What is time, and why does it only seem to move forward?

Nobody knows yet what time is at its deepest level. That may sound strange, since we measure it every day. But measuring something and knowing what it is are not the same. Here is what is well measured, what is well explained, and what is still open.

Start with clocks. Today the official second is set by atoms: it lasts exactly 9,192,631,770 wave cycles of a particular radiation from cesium atoms. Einstein’s relativity says clocks do not all tick at the same rate. A clock that moves fast ticks slower, and a clock higher above Earth ticks faster. This has been measured again and again. In 1971, atomic clocks flown around the world on airliners came back slightly out of step with clocks on the ground, by the amounts relativity predicted. GPS satellites must allow for this, or the system would not work. In 2010, NIST measured the effect between clocks only 33 centimeters (about a foot) apart in height.

Why does time seem to move only forward? The basic laws of physics barely care which way time runs. Yet many things happen in only one direction. Heat flows from hot to cold. The smell of coffee spreads through a room and never gathers back into the cup. Physicists tie this to entropy, roughly a measure of how spread out and mixed things are. There are far more mixed-up arrangements than tidy ones, so mixing is overwhelmingly likely. A widely discussed idea, called the Past Hypothesis, adds one more piece: entropy keeps rising because the universe began with extremely low entropy. Why it began that way, and whether that needs explaining, is still debated.

Then there is time as we feel it. Stress, boredom and busy days can change how long things seem, and memory plays a part. Philosophers also still debate a deeper question: does time truly flow, or are past, present and future all equally real? That question has been argued for more than 2,000 years, and experiments so far have not settled it.