[HN Gopher] Gravity is unlikely to be the cause of quantum colla...
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Gravity is unlikely to be the cause of quantum collapse, experiment
suggests
Author : tdhttt
Score : 78 points
Date : 2020-09-07 18:34 UTC (4 hours ago)
(HTM) web link (www.sciencemag.org)
(TXT) w3m dump (www.sciencemag.org)
| TheRealPomax wrote:
| "Now, one of the most plausible mechanisms for quantum collapse--
| gravity--has suffered a setback." no it hasn't: stop trying to
| inject drama where there is none.
| corey_moncure wrote:
| Lazy evaluation.
| CamperBob2 wrote:
| https://en.wikipedia.org/wiki/Potentially_visible_set
| jng wrote:
| indeed :)
| akvadrako wrote:
| 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.
| gus_massa wrote:
| I agree. There are a (very) few experiments that show the
| effect of gravity in a system with strong quantum effects.
|
| Fox example you can split a ray of neutrons, direct each beam
| through a different path with different height and then make
| them collide and see the interference pattern. The idea is that
| the split creates a superposition and each half has a different
| gravity potential, changing the orientation of the experiment
| produce different interference patterns. (The details are in
| the book of Sakurai "Modern Quantum Mechanics" pp127-129, with
| data from an experiment of Colella, Overhauser, Werner (1975).)
|
| I don't understand why the old experiment was not enough to
| falsify this theory.
| macspoofing wrote:
| >Gravity was never a leading explanation for collapse; it was
| always a fringe idea.
|
| Why not? Gravity is (probably) mediated by a particle, and
| therefore all matter will interact with all other matter ... so
| why shouldn't gravity therefore cause a collapse?
| akvadrako wrote:
| My second sentence?
| gus_massa wrote:
| (Nobody is sure about the graviton, but IMHO it is a good
| bet.)
|
| Electromagnetism is also mediated by particles (photons) and
| the quantum states can survive a lot of electromagnetic
| interactions without collapsing. One of my favorites https://
| en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experime...
| alpineidyll3 wrote:
| 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.
| macspoofing wrote:
| >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?
| nabla9 wrote:
| Wave function collapse is not experimentally verified or
| observed physical phenomenon (so far). It's postulated by
| some interpretations of QM.
|
| Apparent wave function collapse can be explained using
| quantum decoherence.
| btilly wrote:
| 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 hand.
|
| But as we create ever more complex but controlled systems,
| we can perform ever more elaborate experiments verifying
| that quantum mechanics works exactly as predicted. At some
| point if we take seriously the idea that the most
| successful scientific theory of all time is an accurate
| description of ourselves, then we have to accept that
| perhaps there is no collapse after all.
| gmkiv wrote:
| 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?
| LittleTester wrote:
| 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.
| tsimionescu wrote:
| 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?
| nabla9 wrote:
| Different worlds don't exist in and extra space or
| dimension. They are orthogonal quantum states of the
| whole universe.
| ben_w wrote:
| 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 Schrodinger'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 opens the box
| and Carol can now demonstrate that Bob is in a
| superposition of |observing Alice+> and |observing
| Alice-> _instead of_ the combination of 100%|observing >
| and a superposition of |Alice+> and |Alice->.
| akvadrako wrote:
| 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.
| trenchgun wrote:
| The first paragraph seems to be pretty much all misleading or
| wrong:
|
| _It's one of the oddest tenets of quantum theory: a particle
| can be in two places at once--yet we only ever see it here or
| there. Textbooks state that the act of observing the particle
| "collapses" it, such that it appears at random in only one of
| its two locations. But physicists quarrel over why that would
| happen, if indeed it does. Now, one of the most plausible
| mechanisms for quantum collapse--gravity--has suffered a
| setback._
| ksaj wrote:
| As soon as it says "here or there" the writer is clearly
| trying to make it binary again. It isn't "here or there" but
| a function of probability between those two "places." It's an
| infinite number of possible "locations," but we narrow it
| down to a large number of decimal places because otherwise we
| wouldn't be able to make use of it at all. If things were as
| written here, you could do all quantum calculations with a
| straight up coin toss (although some might argue that can
| with an adequately tiny coin, but that still only gets at the
| most basic understanding of "spin" and only becomes an
| accurate analogy if the coin is flipped randomly in zero-g).
|
| Our higher level concept of gravity (at least in the
| classical sense) has been known to break down to the point
| that it doesn't apply to quantum mechanics for quite a long
| time now. The writer should just google "quantum gravity" to
| discover what a complicated subject that becomes.
| trenchgun wrote:
| Indeed.
|
| The weird thing is that the writer of that article has
| apparently written a popular science book about quantum
| mechanics too: https://us.macmillan.com/books/9780374536619
| andi999 wrote:
| Yes, but at least a in principle testable fringe theory. (And
| by penrose!)
| alpineidyll3 wrote:
| For the past several decades Penrose has been a fountain of
| nothing but frivolous fringe theories. Physicists are like
| investments: past performance is not indicative of future
| returns ;P
| andi999 wrote:
| As long as they are testable, it is good enough.
| zackees wrote:
| Tesla was right about Einstein. He was a fuzzy hair crackpot and
| his mathematical model for the Universe is riddled with
| contradictions yet the media continually pushes this guy. Why?
|
| It's funny how science is portrayed as careful and thoughtful but
| then the main stream media pushes theories like string theory and
| super symmetry, which fall to pieces as soon as the large
| particle accelerator machines came online.
|
| When are we going to declare particle physics as fundamentally
| broken and start over at N. Tesla?
| CodesInChaos wrote:
| Which model do you prefer? Both special and general relativity
| are clearly much better models of reality that Newtonian
| physics, even if they break down under extreme conditions.
|
| You're right that string theory and super symmetry are dubious,
| but it's not like they're anywhere close to being accepted.
| eitland wrote:
| > Tesla was right about Einstein. He was a fuzzy hair crackpot
| and his mathematical model for the Universe is riddled with
| contradictions yet the media continually pushes this guy. Why?
|
| Why?
|
| Because it works for a lot of stuff. Example:
|
| We wouldn't have working GPS without.
|
| But let me admit that if you have a better model I'm all for
| that.
| mrwnmonm wrote:
| I read it "Gravity is unlucky"
| tdhttt wrote:
| The [paper](https://www.nature.com/articles/s41567-020-1008-4)
| Animats wrote:
| Nice. It's an actual experimental result.
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