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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

#321

Post by bowsamic »

edgyquant wrote: post #214
goatlover wrote: post #165

Earlier quoted context omitted.

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…

They work because they use human logic which has evolved to understand the universe just enough to “work” and where that logic doesn’t work we use probabilities to ensure we’re only calculating the odds and deriving from there. You can devise probabilistic systems of equations to describe stock prices and account for when you could be wrong, but this is not you describing some objective set of equations the market em…

> This is quite literally the oldest debate in natural science and it seems like one half of the debate just decided they’re right a few centuries ago and stopped caring about the fact that’s not something they have (or can) ever proved.

Yet earlier you asserted one side of the argument without proving it nor even acknowledging that there is a debate

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

#322

Post by bowsamic »

codethief wrote: post #232
bowsamic wrote: post #14

Earlier quoted context omitted.

I’m a quantum physicist and yeah you’re totally right. It’s already obvious that a quantum theory of gravity is needed because we need a way to talk about superpositions of spacetime curvatures. Either QM is wrong in general or we need a way to treat spacetime that is in a quantum superposition Also my old supervisor actually suggested an experiment to observe this gravity induced entanglement but it would require ex…

> It’s already obvious that a quantum theory of gravity is needed because we need a way to talk about superpositions of spacetime curvatures. But it doesn't seem to be that way, at least not necessarily, and that's what the article is about. What makes it obvious to you that there is no room for unification theories (of GR and QM) that don't involve a quantized version of gravity? Note that I'm not claiming gravity i…

The only way it "doesn't seem to be that way" is if we determine that it's actually wrong that QM tells us that we can have entanglement between spatially separated objects. If QM is right about this, then we need quantum gravity. So either QM is wrong in some sense and/or we need to limit ourselves to a specific interpretation, or gravity must be quantum

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

#323

Post by bowsamic »

skinner_ wrote: post #213
eigenket wrote: post #174

Earlier quoted context omitted.

Thats definitely a good question, and one that people have thought about before. I think its amazingly difficult to test it experimentally, even compared to something like directly measuring to see whether a massive object in superposition is forced to decohere faster than we expect, which is already incredibly difficult.

I was taught that choosing a QM interpretation is a matter of taste. Am I understanding correctly that we do have proposed quantum gravity experiments that can falsify various QM interpretations, it's just that they are all very hard to execute?

> I was taught that choosing a QM interpretation is a matter of taste

You were taught wrong

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

#324

Post by bowsamic »

nocoolnametom wrote: post #182
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.

When you say superdeterminism are you referring to something like Pilot Wave theory, where what appear to our measurements as probabilistic yet random interactions are merely expressions of a more complex yet non-random underlying system that we cannot, as yet, measure? (I don't even know if that's the proper description of the hypothesis.)

Superdeterminism is a loophole in Bell's theorem that allows local hidden variables, so there could just be a fully deterministic theory that determines everything, including your detector settings in advanced. Classical physics has generally assumed that we at least have free choice to choose what to measure. If we don't, we can trivially retain classical gravity

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

#325

Post by bowsamic »

tsimionescu wrote: post #297
consilient wrote: post #104

Earlier quoted context omitted.

> Quantum mechanics tells us that, in order to predict the outcome of a measurement, we have to compute a specific probability based on the amplitude of the wavefunction Even this is already wading into interpretational waters. The math says nothing about whether a given POVM should be thought of as a measurement or an interaction (or both, or neither).

Sure, it's not part of the math, but it is part of the physics. There is a very clear distinction in the theory between "interactions", governed entirely by the Schrodinger equation (or its relativistic versions, or QFT), and a "measurement", where you have the apply the Born rule not only to predict what the measurement device will show, but also to correctly predict what will happen in a subsequent experiment on th…

> Sure, it's not part of the math, but it is part of the physics.

"the physics" here is a specific interpretation

> Decoherence helps explain certain things directly from the math without appealing to the extra Born rule, but not fully, and not quantitatively (not the specific probabilities).

You can derive the Born rule using decoherence

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

#326

Post by Mizza »

treeman79 wrote: post #4
btilly wrote: post #2

If you believe in the Everett Interpretation, then he is wrong. The Everett Interpretation, aka Many Worlds, holds that both observer and observed are quantum mechanical systems. From this it follows that the act of observation does not produce a "collapse", but it does separate the observer into multiple observers that can't interact with each other again. So Schrödinger's cat is in a superposition of alive and dead…

Where would energy come from for creating each universe?

Where do the eggs come from when you cut a cake?

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

#327

Post by strogonoff »

Timwi wrote: post #263
strogonoff wrote: post #208

Earlier quoted context omitted.

A simulation produces philosophical zombies, yet you are not one.

The idea of a philosophical zombie is logically incoherent. The only way to even get the idea is to make unscientific assumptions such as souls or a metaphysical life force, which is itself incoherent as it can't even decide what is “real”.

Cartesian dualism vs. naive materialistic monism is false dichotomy.

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

#328

Post by tim333 »

winterismute wrote: post #211

I have always been fascinated by the problem of quantum gravity but, well, it somehow happened "too late", after I got also very deeply into "computing" (mostly HPC, GPGPU, rendering). Does anybody know if there is a way for somebody with my background to actually help/contribute in advancing this field?

I have a personal theory that we have not been able to crack quantum gravity and similar because it's too much for human brains in the same way that we have a job visualising 5d space and dogs don't understand relativity. In which case AI going beyond human brain limits might be the way to crack it. Go build it!

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

#329

Post by GoblinSlayer »

naasking wrote: post #119
H8crilA wrote: post #82

Earlier quoted context omitted.

Oh no I get that, but two comments above inciampati was suggesting that we will end up in a situation where some variable will have to have an infinite value. Like, famously, with the ultraviolet crisis: https://en.wikipedia.org/wiki/Ultraviolet_catastrophe Detecting (something about) the particle through gravity would at best remove the interference, not create an \infty somewhere in the model, implying that the mod…

> Oh no I get that, but two comments above inciampati was suggesting that we will end up in a situation where some variable will have to have an infinite value The black hole information paradox from that article presumably fits. The conclusion from GR is that no information can escape, which is ultimately incompatible with QM, and that conclusion ultimately depends on the infinite density of the singularity. I think…

Problems 1-3 there are caused by point particles, not by space. They mention ultraviolet catastrophe was caused by continuous space, but it was caused by non-quantum radiation, so there's no classical electrodynamics with discreet space, instead there's quantum electrodynamics with continuous space.

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

#330

Post by tsimionescu »

bowsamic wrote: post #325
tsimionescu wrote: post #297

Earlier quoted context omitted.

Sure, it's not part of the math, but it is part of the physics. There is a very clear distinction in the theory between "interactions", governed entirely by the Schrodinger equation (or its relativistic versions, or QFT), and a "measurement", where you have the apply the Born rule not only to predict what the measurement device will show, but also to correctly predict what will happen in a subsequent experiment on th…

> Sure, it's not part of the math, but it is part of the physics. "the physics" here is a specific interpretation > Decoherence helps explain certain things directly from the math without appealing to the extra Born rule, but not fully, and not quantitatively (not the specific probabilities). You can derive the Born rule using decoherence

> "the physics" here is a specific interpretation

No, "the physics" here is the way we are supposed to tie the math to physical observations. You can't explain all of our experiments if you don't make a distinction between interactions which preserve quantum properties and measurements which lead to classical observations. For a specific example, you can't explain the double-slit experiment with a detector at one slit if you don't make this distinction.

> You can derive the Born rule using decoherence

No, you can't. I have actively looked for such derivations and found none (at least none that are considered credible), and there is literature that explicitly says that this hasn't been convincingly done, e.g. [0]:

> My main conclusion is that there is no way to derive the Born rule without additional assumptions. It is true both in the framework of collapse theories and, more surprisingly, in the framework of the MWI. The main open question is not the validity of various proofs, but what are the most natural assumptions we should add for proving the Born rule.

[0] https://pages.jh.edu/rrynasi1/HealeySeminar/literature/Vaidm...

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