[The_Reader_David]

It seems to be called the dilaton or radion, and I can't describe it physically because it has never been observed (doesn't exist?). It seems to show up in all string field theories, and a great deal of effort has been put into explaining why we don't observe it (under the assumption that string theory is right).

Personally, I expect the GUT (in Einstein's sense--*all* of the forces, not all but gravity) will be purely algebraic, along the lines of A. Connes reconstruction of the standard model from non-commutative geometry and the Barrett-Crane model for QG, with the appearance that we live in a Ricci-flat smooth Minkowskian 4-manifold being an artifact of the theory.

I myself am a pure mathematician who works in areas influenced by quantum physics (often collaborating with Crane) so I don't think I can do justice to the explanation for a physicist.

[grey_whiskers]

"with the appearance that we live in a Ricci-flat smooth Minkowskian 4-manifold being an artifact of the theory. "

OK, but no pics of [Harvard Physicist and some-time goddess] Lisa Randall?

Cheers!

[grey_whiskers]

...and why no obligatory Star Trek references?

[Flightdeck]

"It seems to show up in all string field theories, and a great deal of effort has been put into explaining why we don't observe it (under the assumption that string theory is right). "

I met Brian Greene from Columbia a few months ago and asked him if this GUT that he feels is on the (his) horizon would describe how the human mind works and survival after death of the body (afterlife basically). He wasn't even polite when he dismissed the afterlife. Therefore I know that at least one string theorist is far too close-minded to come up with a theory of everything.

[spunkets]

The dilaton is a scalar field whose vacuum expectation value is related to the gravitational coupling constant. The 4d world isn't exactly Ricci flat, because the cosmological constant is small and positive. It appears that the dilaton field is involved in fixing that value also.

The radion is a dilaton and is a massive scalar field that fixes the size of the 5th dimension in antiDeSitter space as a function of the 4 dimensions. Randall-Sundstom models... It has a massive particle that resides on the branes at the ends of the universe. It wouldn't be observed in 4d, other than effects on g and the cosmological constant.

In any case Einstien's tensor theory has been experimentally verified and a scalar-tensor theory is out. If the effect of the dilaton field is simply to fix the coupling constant and the value of the cosmological constant, then it is certainly consistent with GR.

I'm not sure what a GUT would look like. I'm not familiar enough with LQG, or M theory.