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James Webb Space Telescope finds evidence for alternate theory of gravity

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355 points jchanimal | 7 comments | 16 Nov 24 18:33 UTC | HN request time: 0s | source | bottom
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samsartor ◴[16 Nov 24 20:21 UTC] No.42158987[source]▶
My hangup with MOND is still general relativity. We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold. Any model of gravity based on an inverse law is simply wrong.

Another comment linked to https://tritonstation.com/new-blog-page/, which is an excellent read. It makes the case that GR has never been tested at low accelerations, that is might be wrong. But we know for a fact MOND is wrong at high accelerations. Unless your theory can cover both, I don't see how it can be pitched as an improvement to GR.

Edit: this sounds a bit hostile. to be clear, I think modified gravity is absolutely worth researching. but it isn't a silver bullet

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throwawaymaths ◴[16 Nov 24 23:35 UTC] No.42160543[source]▶
> My hangup with MOND is still general relativity.

Fwiw, we know for a fact also that for edge cases GR is wrong because it doesn't agree with quantum mechanics (unless QM is wrong), so it's maybe not right to take GR as gospel, especially for a theory that only seems to also change GR in edge cases, and the only reason why "it doesn't agree" might amount to "the math is hard and the physicists haven't put enough work in yet"

To wit, accepting a mond-ified GR is probably not going to change how GPS works so the claim that "GR has withstood the test of time and engineering" is not a totally solid refutation of MOND

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pdonis ◴[17 Nov 24 06:44 UTC] No.42162410[source]▶
> for edge cases GR is wrong because it doesn't agree with quantum mechanics

What "edge cases" are you talking about? I agree that GR is not a quantum theory, but it's not established that that has to be a problem, nor is it a matter of "edge cases".

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naasking ◴[17 Nov 24 14:28 UTC] No.42164350[source]▶
GR has singularities. It's definitely wrong in those regimes.
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1. pdonis ◴[17 Nov 24 17:02 UTC] No.42165273[source]▶
> GR has singularities.

More precisely, GR allows spacetime solutions which are geodesically incomplete.

> It's definitely wrong in those regimes.

No, that's too strong a claim. Most physicists believe that the solutions that are geodesically incomplete will turn out not to be valid in the regimes close enough to the endpoints of the incomplete geodesics. But that is a belief, not a proven fact. The solutions themselves are perfectly consistent mathematically.

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2. naasking ◴[18 Nov 24 14:34 UTC] No.42172586[source]▶
> But that is a belief, not a proven fact. The solutions themselves are perfectly consistent mathematically.

Every physical theory with singularities has has broken down in that regime. It's not even clear what it would mean for reality to permit singularities. That's a bit more than just a belief.

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3. pdonis ◴[18 Nov 24 15:21 UTC] No.42173142[source]▶
> Every physical theory with singularities has has broken down in that regime.

Can you give some examples? Note that GR has not even been tested anywhere close to the regime you are talking about.

> It's not even clear what it would mean for reality to permit singularities.

GR doesn't "permit singularities" in the sense I think you are using that phrase. "Singularity" in GR actually does not mean what I suspect you think it means, that things like spacetime curvature "become infinite". Notice that in my previous post I was careful to use the term "geodesic incompleteness", since that's what "singularity" actually means in the GR literature. And even in particular cases where there are invariants that increase without bound along incomplete geodesics, the limit points, such as r = 0 in Schwarzschild spacetime, are not actually part of the spacetime in GR. All invariants are finite at every point in the actual spacetime.

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4. naasking ◴[18 Nov 24 15:44 UTC] No.42173450{3}[source]▶
> Can you give some examples?

See Baez's paper, Struggles with the Continuum, https://arxiv.org/abs/1609.01421

The UV catastrophe is probably the most well known.

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5. pdonis ◴[18 Nov 24 15:55 UTC] No.42173565{4}[source]▶
The issue in that paper isn't really limited to "singularities". The basic issue is that we use the continuum in our physical models, and "reality" might not actually be a continuum, so the continuum math we use is just an approximation. But if "reality" isn't a continuum, it isn't a continuum everywhere, not just near "singularities", so the continuum is an approximation everywhere, not just near "singularities". The approximation would just become unworkable near "singularities", while remaining workable in other regimes.

Most physicists believe that our best current theories, GR and quantum field theory, are approximations anyway ("effective theories" is the term often used in the literature), so that in itself is not a new idea. Baez's paper points at one fairly common hypothesis for why they are approximations and what the underlying theory they are approximations to might look like. I don't have an issue with that as a hypothesis; it's just something we aren't going to be able to test by experiment any time soon, since the most likely scale for where the approximation will break down, the Planck scale, is some twenty orders of magnitude away from the scales we can currently probe with experiments.

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6. naasking ◴[20 Nov 24 21:17 UTC] No.42198211{5}[source]▶
Yes, there are many issues in our theories and even our formalisms. All I was trying to is point out, in the simplest way possible, was at least one way we know that GR is "wrong" (incomplete), which is its singularities.
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7. pdonis ◴[21 Nov 24 03:08 UTC] No.42200718{6}[source]▶
> we know

No, we don't know. Most physicists believe it, but that's not the same as knowing. We won't know unless and until we are able to actually do experiments in the relevant regime.