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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

#141

Post by eigenket »

CuriouslyC wrote: post #94
naasking wrote: post #69

> But when they tried to quantize gravity, they ran into unnatural infinities that had to be sidestepped with clumsy mathematical tricks. Maybe they run into unnatural infinities because all of our formalisms in physics are still fundamentally continuous rather than discrete. Uncountable infinities are baked right into the foundations of how we use reason about these systems, so infinities will naturally result. Phys…

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.

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

#143

Post by dawnofdusk »

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…

Yeah the "gravitational hum" would not seem to be the same as gravity itself having some inherent randomness. On the flipside, in a very real sense if the cosmological history of our universe is such that we exist in a universe with an always present indeterministic hum, then for our intents and purposes this is the same as gravity itself being random... unless we can devise experiments to study gravity in other universes with different cosmological initial conditions.

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

#144

Post by kgwgk »

consilient wrote: post #106
tgv wrote: post #84

Earlier quoted context omitted.

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…

> Assuming many worlds, and many here means: enormous amounts, far exceeding the number of particles in the universe, is everything but parsimonious. "Many worlds" is a misnomer. MWI is just wavefunction realism + unitary evolution. There's only one world, and really only one dynamical object: the wavefunction. It evolves according to some unitary operator, and that's the whole story. No splitting, no collapse, no ob…

Is it possible to observe anything at all in MWI? If yes, what does it mean to "observe something" in MWI? If not, is there any physical content in MWI?

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

#145

Post by zerodensity »

fsh wrote: post #109
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.

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?

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

#146

Post by eigenket »

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.

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 different states rather than a superposition. 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.

Essentially from the point of view of the massive object, the interaction with space-time acts as some decoherence process.

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

#147

Post by goatlover »

317070 wrote: post #140
FollowingTheDao wrote: post #113

Earlier quoted context omitted.

I wish any of you with the down vtes would at least have a conversation with me...

> our minds collapse the wave function While I know there are many sources available of people saying this exact thing, this is a common misunderstanding of the Copenhagen interpretation of quantum physics. There is nothing special about our minds.

It was Wigner who speculated that consciousness causes collapse. But Neils Bohr had a Kantian approach to science, so he did think it mattered how our minds understood the world, and what we can and can't say about physical formalisms.

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

#148

Post by eigenket »

inciampati wrote: post #61

> So if gravity is quantized, that means space-time is also quantized. But that doesn’t work, But... How could space-time not be quantized? That would imply the existence of infinities in the structure of the universe. It is like the ultraviolet catastrophe but in space-time.

I think this entire comment thread is using the wrong definition of the word "quantized". There are two relevant meanings of this word

1. Something split up into discrete chunks (like integers vs real numbers)

2. Something which acts quantum mechanically (i.e. with superposition and entanglement and all that stuff)

In this context "space-time is also quantized" means space-time is acting like a quantum system.

Whether space-time is discrete or continuous is a wholly seperate question. There is no particular evidence that it is discrete, but if you make the discretization scale small enough it theoretically could be. Most things work more nicely if it is continuous though.

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

#149

Post by edgyquant »

Gordonjcp wrote: post #101
inciampati wrote: post #61

> So if gravity is quantized, that means space-time is also quantized. But that doesn’t work, But... How could space-time not be quantized? That would imply the existence of infinities in the structure of the universe. It is like the ultraviolet catastrophe but in space-time.

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.

You’re thinking of Planck scale which is just a mathematical abstraction not some physical thing we’ve observed.

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

#150

Post by FollowingTheDao »

317070 wrote: post #140
FollowingTheDao wrote: post #113

Earlier quoted context omitted.

I wish any of you with the down vtes would at least have a conversation with me...

> our minds collapse the wave function While I know there are many sources available of people saying this exact thing, this is a common misunderstanding of the Copenhagen interpretation of quantum physics. There is nothing special about our minds.

First, thank you for responding.

I’m not saying there’s anything special about our mind, it’s just a function of our mind.

And is it misinterpretation or is it my interpretation that you disagree with?

How can one rationalize the fact that a photon can be a wave and a particle at the same time?

I do not believe, for example, that the moon exist, only because I look at it. I think it exists in a probabilistic state in that probabilistic state turns to certainty when I look at it. But that is only my certainty. How do I share my mind’s interpretation of what the moon looks like with someone else? Exactly? I can’t, it’s impossible. The moon both exist, and does not exist at the same time. And what I mean it does not exist. I mean it does not exist in any sense of certainty. Hence the moon is a wave function. The fact that my brain in your brain can collapse the way function in an apparent 100% similarity provides no proof that the moon exists in a single state outside of our own minds.

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