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Comment by Raro | Hacker News Reader

For some context: the article only interviews quantum gravity researchers, so it has that particular slant. They are concerned with coming up with different strategies to mitigate the issues general relativity has with singularities, and these strategies range from "sensible deviations from general relativity" to "fun, but highly unlikely".

In terms of how the theory of gravitation is not like the other fundamental forces, I think the most salient point was made by Sera Cremonini: gravitation is not a renormalizable theory. At least in the sense of how we have approached renormalization with the other forces. It is intrinsically non-linear, because the source is the field (and vice-versa).

To get anywhere close to a grand unified theory that incorporates gravitation requires a rethink of the normalization procedure (and likely requires an entirely new geometric framework—this was a pre-requisite for both general relativity and Newton's Universal Law).

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> For some context: the article only interviews quantum gravity researchers, so it has that particular slant.

I would go even further: Natalie Wolchover only interviewed string theorists and quantum field theorists. They all assume that gravity in some way or another will just be yet another quantum field theory.

There is not a single piece of evidence for this, though. Personally, I also don't think it is a particularly promising approach. As Hawking so eloquently put it:

> But I believe [gravity] is distinctively different, because it shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background.

(Hawking in Hawking & Penrose: The Nature of Space and Time, chapter 1)

> I would go even further: Natalie Wolchover only interviewed string theorists and quantum field theorists. They all assume that gravity in some way or another will just be yet another quantum field theory.

I didn't want to push that point too much. I am personally uncomfortable with the lack of emphasis on empirical evidence. However, that's not to say there can be fruitful results.

In terms of re-thinking geometric approaches, I quite liked (what I understood of) Nima Arkani-Hamed's suggestion for avoiding issues with localization. Similarly for Penrose's twistor theory.

> I quite liked (what I understood of) Nima Arkani-Hamed's suggestion for avoiding issues with localization.

Which suggestion are you referring to? Do you happen to have a link?

The 'Amplituhedron': https://arxiv.org/pdf/1312.2007.pdf

The idea is to focus on the amplitude of scattering interactions from the momentum-space twistor perspective, rather than perturbations about a point in space-time.

For background: Nima made a name for himself by greatly simplifying complex particle interactions, from tens of pages of Feynman diagrams to around a page. I think of the above as an extension of that project. However, I don't recall all the details, as it's been six years since I went to his talk on this.

Oh yes, I've heard a bit about the Amplituhedron. Unfortunately, it only serves to simplify the perturbative calculations in Yang-Mills theories order by order, i.e. the Feynman diagrams. It doesn't describe the full (non-perturbative) theory, so it's not really a candidate for a replacement of "classic" local QFT.

Thank you for pointing this out; this accords with my recollection, but you clearly stated a fundamental issue with the approach in its current state.

I thought of the idea as more a sketch of how things might be looked at with a perspective change, rather than a full-blown theory (at least in the form I saw it in 2014). I guess I was holding out for the (highly unlikely) possibility of an asymptotic limit or something similar. It just seemed to be rooted more in reality than most of string theory/LQG. Of course, that could be my bias towards novelty!

> I guess I was holding out for the (highly unlikely) possibility of an asymptotic limit or something similar. It just seemed to be rooted more in reality than most of string theory/LQG. Of course, that could be my bias towards novelty!

I totally understand! I was quite excited about the Amplituhedron, too, when I first read about it!

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