This past fall, the world of physics celebrated the 100th anniversary of Albert Einstein’s general theory of relativity, which forms the foundation of our modern understanding of the force of gravity. Einstein’s creation has been the ultimate antidote to a blasé, seen-it-all attitude that sometimes infects even scientists.Yet the ink was barely dry on the theory when Einstein saw a problem. It contradicted quantum mechanics, suggesting that physicists needed an even deeper theory to unify these two pillars of fundamental physics. In June 1916 Einstein wrote: “Quantum theory would have to modify not only Maxwellian electrodynamics but also the new theory of gravitation.” That was quite an insight when you consider that quantum theory didn’t even exist yet. It was still a nebulous idea that wouldn’t coalesce for another decade. So, we have been celebrating the centenary not only of Einstein’s theory, but also of the long slog to supersede it.
A theory of gravity is also a theory of space and time:
Whereas general relativity took a single genius a decade to create, that deeper theory — known as a quantum theory of gravity — has flummoxed generations of geniuses for a century. In part, physicists are victims of their past successes: when you accomplish anything in life, you raise the bar, making it that much harder to take the next step. But quantum gravity also poses difficulties that are unique in the history of science. A theory of gravity is also a theory of space and time — that was Einstein’s great insight. Yet physicists have always formulated their theories within space and time.
So, a theory of gravity swallows its own tail. It supposes, for example, that the passage of time varies, but the word “varies” connotes a temporal process. If time is varying, then the very standard by which it is varying also varies. The whole situation threatens to become paradoxical. This conceptual circularity creates weird mathematical difficulties. For instance, the little ‘t’ that physicists use to denote time drops out of their equations, leaving them at a loss to explain change in the world. To describe what happens, physicists need to go beyond space and time. And what is that supposed to mean? Such an idea forces us into (literally) uncharted territory.