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Comment by MathMonkeyMan | Hacker News Reader

Here's the [paper][1] on the arxiv.

I found it hard to believe that they accounted for other forces precisely enough that they could attribute the phase change to gravity, but this is beyond me so I trust the result.

At first I thought "they showed that you can measure a particle falling in gravity," which seemed dumb because we already know that particles fall in gravity. But they showed that you can measure a single (aggregate) particle falling in gravity, which is pretty cool because if gravity is quantum then that means that they observed an interaction between the graviton and their rubidium atom.

[1]: https://arxiv.org/pdf/2502.14535

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There's no evidence for gravitons.

> ...if gravity is quantum...

You don't need evidence to support a premise.

Their point is measuring gravity's influence on a single particle is not necessarily evidence of the existence of the graviton.

Nobody's claiming it is.

“ if gravity is quantum then that means that they observed an interaction between the graviton and their rubidium atom.”

'if'

That's pretty pedantic. They were simply addressing one branch of the if, so their statement was relevant.

Actually, we do need evidence for premises which are not definitions or basic assumptions (axioms). Graviton is still a hypothetical particle and falls under neither of two categories - Definitions or axioms.

The statement "if gravity is quantum, this was an interaction with a graviton" is true even if it ultimately turns out that gravity is not quantum. The statement is about the consequences of a premise, and is not falsified if that premise is false.

The only way this statement could be false would be to have quantum gravity but no particle that mediates this interaction - which doesn't seem plausible almost by definition.

>> The only way this statement could be false

That's quite an exotic claim! If Sound and Temperature can emerge without a sound particle or temperature particle, why is it not plausible for gravity to exists without graviton?

Though, I get it that mainstream view from physicists is Graviton is the most 'likely' cause, if the gravity is proven to be quantized. But even they would have the humility to accept that this is a theory yet to be proven and observed!

Their statement has two possibilities:

A: If gravity is quantum, then (B) there must be some particle-like think that we call a graviton that mediates the interactions, and then seeing a rubidium atom fall must have been an interaction with this particle.

not A: If gravity is not quantum, then they are making no claims about a graviton.

If you think gravity is not quantum, then you go to the "not A" branch, and they make no claims about that branch - so there is no contradiction with their IF.

Maybe I should have been clearer! There is also a possibility for Gravity to be quantum without gravitons https://arxiv.org/abs/gr-qc/0204062 https://en.wikipedia.org/wiki/Induced_gravity https://en.wikipedia.org/wiki/Entropic_gravity

However, for me to claim that quantum Gravity is only emergent without a particle like graviton's mediation, I would need to present evidence. Just like this claim - quantum gravity is only possible from gravitons.

Ohh, yes, I had not at all understood that this was what you were referring to, especially as I had already suggested this is a real, though to my mind implausible, possibility.

Now, as to how plausible this is, we'll actually get into semantics. To me, saying that "gravity is quantum" would mean that it is a real force/interaction, and that this force obeys the Heisenberg uncertainty principle. So, I wouldn't say "gravity is quantum" if it turns out that the Entropic gravity theory is true, as in that theory gravity is only an apparent interaction. However, I admit that this may only be my perception of what these terms mean together, and that others may use "gravity is quantum" to refer to any theory that explains the macroscopic effects we call gravity in a way that matches quantum theories.

So, basically Gravity is quantum if Gravity is fundamental (not emergent)?

If I am understanding your counterpoint correctly, then this argument can only be concluded satisfactorily if either Graviton is observed (proving is existence) or something more fundamental and deeper is observed (proving that quantum gravity can emerge without graviton).

That doesn't stop us from imagining what something could mean in the context of gravitons if evidence of them ever does turn up.

This is a global issue with modern particle physics - scientists imagine something and then blink, and two decades and a hundred of books had been published while that something is still not present in any test. It would be fine if that happened rarely, but it seems that it's just par for course nowadays.

This is literally everything ever, not just particle physics or even physics in general.

It's a quiet test of Penrose's idea that gravitational interactions cause objective collapse of superposition.

Maybe there's something subtle in the details which explains why it isn't that, but it certainly looks adjacent to it - although maybe not deliberately?

Is this the basis of gravitational propulsion engines?

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