Comment by jprete | Hacker News Reader

Comment by jprete | Hacker News Reader

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I'm not a physicist but 1/r^2 strikes me as conceptually very important, because it's the relative contribution of any fixed area of spherical surface to the total area of that surface. So the total strength of gravitational field emanating from a particular object, at a given distance from that object, is a constant.

It's somewhat weird to think of the total gravitational "force field" _increasing_ in magnitude with distance. Decreasing, sure. Increasing? That makes no sense. Certainly not at a large enough function of distance for the rotational curve to be _flat_. That's got to be some kind of wonky power term over distance which implies potential energy from the field goes up with distance as well.

As above, I'm not a physicist, but a linear rotational curve breaks every intuition I've ever gleaned from physics about the nature of what's really going on with relativity, particle mediation of forces, or even the concept of a field.

Maybe it means spacetime curvature is way higher than we think.

Well the general solution is to invent a field with force mediating particles that have the exact same properties as dark matter but insist it's not dark matter.

Maybe not so weird if gravity isn't the curvature of spacetime but a symptom of there being either more or less of it, and mass creates spacetime.

Replace the highly curved spacetime region close to a blackhole with the idea that huge amount of spacetime is being created by the mass of the blackhole, so there is more spacetime near the blackhole.

The more spacetime being created and 'flowing outwards' away from the mass, the faster the apparent 'velocity' of an object through that region of spacetime ner the blackhole (and have this work out that the spacial component handles the physical motion and time slows down to compensate - just like it does in highly curved spacetime), and consequently the slower it moves relative to an external observer.

Areas further from mass see much more 'dilute' spacetime (whatever the heck that means) and travel with relative slower spacial velocity but faster in time, so it appears to be travelling faster up. This would be doubly obvious at the scale of galaxies.

I think this ridiculousness would rely on the relativity of simultaneity in rather a large way!

The other interesting thing is, if mass does create spacetime then pockets of mass like galaxies should move away from each other faster and faster as they make more of it in between themselves.

(NOTE - this is just a silly thought experiment, don't take it seriously)

I don't think this sounds silly at all. Virtually everywhere we look in cosmology the last decade or so, you get a sensation of things unseen. Like we're looking at one of those optical illusions that change shape when you cover your left eye, or like the McGurk Effect, when your audio perception shifts completely because you can see someone's lips.

Another element that's being discussed is, like with mass and spacetime, that the spatial dimensions themselves are emergent phenomenon arising from bulk entanglement. Sean Carrol has talked about it a fair amount, and it's been surfing around maybe harder than it would ordinarily, because it provides some edge cases that are, at least conceivably, testable without solar-system-sized accelerators or a DeLorean to the beginning of the cosmos. It's an evocative thought. In one interpretation of this, Double Slit restricts many of the spatial dimensions, resulting in a particle that might seem to be in different places, but which is, in some respects, the same particle. Another interesting notion is that singularities, in some dimensions, might be the same place.

Combined with your notion, it almost re-frames mass as - forgive me for getting poetic here - a measure of fate. How much does this resist doing that?

It’s not at all silly. There’s some nice visualisations[1] of GR in YouTube that look like space is being swallowed up by matter.

A toy model I like to use in my mind is that matter absorbs spacetime. It is literally sucked in!

A possible extension of this model is that the tension introduced in the vacuum causes it to stretch out. That could potentially explain the non-r^2 terms in galactic rotation curves.

[1] https://youtu.be/DYq774z4dws?si=6vDWZ8jPzgjxSBb1

The strong force increases with distance, fwiw.

I'd thought that was because gluons are color charged. Unlike neutral photons

> spacetime curvature is way higher than we think.

You mean way weirder.

Remember 1/r^2 does not work quite right for, e.g. mercury.

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