PM UTC

mrwnmonm wrote: post #12

I read it "Gravity is unlucky"

Why in the hell I get downvoted for this comment?

The world is full of freaks.

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

#86

Post by mgraczyk » Tue, Sep 08, 2020, 8:59 PM UTC

Orlanthai wrote: post #69

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.

I interpret KS backward from what you're saying. From my understanding, KS says that there is no sense in which the experiment involves an objective revelation of hidden state.

After the experiment, the experimenter did not "learn" or "reveal" some objective fact about reality, ie that the true state was UP rather than DOWN. Instead, after the experiment the experimenter becomes entangled with the UP/DOWN system in such a way that the experimenter measured both UP and DOWN, but all observables relating to the experimenter are either wholly consistent with UP or wholly consistent with DOWN.

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

#87

Post by Orlanthai » Tue, Sep 08, 2020, 10:24 PM UTC

mgraczyk wrote: post #86

Orlanthai wrote: post #69

Earlier quoted context omitted.

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.

I interpret KS backward from what you're saying. From my understanding, KS says that there is no sense in which the experiment involves an objective revelation of hidden state. After the experiment, the experimenter did not "learn" or "reveal" some objective fact about reality, ie that the true state was UP rather than DOWN. Instead, after the experiment the experimenter becomes entangled with the UP/DOWN system in s…

The lack of noncontextual hidden variables is the main implication of the Kochen-Specker theorem from which the inability to condition follows. It's not "backward" from the conclusion of no noncontextual hidden state, it's just another consequence there of.

So the Kochen-Specker theorem says there was no pre-existent noncontextual state for the particle. However that doesn't in any way imply the particle was both UP and DOWN or that the device measured both UP and DOWN. Especially for the device as the Kochen-Specker theorem is proved in a context where observable outcomes are assumed to be single-valued.

However in real practice we can condition on the states of our devices following measurement, hence they don't seem to be susceptible to a Kochen-Specker result when viewed as the system for some "super"-observational device. Which they would be if they simply entered an entangled state. Thus the assumption of measurement as simple entanglement does not match actual observed reality. This is a point made in many texts such as those of Schlosshauer, Omnès, Peres and at a very rigorous level in the theory of C* algebras and Category theory by Fröhlich and Landsman respectively.

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

#88

Post by Orlanthai » Tue, Sep 08, 2020, 10:27 PM UTC

alpineidyll3 wrote: post #78

Orlanthai wrote: post #66

Earlier quoted context omitted.

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.

One could argue even classical mechanics isn't deterministic as we think of it because of chaos, which has fascinating connections with QM. W Hoover (of the Nose-Hoover thermostat fame) did some great work with reversible thermostats exploring the instability of Newtons equations of motion.

I think that's different. Chaos still uses classical probability and the randomness is just ignorance of underlying initial conditions. This is very different from QM.

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

#89

Post by alpineidyll3 » Tue, Sep 08, 2020, 10:48 PM UTC

Orlanthai wrote: post #88

alpineidyll3 wrote: post #78

Earlier quoted context omitted.

One could argue even classical mechanics isn't deterministic as we think of it because of chaos, which has fascinating connections with QM. W Hoover (of the Nose-Hoover thermostat fame) did some great work with reversible thermostats exploring the instability of Newtons equations of motion.

I think that's different. Chaos still uses classical probability and the randomness is just ignorance of underlying initial conditions. This is very different from QM.

You'd be surprised. You should read about many-body localization and the eigenstate thermalization hypothesis.

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

#90

Post by lmm » Wed, Sep 09, 2020, 1:09 AM UTC

Orlanthai wrote: post #75

lmm wrote: post #72

Earlier quoted context omitted.

What would you expect "experiencing along those other bases" to look like? If you expand along a different basis you just get something like: half a chance of experiencing (1/sqrt(3)|x> + 2/sqrt(3)|y>), and half a chance of experiencing (1/sqrt(3)|x> + 2/sqrt(3)|y>), so it amounts to the same thing.

Those are the same states so I'm not sure what you mean. The point is that there is no reason to select out any particular basis over another. You can't just retreat into "well this is the only basis I can experience" because the human sensory apparatus would be able to select out a range of bases in a full unitary account and also the ambiguity of basis decomposition means you can't perform conditioning which we do…

> Those are the same states so I'm not sure what you mean.

I mean that if you decompose along a different basis than experiencing x/experiencing y, you just get an ensemble of states each of which is a superposition of experiencing x and experiencing y. So you end up with the same thing.

It's like looking at an entangled state (because that's exactly what it is) - if we have a two-particle state like 1/sqrt(2)(|x>|x> + |y>|y>), that behaves like the first particle being in |x> and experiencing the other particle being in |x>, or being in |y> and experiencing the other particle being in |y>, and it might look like that's an artifact of this particular basis decomposition, but it actually isn't - the structure of the wavefunction is that it divides cleanly into those two branches, and that's true in any basis.

> You can't just retreat into "well this is the only basis I can experience" because the human sensory apparatus would be able to select out a range of bases in a full unitary account

A system that's freely interacting will become entangled; whatever we consider ourself is constantly interacting with the rest of ourself, almost by definition.

> also the ambiguity of basis decomposition means you can't perform conditioning which we do all the time in experiments.

Of course you can, and it works exactly the way you'd expect - we already do experiments where some isolated apparatus inside the experiment does something if it detects one thing and something else if it detects something else. Choice of basis is a tool for understanding the wavefunction, not a physically real thing.