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.
