Relentless, elusive and infuriatingly hard to define. Not only is time one of the great existential mysteries, it also holds the key to the most ambitious challenge in theoretical physics - to express the complex workings of this vast universe in a single, elegant theory, a theory of everything.
Such a theory would unite general relativity, Einstein's theory of gravity which describes the workings of space and time, with quantum mechanics, the endlessly weird but entirely compelling theory that describes the physics of matter. And it is here that we have been struggling for nearly a century.
General relativity and quantum mechanics offer radically different descriptions of time, and this difference is a prime source of their stubborn refusal to be unified. If we are to make progress, something's got to give - and many suspect that it will have to be time.
Almost a century ago, Einstein showed that time is not the fundamental ingredient of reality we once thought it was. His theory of general relativity sewed space and time into a unified entity called space-time, which can stretch and wrinkle in the presence of matter or energy, producing the space-time curvature we feel as the force of gravity. But the problem with space-time is that it is frozen. Space-time as a whole cannot develop over time because it is time - no clock can sit outside the universe.
According to general relativity, what we experience as the flow of time is a kind of illusion generated by the patchwork ways in which different observers slice the unified space-time into space and time with their individual points of view.
To read the rest of the article, click here.