## Thursday, October 20, 2005 ... //

### The illusion of gravity

All readers who like or want to read popular texts are recommended the article "Illusion of gravity" about holography in quantum gravity in the November issue of Scientific American. It is written by a person who has done the maximum to prove the concept of holography quantitatively - namely Juan Maldacena. His article offers Escher's pictures and rather detailed explanations of anti de Sitter space and other relevant concepts.

#### snail feedback (5) :

The temperature of the black hole is inversely proportional to its mass, so the black hole gets hotter and hotter as it decays.

All the time the collapse is taking place, we know that it has a valuation in this measure. So, if a string theorist can take this from a ten dimensional perspective down to four, has this not indeed accomplished something string theorists can do?

So while the "mathematical crowd" has found a method to determination, why can we not settle minds around the fact that as a string theorist, one should be able to tell us something about the theoretics of blackholes in ways that we hear of, a large proportional of science(?), relegates to absurdity?

A layman, concerned about the physics.

So is Juan speaking to this, and will you do a summation of the article Lubos?

If you wanted to erase other "same comment" following Peter's comment I would like this one to stand.

Hi Plato,

I've no confidence anymore in the idea that guessing with abstract mathematical models will really help with gravity, either in popularising new ideas or actually getting the work done and tests made to prove models.

Feynman gravity blog

The approaches above (links) are working from experimentally determined facts, instead of from beautiful mathematical guesses about strings, extra dimensions, parallel universes, etc. Quite frankly, I had hoped for more understanding from t'Hooft if not Plato, but am coming to see them as in love with awesome abstract maths and less interested in simple ideas which can be developed with a lot of persistence (trial and error, which Kepler used to get a few working laws of planetary motion).

You seem to all be working on the assumption that God is, like yourselves, a pure mathematician. Haldane, the biologist, said God must love beetles, because there are far more species of beetles than anything else. An astronomer might say that God loves stars, since there are so man stars. Just because you have an immense number of empirical guessed equations which have yet to be explained heuristically, does not constitute evidence that the universe us ultimately string theory maths.

Cheers,
Nigel

Hi Nigel,

I think we are all on a time schedule?

What science do you have that will take us to the beginning of this universe? Or does it not matter?

Brian Greene:Time is far more subtle than our everyday experience would lead us to believe. In many ways, time may simply be a psychological construct for organizing the world. It is a device we scientists have found useful, but it may in fact be a dim approximation of something far more complex."

Dear Lubos,
I have a question for you. The boundary space time in AdS/CFT does not contain gravity. What does this tell us about the properties of spacetime on the boundary? For example is general covariance not holding on the boundary spacetime? Does this then mean that there is one preferred coordinate description of the boundary spacetime etc.?

Dear Mikael, I think that while your question may sound intriguing, it is fundamentally misguided.

The CFT description is non-gravitational which *exactly* means that you shouldn't consider general diffeomorphisms a symmetry. They're not a symmetry in general - it's not a theory of gravity.

It doesn't mean that the boundary CFT isn't invariant under any diffeomorphisms. Indeed, it is a non-gravitational theory invart under (super)conformal transformations - translations, rotations, scaling transformations, some extra hard transformations that preserve angles, and fermionic counterparts of all of those.

If you think about it rationally, you will have to notice that the previous paragraph says *everything* about the action of diffeomorphisms on the boundary theory.

What I implicitly see in your question is an attempt to imagine that the boundary CFT is just a *part* of the bulk gravitational theory. But it's not a part. It's the whole thing. It's an equivalent description of it. In this description, gravity is frozen. In AdS/CFT, it's frozen because you would need an infinite energy to create big disturbances at the AdS boundary which is an infinite-volume region. But it's inconsistent to look at the boundary CFT as if it were a theory of the same type as the bulk gravitational theory. It's not.