Einstein's theory of general relativity has been tested to remarkable precision. Yet from the theoretical problems posted by singularities to the unexplained nature of dark energy, some serious physicists are troubled by the theory's shortcomings. Will the theory continue to stand the test of time? In this article, Clifford M. Will and Nicolás Yunes survey the new and future tests of Einstein's greatest achievement.

In the 1990s, the TV series “The X-Files” told the story of an FBI detective, who was obsessed with finding the truth about a government conspiracy to hide the existence of aliens. Agent Mulder truly believed that aliens existed and that if he looked hard enough he would find the evidence he needed. Unlike Mulder, most people do not think that aliens are visiting Earth on a weekly basis, communicating through our microwave ovens or abducting everybody they spot, even if they accept that intelligent life probably does exist elsewhere in our galaxy. But the wide popularity of the show suggests that we all have a side that enjoys seeing conventional wisdom upended.