The theory, which was published nearly a century ago, had already passed every test it was subjected to. But scientists have been trying to pin down precisely at what point Einstein's theory breaks down, and where an alternative explanation would have to be devised.

Einstein's framework for his theory of gravity, for example, is incompatible with quantum theory, which explains how nature works at an atomic and subatomic level.

Consider that for a black hole, Einstein's theory "predicts infinitely strong gravitational fields and density. That's nonsensical," said Paulo Freire, an astrophysicist at the Max Planck Institute for Radioastronomy in Germany and co-author of the study, which appears in the journal Science.

And so scientists are testing the general theory not because they think it is wrong but because they are certain it can't be the final explanation—just as Isaac Newton's notion of gravitational force was superseded by Einstein's.

Einstein's general theory of relativity states that objects with mass cause a curvature in space-time, which we perceive as gravity. Space-time, according to Einstein's theories of relativity, is a four-dimensional fabric woven together by space and time.