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A physicist who bets that gravity can’t be quantized

quantamagazine.org

251–260 of 378 posts

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

#251
Fields aren't quantised, that's the problem, that's why gravity can't be quantized. It's a field!

Immediately, dozens of people are about hit the reply button and say something like: "But all of modern physics is based on quantization!" or something to that effect.

The issue is that there's two ways to think of quantization, and for almost all practical experiments, there's no way to distinguish between the two.

Essentially, you can think of either the fields being quantized, or the interactions with fields being quantized. Unfortunately almost all experiments measure only interactions with fields, so there's few practical ways to distinguish between the two. The mathematics is largely equivalent as well, so physicists "picked one" of the two options, and forgot about the other, equally valid option.

This is similar to the Veritasium video titled "Why No One Has Measured The Speed Of Light".[1] From inside the universe, it's basically impossible to measure the one-way speed of light, all experiments measure the two-way speed of light. So... we just assume that it's the same speed both ways. Right now, that works well enough. If it stops working, then we need to revisit that assumption instead of devising ever more complex mathematics to explain away our faulty assumption.

One difference between the "fields are quantized or not" issue and the "one-way speed of light" issue is that the former is testable. It's just that most experiments don't happen to test it.

Any experiment that uses atomic orbitals is conflating the quantization of atomic orbital levels with field quantization. There are experiments that don't involve orbitals, such as free electron lasers or radio waves. All such experiments show no quantization of fields.

Unfortunately, those experimental results are cheerfully ignored and hand-waved away. But stop and think about it: how exactly would you model a five kilometre long radio wave quanta as a point? How could something like that be instantaneously absorbed? How would the "rest of the wave" know that the tiny detector had made it vanish? It's madness, clearly, but that hasn't stopped thousands of physicists using this flawed model of quanta buzzing around in free space, because for short wavelengths like UV light you can make the mathematics work without paradoxes.

The channel Huygens Optics has some great videos[2] on the topic.

Another aspect of quantization is that for any wave in a continuum, you can model its behaviour in several ways mathematically that all arrive at the same numerical result, but "paint a different picture" in the imagination. For example, rendering something like light waves or sound waves bouncing around in a room can be done in two distinct ways: Either using local simulations with little oscillators at each volumetric point interacting only with their neighbours, OR as points moving around the space, bouncing around like particles, and carrying properties around with them such as intensity, wavelength, and phase.

The former is the "wave pool" approach, the latter is the "Monte Carlo" approach.

All of modern computer raytracing graphics uses the latter, not the former. Why? Because it requires less memory. The former scales a x^3 as the side-length of the volume 'x' goes up, the latter requires x^2 memory to accumulate the rays on the surfaces of the simulated volume, ignoring the intermediate states in the middle.

Richard Feyman's QED is famously successful, and in large part it is practical because it uses the more efficient Monte Carlo simulation approach, treating fields as little particles bouncing around. This has entrenched the "fields are made up of little quanta" in the minds of entire generations of physicists.

It's just a mathematical trick! An efficient way to do integration! It's not the One True Path, an insight into the truth of the universe.

We all need to take a step back and revisit our assumptions, and try to get away from thinking of integration tricks as having explanatory power in and of themselves.

[1] https://www.youtube.com/watch?v=pTn6Ewhb27k

[2] https://youtu.be/ExhSqq1jysg?t=283

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

#252
post #212

Earlier quoted context omitted.

Any reason you are specifically picking the Higgs field in your question? I am asking, because this sounds a bit like you are riffing on a common misconception that the Higgs field has something to do with gravity, which is not the case. The interaction with the Higgs field is the reason some (only some) of the particles have a mass, but explaining gravity does not need to have anything to do with the Higgs field. Bu…

My understanding is that the Higgs field should be simpler than gravity because it's just a static value whereas gravity is SU(1)? At least to me it seems logical that if the Higgs is quantized, surely more complicated fields would be as well?

the way the quote reads it sounds like this physicist is saying that gravity is not a field at all.

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

#253

Earlier quoted context omitted.

Is that not what it does?

General relativity requires singularities, QM prohibits them. It’s a puzzle.

Singularities aren't believed to be real by physicists to my understanding. Which if true, would mean the existence of them in GR is an error of the formulation or an error of comprehension.

If you look at the tangent lines facing toward the middle of the torus they would point to a single middle point, a singularity. But if you follow those lines around the surface they would never get to that extrapolated point of singularity. Probably nonsense, but the 2d creatures can't see in 3d and all that philosophy stuff.

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

#254

> It’s become dogma. All the other fields in nature are quantized. There’s a sense that there’s nothing special about gravity — it’s just a field like any other — and therefore we should quantize it. I keep on citing Stephen Hawking here on HN, but it again seems very appropriate: > It would be rather boring if this were the case. Gravity would be just like any other field. But I believe it is distinctively different…

If gravity is not quantized, would that not mean that it has infinite information (the accuracy to represent its values to infinite digits of precision), and thus cause a black hole due to such high information density?

IIUC, quantized doesn't mean finite, it just means discrete. Energy being quantized in bound states in quantum mechanics means that the eigenvalues are discrete, but a state can still be any linear combination whatsoever of the eigenstates.

And since spacetime is continuous, it's determines by its values on a dense subset, in particular a countable sense subset, which would make both sets of possibilities, gravity and quantum, have cardinality something like R^N.

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

#255
post #76

Earlier quoted context omitted.

> Physics has repeatedly had to tame infinities by elaborate tricks, or by eliminating them entirely or In case of black holes by actually interpreting infinity as a real place in the universe.

If I were a betting man I'd say black holes have no singularity, but rather a core of extremely dense exotic matter (probably formed from top/bottom quarks) which we haven't detected because it decays quickly under less extreme circumstances.

I think it’s true that once an event horizon forms no physical force (known or hypothetical) can stop a singularity forming.

There may be some form of very dense matter that stops large stars from collapsing to the point where an event horizon forms in the first place, but that doesn’t seem to apply to super massive black holes.

For super massive black holes the event horizon grows too fast.

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

#256

Earlier quoted context omitted.

> possibly even without creating a particle accelerator the size of the solar system. Sure, you just need to entangle two objects big enough to exert a noticeable amount of gravity on one another but somehow do not interact gravitationally with the rest of the set up. Anyway, let's try a cat sized object first, then we'll finally know if Schroedinger had a point.

> somehow do not interact gravitationally with the rest of the set up. And how would you do that? If we had a way to shield a region of space from gravitational influence from something else we'd have a very useful technology

No force can be shielded fully, but that's fine, because we can make the other forces smaller using distance of time averaging or averaging over many repetition etc like we do in other physics experiments.

A scattering experiment in which we send entangled masses to fly by each other and measure their interactions sounds very possible, even if it is very hard technically.

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

#257

Earlier quoted context omitted.

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?

Here are a some papers:

Avoiding Ultraviolet Divergence by Means of Interior-Boundary Conditions https://arxiv.org/abs/1506.00497 This is perhaps the first of the papers and so may be a good place to start.

Bohmian Trajectories for Hamiltonians with Interior-Boundary Condition https://arxiv.org/abs/1809.10235 This is the Bohmian part of the story.

Multi-Time Wave Functions https://arxiv.org/abs/1702.05282 This explains the trickiness of having interactions with multi-time wave functions (space-time suggests having multi-time wave functions and no single time).

Creation Rate of Dirac Particles at a Point Source https://arxiv.org/abs/2211.16606 This seems to suggest that there is a kind of spiral approach from/to the point of creation/annihilation.

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The authors have done a variety of papers on this. A key phrase they use is Interior-Boundary Conditions.

They also released a book based on a course covering some of these ideas: Multi-time Wave Functions, An Introduction https://link.springer.com/book/10.1007/978-3-030-60691-6

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

#259
post #248

Earlier quoted context omitted.

General relativity requires singularities, QM prohibits them. It’s a puzzle.

Keep in mind in GR singularities don't really make sense either: i.e. when you pass the event horizon of a black hole, under GR the rules say that you must always been traveling towards the singularity. But the corollary of this is that it's not actually possible - mathematically - to arrive at the singularity (because then you'd be moving parallel with it rather then towards it). So while we can define what happens…

Can they reach the center? And how long would that take? I ask because time will slow down for them as they accelerate towards the center.

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

#260

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 simply the idea that all quantum experiments results could have been known before performing them, assuming a perfect knowledge of the state of the universe.

It's one of those things that could be true and explain all of QM but also is kind of a cop out. "Of course your two detectors are giving correlated results, they were tightly coupled 13.8 billion years ago and now they are forever linked like all things."

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