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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

#231

Post by [deleted] »

meindnoch wrote: post #224
kromem wrote: post #185

In the decades since the establishment of these theories, both the continuous (classical) spacetime of general relativity and discrete matter of quantum mechanics, the world has changed in rather significant ways. One of those has brought forth advances in technology which led to creating virtual world geometry with continuous function derivation which then gets converted into discrete voxels in order to track state…

>virtual world geometry with continuous function derivation which then gets converted into discrete voxels in order to track state around free agent interactions What?

[deleted]

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

#232

Post by codethief »

bowsamic wrote: post #14
eterevsky wrote: post #8

(Not a quantum physicist. Please correct me if I am misunderstanding.) From what this article says, his assumption is that gravity is classical, but "fuzzy" or probabilistic: you can't precisely measure the gravitational field of a sufficiently small object. In the last years we've seen progressively bigger objects being put in quantum superposition. The theory from this article is incompatible with this process cont…

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 is not quantized. I'm just saying it cannot be ruled out yet.

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

#233

Post by codethief »

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…

> Then the interaction between space-time and the massive object end up pushing the object from the superposition state you tried to produce into a mixed state just like the space-time.

So we'd be evolving from a pure to a mixed state?

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

#234

Post by goatlover »

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…

> Atoms are still just a human created mathematical abstraction we use to describe and make predictions about matter. All there really is is energy and atoms are just a mathematical tool we use to study energy.

We can see atoms with electron microscopes. A mathematical tool to study energy doesn't say anything. A tool describing how chemistry arises from atomic structure does.

> 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.

Not proved, but imagine using this sort of argument against evolution being a true account of life's history on Earth. Of course it won't get everything right, but in general there is simply no other way to explain how life forms changed over time. Similarly, there's no other way to understand microscopic physics than quantum mechanics. Maybe a more complete theory combining gravity and QM would be more true, just like Newtonian physics was incomplete and superseded by relativity.

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

#235

Post by codethief »

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 don't know about quantum gravity experiments but there are already (proposed) experiments that don't involve gravity which might help distinguish between different interpretations:

https://www.scientificamerican.com/article/this-simple-exper...

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

#236

Post by Gordonjcp »

jfengel wrote: post #171
Gordonjcp wrote: post #101

Earlier quoted context omitted.

But space is quantised, that's what Planck's Constant is. The oddity we see with things at relativistic energies is simply because - as with all discretised representations of continuous-time systems - things get a bit fucky close to Nyquist, and the maths breaks down.

That's not what Planck's Constant is. Planck's Constant connects energy to length (in space or time). It's just a way of shifting units around. You're thinking of the Planck Length, which is the result of combining the Planck Constant with the speed of light. It also happens to be the wavelength of a photon so short that its energy makes it a black hole, connecting it to gravity. It's often presented as if it were th…

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

#237

Post by goatlover »

VirusNewbie wrote: post #215
slowmovintarget wrote: post #28

Earlier quoted context omitted.

In Everettian Mechanics, decoherence occurs long before the box is opened. The photons in the box interact with the cat. That entanglement with the environment which causes the branching of the wave function occurs then. The observer that opens the box is already either in a branch where the cat is asleep (to borrow from Sean Carroll) or awake. The interesting thing to come to understand is that probability doesn't a…

But the observer is in all “probabilities” no?

There are observers in all probabilities of the experiment, but each observer only observes their world because decoherence interferes with the superposition of the other worlds. Worlds being the entanglement of the entire experimental setup spreading to everything else.

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

#238

Post by codethief »

jostylr wrote: post #228
meroes wrote: post #169

Earlier quoted context omitted.

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?

To my knowledge, it does not. There are various items to address with only item 4 being currently with no clear direction: 1) Bohmian mechanics seems to require some kind of simultaneity. Various proposals have been put forth for making natural foliations, possibly using the wave function to do so, that allow one to evaluate the positions of all the particles at a given time so as to know which configuration point to…

Thanks so much for your enlightening comment!

> The difficulty is purely in having a properly defined wave function evolution and that is on its way to being solved.

Interesting, could you provide some references?

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

#239

Post by cornel_io »

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?

This isn't about a QM interpretation, rather it's about a non-quantum model of gravity with some features that would make it distinguishable in principle from any quantum gravity.

Edit: I maybe semi take this back, it actually may be the case that this particular weird model of gravity would provide a way to distinguish between the interpretations of QM because of how grossly it violates the usual assumptions of QM. It seems too implausible for me to really dig into but I don't entirely grok how they propose melding the proposed inherent but non-quantum randomness with the rest of quantum physics in a way that still makes things line up well in the face of observation/collapse (or the equivalent in many-worlds). They might lean enough on assumptions about how that happens to break the observational equivalence?

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

#240

Post by drdeca »

gerdesj wrote: post #230
skinner_ wrote: post #205

Earlier quoted context omitted.

I believe you are misparsing that sentence. The dogma and the tradition are the same thing.

Dogma and tradition are two very different concepts. Dogma is forced down from "above" and tradition is consensual. Tradition is "organic", dogma is proscribed. Tradition: "We chase a wheel of cheese down Coopers hill in Gloucestershire and invite severe injury, for a laugh" - won by a somewhat concussed Canadian lass this year, well done her. https://www.theguardian.com/food/2023/may/29/woman-wins-coop... Dogma: "Th…

Skinner_ is saying that the description which nomel interpreted as describing "an alternative to the thing being called dogma" and as "tradition", was a description of the thing being described as "dogma", and not, as nomel interpreted, as an alternative thing to the thing that the quote was considering "dogma".
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