Brian Greene in the New York Times:
After Einstein’s death, the torch of unification passed to other hands. In the 1960’s, the Nobel Prize-winning works of Sheldon Glashow, Abdus Salam and Steven Weinberg revealed that at high energies, the electromagnetic and weak nuclear forces seamlessly combine, much as heating a cold vat of chicken soup causes the floating layer of fat to combine with the liquid below, yielding a homogeneous broth. Subsequent work argued that at even higher energies the strong nuclear force would also meld into the soup, a proposed consolidation that has yet to be confirmed experimentally, but which has convinced many physicists that there is no fundamental obstacle to unifying three of nature’s four forces.
For decades, however, the force of gravity stubbornly resisted joining the fold. The problem was the very one that so troubled Einstein: the disjunction between his own general relativity, most relevant for extremely massive objects like stars and galaxies, and quantum mechanics, the framework invoked by physics to deal with exceptionally small objects like molecules and atoms and their constituents.
More here. [Thanks to Christine Klocek-Lim.]