The discoveries of general relativity and quantum mechanics began a revolution in physics. But the problem remains of completing the revolution with a theory of quantum gravity that joins those two pillars of modern physics. To have a chance to succeed, the new theory must not only make sense, it must make new predictions that can be tested by doable experiments. Physicists have made a great deal of progress in achieving the synthesis and have even proposed new experiments. That progress is the subject of this article.
The need for a quantum theory of gravity was first mentioned by Albert Einstein in 1915, in his initial paper about gravitational waves. The first PhD thesis on the subject, by Matvei Petrovich Bronstein, appeared in 1935. Still, the subject needed a long time to develop into a real branch of physics. Only in the past 20 years has the physics community seen...
