Hacker News Reader

Comment by tus88 | Hacker News Reader

> When bootstrapping, physicists determine how elementary particles with different amounts of “spin,” or intrinsic angular momentum, can consistently behave. In doing this, they rediscover the four fundamental forces that shape the universe.

They have derived gravity through quantum mechanics? What rock have I been living under?

View on HN

Not a physicist by any means, but from the article it talks about gravity in the context of the graviton (as one plausible solution to the problem mentioned). Which is still hypothetical, so it's not like they've proven gravity from first principle or anything.

As a layman reading this, it’s funny. Because I do not need gravity proven to me from first principles ;)

Proven is the wrong word. You cannot prove a theory.

Deriving a description of gravity from first principles would be more appropriate

*an empirical theory.

I can certainly prove things about, say, addition.

if you mean you can prove 2+2==4, it's actually tautology: | | + | | == | | | |

no like, "addition is closed over the integers" is something you can show to be True (for all members of an infinite set, no less -- so it's not empirical.)

And yet, funnily enough, your phone needs a more thorough understanding of gravity than you do. The GPS system must correct for relativistic effects that cause the atomic clocks to shift relative to ground.

Isn’t this corrected by the satellites themselves?

Mostly, but there are still correlations between the satellites that are corrected by the receiver.

The Earth's gravity field is not deterministic.

There is a nice derivation of Newton’s law from quantum field theory in Zee‘s book „Quantum Field theory in a nutshell“. You can also arrive at Einstein’s field equations by deriving the beta function of bosonic string theory and (2-2 scattering to be precise) and requiring that it is constant.

Interesting, is there any reading material on the second point?

David Tong‘s lecture notes are pretty good: What I am talking about is covered in lecture 7: http://www.damtp.cam.ac.uk/user/tong/string/seven.pdf

> They have derived gravity through quantum mechanics?

Sort of! The problem is that the solutions of the existing quantum field theories are not renormalizable so infinities arise in the solutions. The whole point of quantum gravity is to find realizable solutions.

Asymptotically safe quantum gravity may not be a dead end, incidentally.

Feynman lectures on Gravitation, 1962

Thanks.

Basically, Feynman starts with microscopic approach. We want to describe interaction where each piece of mass/energy is attracted to each other. So lets introduce a boson (whole number spin - 0, 1, 2, ...) to carry interaction. Then he talks about spin properties, rejects spin-1 boson (aka photon), somehow rejects spin-0 boson (I don't remember arguments) and start building microscopic theory of interacting via spin-2 massless bosons. Because they carry energy there is a self-interaction. He builds interaction rules, Feynman diagrams, and then goes to classical limit, where he derives macroscopic classic GR from microscopic quantum interactions. That's what I remember from reading this book. Apparently he did it on the fly, lots of errors in derivations which were noted and corrected in footnotes. Nevertheless, very entertaining approach...

Reply on news.ycombinator.com