Science

Time feels like the most basic feature of reality. Seconds tick, days pass and everything from planetary motion to human memory seems to unfold along a single, irreversible direction. We are born and we die, in exactly that order. We plan our lives around time, measure it obsessively and experience it as an unbroken flow from past to future. It feels so obvious that time moves forward that questioning it can seem almost pointless.

And yet, for more than a century, physics has struggled to say what time actually is. This struggle is not philosophical nitpicking. It sits at the heart of some of the deepest problems in science.

Modern physics relies on different, but equally important, frameworks. One is Albert Einstein’s theory of general relativity, which describes the gravity and motion of large objects such as planets. Another is quantum mechanics, which rules the microcosmos of atoms and particles. And on an even larger scale, the standard model of cosmology describes the birth and evolution of the universe as a whole. All rely on time, yet they treat it in incompatible ways.

When physicists try to combine these theories into a single framework, time often behaves in unexpected and troubling ways. Sometimes it stretches. Sometimes it slows. Sometimes it disappears entirely.