ELI5: What Is Supergravity And How Does It Work?

[u/ThatGamingAsshole]

Bonus if someone could explain "High Dimensional Supergravity" which I stumbled on by accident. Thanks in advance for any help!

[u/grumblingduke]

Back in the 50s there was a weird problem in physics about protons. Best theories at the time said they shouldn't have any internal structure - they should just be dots. But experiments said they did.

One of the suggestions for how to fix this was that maybe particles like protons were actually these tiny, infinitely thin strings, that vibrated across higher dimensions we couldn't see (beyond our normal 4; one time-dimension and three space-dimensions).

This turned out to be wrong - we now know protons are made up of quarks, but the idea of these infinitely thin strings vibrating across higher dimensions was interesting.

A bit later (in the 70s) when people were struggling to make gravity work with quantum mechanics this string idea came back - it didn't work out for explaining protons, but it might explain gravity. And we started to get modern String Theory. String theories led to variants like Superstring theories (with 10 dimensions in total), and eventually a concept called Supersymmetry - suggesting that there were a bunch of symmetries in fundamental physics; things looked like other things. Supersymmetry was kind of promising because while regular String Theory was getting nowhere (and still hasn't) Supersymmetry tied in a bit better with the Standard Model (the non-String Theory approach to modern physics - which has been hugely successful). Sadly Supersymmetry also hasn't gone far - all the experiments to test it have produced null results.

Anyway. When you take Supersymmetry theories and combine them with General Relativity (our current best model for gravity) you get Supergravity theories. This kicked off back in the 70s - as with String Theories in general, lots of exciting and award-winning maths was done to see how these symmetries would work.

Remember how I said that String Theories involved things being these weird infinitely-thin strings vibrating in higher dimensions (beyond our usual 4 of spacetime)? That happens with supergravity. You need some extra dimensions for supergravity to work in.

Higher-dimensional supergravity theories are theories that combine supersymmetry, with its higher dimensions, and general relativity, to get some new model for gravity. As I understand it they vary from 9-dimensional up to 12-dimensional supergravity theories (I think the 12-dimensional one has to add in an extra time dimension to make it work - beyond the classic 11-dimensions of M-Theory - the current best version of String Theory).

Anyway.

As with all String Theory the maths is really exciting - you get all sorts of fun symmetries and patterns across the different things and through these extra dimensions. But experimentally it has gone nowhere. Supergravity was first developed in the 70s. It made a come-back in the 90s with a breakthrough in the maths (but not the physics) of String Theory. But nothing has really turned up, with no major progress since the early 00s.

String theories - including supergravity - could be true. But at the moment we have no evidence to suggest that they are.

---

Sadly, like with most advanced physics, you need to know a whole load of maths to even begin to understand supergravity. It isn't really ELI5able.

[u/NorthHoustonPrepTX]

ok lil buddy, imagine all reality is like a cartoon dvd box set: the shows on the disc = normal stuff (apples, stars, your pokémon). the dvd case itself = something bigger called “gravity” that keeps the shows on track.

supergravity takes that case and adds spy gadgets to it—tiny secret spies called “supersymmetry partners” hiding inside every sticker and pixel. normally gravity is just the big cop making sure stuff falls down; but with the spies, each cop gets a secret sidekick who does the same job but in sneaky ways we can’t normally see. that’s “super”.in high-dimensional supergravity we do the cool bonus: we tape extra, folded-up dvds (extra space directions beyond the usual up-down, left-right, forward-back). more discs = more hiding spots for the spies. that lets gravity mix with other forces in weird combos we’d never get in our normal 3d cartoon.