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A physicist who bets that gravity can’t be quantized

quantamagazine.org

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Re: A physicist who bets that gravity can’t be quantized

#161

Post by bowsamic »

eigenket wrote: post #146
bowsamic wrote: post #46

Earlier quoted context omitted.

The problem is that it doesn't act like a mass distribution, it acts as two non spatially overlapping possibilities. I think that the only way to save classical gravity would be superdeterminism. If quantum states correspond to anything other than our ignorance, i.e. if superposition states are actual physical states of reality, then gravity will need to be quantum.

I attended a talk by Jonathan Oppenheim a while back on this subject, and my understanding based on that is that in his model something (very) roughly like this happens. You put your massive object in superposition, it interacts with space-time and "tries" to put space-time in superposition but since space-time is fundamentally classical (in his model) what happens is space-time ends up in a probabalistic mixture of…

> the interaction with space-time acts as some decoherence process.

Well, the question is, can we use gravity in the same way that we use quantum processes, i.e., in a coherent way? If we can, e.g. using it to establish entanglement, then the theory you describe cannot hold.

Re: A physicist who bets that gravity can’t be quantized

#162

Post by bowsamic »

H8crilA wrote: post #86
bowsamic wrote: post #79

Earlier quoted context omitted.

It's pretty simple. You basically have two suspended mirrors on wires and you shoot lasers at each mirror. Due to gravity there will be an interaction between the mirrors. There are then two things you can probe. First, the correlation between the two reflected lasers, and if you can achieve the extreme temperature requirements, entanglement between the two reflected lasers caused by the gravity. Now, the former does…

Those reflected beams would at best be very very weakly entangled, right? I'm not sure what's the name for it, but if you arranged the quantum state of the two beams into a 2x2 matrix then the determinant would be just a tiny bit non-zero.

Yes

Re: A physicist who bets that gravity can’t be quantized

#163

Post by consilient »

eigenket wrote: post #146
bowsamic wrote: post #46

Earlier quoted context omitted.

The problem is that it doesn't act like a mass distribution, it acts as two non spatially overlapping possibilities. I think that the only way to save classical gravity would be superdeterminism. If quantum states correspond to anything other than our ignorance, i.e. if superposition states are actual physical states of reality, then gravity will need to be quantum.

I attended a talk by Jonathan Oppenheim a while back on this subject, and my understanding based on that is that in his model something (very) roughly like this happens. You put your massive object in superposition, it interacts with space-time and "tries" to put space-time in superposition but since space-time is fundamentally classical (in his model) what happens is space-time ends up in a probabalistic mixture of…

Seems like this would show up as an equivalence principle violation, no?

Re: A physicist who bets that gravity can’t be quantized

#164

Post by rapjr9 »

bdamm wrote: post #123
rapjr9 wrote: post #114

I wonder if the recent news about the background gravitational hum of the universe has any bearing on this: https://www.sciencealert.com/breaking-news-physicists-have-d... It essentially means that gravity is always in flux everywhere, at all frequencies of gravity waves, so there is noise in gravity. This is in addition to the background EM radiation in the universe. Gravitational and EM fields are infinite are they…

Do we actually know that the fields are infinite? I thought it was just the easiest way to model what we know and can observe.

I think the question would be, what happens at the edge of the universe? If the universe is finite, though expanding, then the waves can not be infinite unless they reflect at the edge, although one definition of the size of the universe might be that it extends as far as gravity and EM waves have traveled since the big bang. In which case I think the answer would be that if waves define fields then fields are not currently infinite nor will they ever be infinite, but they will always define the edge of the universe and keep traveling forever and over infinite time they will keep traveling infinitely. Which means they are essentially infinite, but not actually currently infinite, but that difference might not be much of a difference, even mathematically. It would suggest that EM radiation and gravity become more dilute in any one place over time though, as the universe gets bigger and the waves spread out more.

There is also the speed of light problem. If EM radiation and gravity can only travel at the speed of light, then when new fields are created they do not propagate to infinity instantly, it takes time. So there is a kind of localness to EM/gravity but given the age of the universe a huge number of waves have propagated very far, which may be close enough to infinite we can't tell the difference.

Matter can be created and destroyed so there can be new gravity fields created? Although I believe current thinking is that energy also exhibits gravity so maybe all the gravity that will ever exist is already here. That would be a difference in the nature of gravity versus EM waves since new EM radiation can be created while perhaps new gravity can not. Is a wave a perturbance that travels at the speed of light in a field that propagates instantly? Or is a field the propagation of a wave that travels at the speed of light? The idea of infinite fields suggests instant propagation, but gravity waves and EM waves certainly do not seem to propagate instantly. Maybe the idea of fields makes no sense and there are only waves?

Re: A physicist who bets that gravity can’t be quantized

#165

Post by goatlover »

edgyquant wrote: post #152
bowsamic wrote: post #17

Earlier quoted context omitted.

Because it’s very straightforward and has less assumptions and weird features than most other interpretations, and it has seen the most advancements in quantum fundamentals research

These are just mathematical tools we use to make predictions. Assuming they represent objective reality and somehow prove an infinite number of universes doesn’t seem very scientific to me.

That's way too instrumentalist. The math also describes how atoms work and many other things. Modern physical understanding is based on those descriptions. Cosmology and nuclear physics would be useless without an understanding.

How do the predictions work if they aren't in some way modeling the way reality is? Why does the technology based on them work? Instrumentalism gives no answers to those questions. Science is about understanding the world, and using that to make predictions.

Re: A physicist who bets that gravity can’t be quantized

#166

Post by consilient »

zerodensity wrote: post #145
fsh wrote: post #109

Earlier quoted context omitted.

There is zero evidence that space is quantized, and this would contradict the Standard Model, as well as General Relativity. The Planck constant gives the relation between the energy of a photon (or more generally a harmonic oscillator) and its frequency. It has nothing to do with the structure of space.

How exactly does quantized space-time contradict the Standard Model and General Relativity?

It would break Lorentz symmetry, for one.

Re: A physicist who bets that gravity can’t be quantized

#167

Post by consilient »

eigenket wrote: post #141
CuriouslyC wrote: post #94

Earlier quoted context omitted.

Digital physics seems so obviously the correct approach from a philosophical perspective, it's a shame it blows up the math. Real numbers are the most ironically named thing ever, and infinity is a thought experiment - not a real thing. Any model of the universe based on such constructs should be heavily suspect outside the range of established observations, and taking predicted limit behavior seriously is just fooli…

Speaking as a physicist I think its very far from obvious that digital physics (or anything else) is the correct approach from a philosophical perspective. Real numbers are so ubiquitous in physics because space-time (and other quantities) look really continuous.

Agreed that digital physics is far from obvious, but the use of real numbers as our default model of the continuum is at least in part historical accident. We could have for instance easily ended up with locale-theoretic foundations instead, though I doubt the finitist crowd would find that any more satisfying.

Re: A physicist who bets that gravity can’t be quantized

#169

Post by meroes »

bowsamic wrote: post #46
phkahler wrote: post #35

Earlier quoted context omitted.

>> It’s already obvious that a quantum theory of gravity is needed because we need a way to talk about superpositions of spacetime curvatures. Is that because of things like the double slit experiment they mention? A particles could be monitored via its gravitational effect on spacetime to determine which slit it went through. What if the particles mass behaves as a mass distribution in such an experiment? Does that…

The problem is that it doesn't act like a mass distribution, it acts as two non spatially overlapping possibilities. I think that the only way to save classical gravity would be superdeterminism. If quantum states correspond to anything other than our ignorance, i.e. if superposition states are actual physical states of reality, then gravity will need to be quantum.

Do you know if Bohmian mechanics with a particle ontology (still with non-local "hidden" variables ofc) or GRWf has potential to save classical gravity as well?

Re: A physicist who bets that gravity can’t be quantized

#170

Post by captainclam »

tgv wrote: post #84
captainclam wrote: post #38

Earlier quoted context omitted.

I think because of the abuse of "many worlds" in science fiction media (especially as of late) as a convenient plot device, people develop this idea that it's an unserious proposal wrt the foundations of physics. As far as I have read, it strikes me as perhaps the most parsimonious interpretation of QM out there. Genuinely curious, what do you mean by "but you didn't say it couldn't be this, nanananana...?" The Evere…

Assuming many worlds, and many here means: enormous amounts, far exceeding the number of particles in the universe, is everything but parsimonious. There happens to be a model that fits some data, but that's it. It's a grotesque assumption to avoid a conflict in a man-made theory. It's a funny thought, but no more than that. There's also nothing special about observing. Our consciousness isn't super-natural, so the i…

Correct, it is an enormous amount...Under Everett, the "number of worlds" practically exists on a continuous spectrum, so basically an infinitude of worlds. But this doesn't violate parsimony in terms of building a model of reality. I don't have the greatest comprehension of the history of science, but I imagine the behavior of water and gas was not initially explained as being the collective behavior of moles (6.02x10^23) and moles of individual molecules. The parsimony of thermodynamics is measured by the simplicity of the underlying equations that describe their behavior, not the staggering number of atoms involved.

As for observation, I didn't suggest there was anything special about observing. In fact, this is part of what makes Everett among the most parsimonious interpretations: While some (not all) other interpretations are tasked with explaining the nature of observation (what counts as observation, how quickly does collapse propagate, etc), under Everett, there is no notion of observation at all. My point about consciousness is that, being conscious, we are forced to have a point of view which depends on where we exist in/on the wave function. I wasn't suggesting that consciousness has any actual effect whatsoever on the wavefunction.

I admit that the nature of probability is among the most difficult parts of Everett to wrestle with, though I'm not sure I understand why subsequent observations should follow a radically different pattern? It's still the same wavefunction with the same distributions as before...Schrodinger's equation is the same wherever you are on the wavefunction.

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