Comment by jerf | Hacker News Reader

Comment by jerf | Hacker News Reader

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Gravity is a "consequence of the curvature of spacetime" in a particular theory that we know can't be the foundational description of reality, because all it has is mass and spacetime in it. Since it's a known-incorrect or known-incomplete theory (pick which one you like), we are not in a position to declare that gravity is absolutely, positively "curvature in spacetime" and nothing else.

Moreover, quantum mechanics comprehensively lacks gravity. It isn't just "oh, well, maybe this system is also accelerating due to gravity but we can ignore it". It's, if atoms are in a superposition of "over there" and "over here", what is the gravity of the resulting system? We know we can produce such superpositions in the real world, and when those systems exist, there must be some answer to that question, even though we don't know what it is. What impact does very, very high gravity have on the non-superposition-related aspects of the standard model? How do energies of all the various bits of the proton change in such a field? And so on.

It doesn't help at all to simply reiterate the theory of relativity. The conflict between relativity and QM is deep, real, and very very challenging.

Recently in pbs spacetime I saw a video about entropic gravity, where some of the researchers in this field seems to explore wether gravity is just an entropic force resulting from thermodynamics. It was an entersting perspective I haven't heard of.

Who knows, maybe in a distant future we'll that that 4th fundamental force is not so fundamental after all.

- https://m.youtube.com/watch?v=qYSKEbd956M

- https://en.m.wikipedia.org/wiki/Entropic_gravity

> the foundational description of reality

How can we ever really know that? Isn't the best we can do is just models?

A model that is able to perfectly describe and (more importantly) predict everything is effectively indistinguishable from “reality”. Yes, there might or might not be a completely different, and maybe even weird, process underneath that gives rise to that model, but assuming that nothing whatsoever of that underlying process “leaks” into the model, it does not matter and may as well not exist for us.

Luckily, we’re still pretty far from a model which fully describes reality, and it may be entirely unobtainable for us. (Also, we only assume that spatial and temporal locality isn’t a thing, i.e. that the laws of physics are the same everywhere and everytime, so there’s that as well.)

Science can never prove that it has reached the bottom, but it could hypothetically yield a model that makes predictions that always match the universe to within all relevant margins of error. It could be that that model is itself a simulation or that there are yet deeper layers we just can't see, whatever, you know, all that jazz. But if there was a complete model, with no internal contradictions, that matched everything we could see, then at least when someone said "in light of this theory, gravity is X", we would be making a coherent statement.

Right now we can't say what gravity is. We know we don't have a complete description of it, because the contradictions between QM and relativity are something explainable to a motivated high school student, as they are pretty fundamental. You don't need to deeply understand either theory to understand enough of the conflict to know we don't have an answer.

This isn't a criticism of either theory per se or the scientists involved. They're the best we've got and they're each on their own pretty good. But we are as justified in saying "gravity is definitely a curvature of spacetime because Relativity says so" as we are justified in saying "gravity doesn't exist because it doesn't appear in the standard model at all". Both are basically the same statement relative to the theory they relate to, it is just more obvious the latter is wrong.

I think that the same way that energy is shed when a particle steps from high energy to low energy is the way that "gravity aka possibility energy" is shed when quantum superposition collapses into position.

I have daydreamed of becoming a physicist just to try testing this.

To directly answer your question, by experimental results.

If you seek a foundational description of reality in physics, you are treating physics like religion.

Physics, as a natural science, deals in “if we do this then we observe that”. Note the “we”: it is inherently subjective. The models resulting from that are, similarly, a product of our minds seeking analogies and stories. Those models are useful for coming up with more experiments but are necessarily simplified maps, and if you have a map that exactly corresponds to the 100% of the territory then the map you have is A) useless, and B) impossible (so if you think you found one, you ought to think again).

At best you will never find such a foundational description of reality. At worst you will believe you’ve found one and be angered at anyone who chooses to use a different map and dares to not treat yours as gospel.

I think you're reading that comment rather uncharitably, I interpreted "description of reality" to be a synonym of "model".

Indeed, I went off a bit of a tangent on a pet peeve rant…

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