But then it all seemed to collapse. While investigating his new equations Einstein committed a fateful technical error, leading him to think that his proposal failed to correctly describe all sorts of commonplace motion. For two long, frustrating years Einstein desperately tried to patch the problem, but nothing worked.
The empirical research frontier? Detecting gravitational waves.
There is one remaining experiment, currently more than 20 years in the making, that many consider the final test of the general theory of relativity. According to the theory, two colliding objects, be they stars or black holes, will create waves in the fabric of space, much as two colliding boats on an otherwise calm lake will create waves of water. And as such gravitational waves ripple outward, space will expand and contract in their wake, somewhat like a ball of dough being alternately stretched and compressed.