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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

#71

Lazy evaluation.

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. All particles are constantly entering new superpositions, not just individual particles in fancy lab experiments. Only the very simplest of particle interactions (like two individual hydrogen atoms interacting) are feasible to simulate with full quantum mechanics on classical computers. * "Observation" happens through any physical interaction at all, including particle collisions. Outside of some individual particles inside carefully controlled experiments, most particles end up transitively interacting with most other particles near them.

If the universe was at all optimized for simulation costs for human experience, we probably wouldn't expect there to be be trillions of galaxies with hundreds of millions of stars each, for the smallest particles to be on the scale of billion-billionths compared to humans, or for QM to work anything like it does.

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

#72
post #60

Earlier quoted context omitted.

> 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.

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.

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

#73
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…

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

There's no proof that it's the only possible way - but no-one's ever been able to come up with a concrete alternative.

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

#74

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?

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.

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 the past or in the future).

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

#75
post #72

Earlier quoted context omitted.

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

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 all the time in experiments.

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

#76
post #73

Earlier quoted context omitted.

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

There's no proof that it's the only possible way - but no-one's ever been able to come up with a concrete alternative.

There are other alternatives such as the derivation of quantum theory within the GPT framework and many other axiomatic derivations.

I've never seen the Born rule derived from unitary evolution and axioms for how subjective experience should work, so I don't even see this as one of the ways.

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

#77

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.

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.

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

#78

Earlier quoted context omitted.

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.

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.

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

#79

Earlier quoted context omitted.

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.

Measurement is a transfer of a state information from a subsystem 'being measured' to a 'measurement device' caused by an interaction. (ie: a quantum bus is a measurement) Just like in a classical situation, further dynamics can depend on the final state in the measurement device. The only difference is that all measurement states will be involved in the quantum measurement device, and all possibilities of dynamics conditioned on the measurement are explored. At any point in the future the whole chain of events can be collapsed, if the measurement device interacts with a classical incoherent object.

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

#80

Earlier quoted context omitted.

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.

Thanks. Do you by chance have a good source for a gentle introduction into axiomatic QM? Like undergrad level is fine, I've taken basic QM and worked through Griffith's intro book on my own, and I've had a lot of math.

I'd love to read more but my google results aren't turning up a good definitive introduction.

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