When Einstein knew gravity is geometry and not force

[dpa]

no gut?

well,

from GR or LQG as well, gravity is not force but geometry right?
When einstein knew gravity is geometry and not force, why did he try to unify them?
Will we have 3 fundamental forces.
Is gut ruled out?

 

[tom.stoer]

Einstein knew already since the twenties that gravity and electromagnetism can be unified; he was very impressed by the ideas of Kaluza and Klein. This idea (to unify the two 'forces' geometrically in higher dimensions and to 'break the unification' via dimensional reduction is still used in SUGRA and string theory). For other forces it's very difficult but as we learn from SUGRA and strings, it's not impossible.

GUT is used for a very special way of unification, namely for unification of standard model forces w/o gravity. Mathematically this is rather simple (no higher dimensions, no SUSY); you take a gauge group which is large enough - SU(5), SO(10), E(6) will do - write down the Lagrangian and introduce Higgses in an appropriate way to break the symmetry down to the SM. The problems are purely phenomenological: the larger gauge group comes with additional gauge bosons mediating new forces which induces obervational effects; and afaik all GUTs have been ruled out due to these new effects (e.g. due to the predicted proton decay which hasn't been observed).

Afaik the current status is that simple GUTs are ruled out experimentally and that string theory as a framework for describing SM+gravity as a low-energy sector (specific certain string vacua) is still not able to make testable predictions like specific interactions, particle masses etc.