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Suman Suhag's avatar

First we need to understand what are the problem with quantum gravity. For this discussion we focus on three issues

Renormalizability - a straight forward quantization of Einstein equation bring uncontrollable divergences. These divergences are related to gravitons loop diagrams. A consistent quantum gravity theory must fix this.

Background independence - As we quantizing space itself we should be able to describe phenomena that are beyond the space-time structure we started with.

Black hole thermodynamics - any theory of quantum gravity is expected to be consistent with the semi-classical description of black holes (entropy, hawking radiaiton) and must include the necessary microstates (ie making moving from thermodynamic toward statistical mechanics)

String theory manage to do it all (see footnotes below):

Renormalizability - this is the big one !

As a perturbative theory, String theory looks like an infinite tower of fields (infinite many particle types, with increasing mass). With some clever math underneath the structure, the tower of fields is tuned in such a way that divergences are canceled. (each particle in the tower makes it’s contributions in such a way that the sum of the contribution is zero).

On top of that, the theory has no anomalies (an non-trivial self consistent check ,that show that the local symmetries of gravity aren’t broken by quantum effects).

Background independence -

this require some non-perturbative calculations (which is hard to explain). But String theory has proven capable in describing process that include topology changes, pinching of small narrow tubes and a bunch of other non-trivial problem in quantum gravity.

One of the most interesting result (and yet a relative simple one) is the way String theory observer narrow tubes. It turns out that when you take a tube like piece of space, and lower it’s circumference, there is a critical size (at the string length) where we cannot differentiate between a very small tube and a very large one. Ie R∼α′RR∼α′R. So for what gravity cares we avoid the case of too narrow tubes, buy proving they are just large tube viewing with a T-duality glasses.

Black hole thermodynamics -

In some scenarios (some models which make calculation easier) we can show the the quantum gravity coming from String theory is holographic, we can correctly enumerate the microstates and count them to get the Hawking-Beckenstein’s entropy.

String theory predicts correction to Einsteins equation that help prevent the singularities from forming in the center of black holes.

Peter Cummings, MD's avatar

Fantastic description!!

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