[guygurari]

As far as I know, string theory is the only complete theory that incorporates these two observations:

1. Nature includes the force of gravity, described at low energies by Einstein's theory with a gravitational field.

2. Matter in Nature is quantum mechanical.

We believe that spin 2 particles will exist in any such theory (as they do in string theory), but even without this assumption it is still difficult to come up with such a theory.

The example you give of gluing the Standard Model to General Relativity does not achieve this goal and does not correctly describe nature. This is because the quantum matter of the Standard Model does not correctly back-react on the classical gravitational field.

When we couple the classical theory of General Relativity to classical matter, the gravitational field pushes matter around, and the matter in turn back-reacts on the gravitational field and changes it. When coupling quantum matter to classical General Relativity, the classical gravitational field can act on matter just fine, but it does not work in the other direction. The problem is that when matter is in a quantum superposition, it is not clear how to update the classical gravitational field (which cannot itself be in superposition). This missing part of the interaction means that SM+GR is not a complete theory.

[evanb]

Hey Guy! We should finish the horizon paper :D

-- Evan