Adam Brown

One problem it solves is...

Gravity is mysterious, particularly once we improve quantum mechanics in various ways that we could go into.

This is why it's hard to quantize gravity.

But if you can say that this theory that involves both quantum mechanics and gravity is exactly Joule, is in some sense the same theory, it's just an alternative description of a theory...

in one few dimensions that doesn't involve gravity, well, that's great because we have much better grasp on how to understand theories that don't have gravity than we do on theories that do have gravity.

So it puts everything on a much clearer footing to have this non-gravitational description because then you can just use the standard tools of non-gravitational quantum field theory in order to define it and understand it.

Yeah, so maybe I should just lead with some disappointing news, which is that ADS-CFT was a tremendous conceptual breakthrough in our understanding of quantum gravity and embodied the holographic principle.

But at the same time, it doesn't describe our universe.

In particular, in ADS-CFT, there is a negative cosmological constant in the gravitational theory, and our universe, as we discussed before, has a positive cosmological constant.

So it's great because it provides an existence proof of a well-defined theory of quantum gravity, not, alas, in the universe in which we live in.

Okay, but having said that, yeah, it's extremely confusing and was a...

very impressive result, precisely because you might think, how could it possibly be the case that two different theories in two different dimensions could turn out to be equivalent?

And the answer to your question is, if you have two people who are living in this negatively curved space and talking to each other, what does that look like

in this other theory.

I say that there's this process going on in the gravitational theory.

That's Joule, which is exactly isomorphic to some process going on in the non-gravitational theory in one fewer dimensions.

But what maybe looks very simple in one theory, like you and I chatting back and forth to each other, would look like some complicated plasma physics in the...

lower dimensional boundary theory and so that the sort of complexity of how it looks like which is a better description does not need to be conserved across the isomorphism so in fact that's often what we use it for we use it to do arbitrage between things that look simple in one theory and things that look simple in the alternative description and we use we use the fact that things look simple in one to understand the sort of complicated version in the other

In fact, it flows in both directions.

You might naively expect that because gravity is so complicated, we would always be using the non-gravitational theory to understand the gravitational theory.