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A physicist who bets that gravity can’t be quantized

quantamagazine.org

221–230 of 378 posts

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

#221
post #196

Earlier quoted context omitted.

> Will other large thermalised system (e.g. rocks) also experience observations that "look classical"? The inner life of rocks is somewhat beyond our reach, I'm afraid. But they'll induce decoherence in the same way as a human, yes. > Is there something special about "large thermalised systems" (and/or humans)? Aside from being large and thermalized? No. > How large have they to be to allow for "our observations"? It…

> But they'll induce decoherence in the same way as a human, yes. From the perspective (?) of the (non-human) thermal bath, will that decoherence result in a single (diagonal, mixture) state or in a particular state of those N separate states that would "look classical"? Decoherence doesn't make things "look classical" by itself - at least until you define what "looking" is.

Depends on whether you're talking about the "perspective" of the whole joint |Rock>|Interaction Eigenstate 1> + |Rock>|Interaction Eigenstate 2> ... system or just one of its components.

> Decoherence doesn't make things "look classical" by itself - at least until you define what "looking" is.

Of course; but that's true of every scientific theory. Decoherence solves the preferred basis problem, not the hard problem of consciousness.

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

#222

Earlier quoted context omitted.

Planck's constant is not the Planck length.

The Planck length ℓ P is defined as: ℓ P = ℏ G c 3 ≈ 1.616 255 ( 18 ) × 10 − 35 m , where is the speed of light, G is the gravitational constant, and ħ is the reduced Planck constant. I know they’re not the same. But they obviously depend on each other, no?

> But they obviously depend on each other, no?

Sure, but so does twice the Planck length. Also half the Planck length. Also 86.173491 times the Planck length. And so on.

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

#223

Earlier quoted context omitted.

Hey, I’m no Einstein, but even he had hypothesis that took decades to be shown evidence to prove they were true. I don’t know how someone is supposed to come up with evidence before a hypothesis, but if you have a new understanding of the scientific process Please let me know.

> I don’t know how someone is supposed to come up with evidence before a hypothesis Sure, this is usually how science works. A good example is the famous Michelson–Morley experiment which was performed in 1887 and was a key piece of evidence directly inspiring Einstein's theory of special relativity in 1905. The way science usually progresses is that there is some piece of evidence which current theories can't explai…

What are ridiculous thing to say. The Michaelson Mosley experiment was sent experiment based on the hypothesis that something called the Luminiferous aether existed.

So someone had a hypothesis, and they tested it. I have a hypothesis, go out and test it if you wish.

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

#224
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?

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

#225
post #18
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…

I’m a “believer” in the Everett Interpretation (on the grounds that it’s the simplest interpretation) but I can’t see how such an experiment could prove anything. This experiment seems to assume we can keep the whole apparatus including the Geiger counter and the mechanism to move the cannon ball from becoming entangled with the environment (decohering). This seems very far outside the realm of current technology.

It shows that when there is a superposition of quantum mechanical states, you also get a superposition of two different space-time structures.

The reason why the Everett Interpretation is needed for the experiment to be valid is that according to it decoherence results is a superposition of quantum mechanical states, and not a collapse. If you believe in another interpretation, there has been a collapse, and so the experiment demonstrated nothing.

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

#226

Earlier quoted context omitted.

Rational numbers can produce "continuous" values to a precision way beyond what any equipment you could get your hands on could differentiate without physically ludicrous postulates.

That is certainly true. In fact you don't even need rational numbers, it is entirely possible that there are a finite number of positions on the universe, for example. Nevertheless physics seems to work very nicely when expressed in the language of calculus. Everything from Schrödinger's equation to the Einstein field equations, and from classical mechanics to the standard model of particle physics are expressed in t…

Unless the resolution of the rational model is fine enough to be indistinguishable from the continuous solution.

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

#227

> 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?

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

#228
post #169

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.

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 use in obtaining the velocity of the particle. This is possible, but it does not feel philosophically satisfactory yet. GRWf does not require a foliation to be relativistic which is a nice feature.

2) Quantum field theory naturally models particle creation and annihilation. Most QFTs are presented in a mathematically incoherent way. While conclusions can be made via renormalization, etc., driving an actual dynamics is tricky from that stuff. By not doing perturbations but rather defining the operators by taking into account the shifting of the probability from one sector of n particles to another of n+1 particles, QFTs can actually be made to make mathematical sense. This has been accomplished in some of the simpler models. It is not a problem whatsoever to define a Bohmian evolution where particles appear and disappear in a probabilistic fashion. The difficulty is purely in having a properly defined wave function evolution and that is on its way to being solved.

3) One needs to define wave functions, their evolution, and the particle evolutions in a curved space-time. This is not a problem whatsoever. It is very easy to translate and interpret what we need into differential geometric language. QM has issues with the usual observables/operators translating (such as a momentum operator), but since these are derived concepts in Bohmian mechanics, no fundamental difficulty arises.

4) In general relativity, the mass distribution is part of the evolution of the space-time metric. The mass is based on where the particles are. To date, the wave function tells the particles what to do, but the particles do not have any impact on the wave function. The wave function is impacted by the space-time metric. Also, there are some suggestions that mass might be entirely a part of the wave function and not associated with the particle, i.e., the particles are really just undecorated points moving about.

Basically, gravity and the wave function need to work it out and the particles will then be guided by both as the space-time metric is what takes the gradient of the wave function or, for Dirac style motion, something analogous to a square root of the metric is floating around. Bohmian mechanics does have the advantage that it just has to be concerned with the evolution of the particle lines and not having to figure out how to define various observables.

In other words, a known space-time metric interfaces just fine with Bohmian mechanics, but figuring out how to evolve the space-time metric is still an open question.

At the current time, I am not aware of any novel ideas coming from a Bohmian or GRWf point of view towards resolving the problem of gravity.

If interested in a good discussion of all these things and much, much more, I highly recommend the recent book Foundations of Quantum Mechanics by Roderich Tumulka which covers what the title says, but also discuss Bohmian mechanics and other interpretations.

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

#229

> 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?

Is that not what it does?

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

#230
post #194

Earlier quoted context omitted.

> it’s just a field like any other — and therefore we should quantize it. So, trade dogma for tradition!

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: "This is what we believe". Here is an example - https://www.catholicnewsagency.com/resource/55423/the-four-m...

Dogma is a religious thing. Originally Catholic but seems to have spread further.

Tradition is something that is done because that is how it was done. "Was hal" - be you hale ... that's where "wassail" came from.

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