What is time, and why does it only go forward?

What the phenomenon is

"What is time" turns out to be two different questions sharing one label. The first is practical: how precisely can it be measured, and against what standard? That question has an exact, settled answer. The second is deeper: why does time move only forward, so that people remember yesterday but not tomorrow, and a dropped glass shatters but never reassembles itself. That question has a well-supported answer for most everyday purposes, but its deepest layer, why the universe was arranged the way it needed to be for that answer to work, remains genuinely open.

The popular claim

A common assumption is that the basic laws of physics obviously point forward, the same way experience does. They mostly do not. Philosophers who study this puzzle point out that the laws of classical mechanics are time-reversible: run the equations backward and they describe an equally valid sequence of events, which is why, in principle, a film of colliding billiard balls looks just as valid played in reverse. If the fundamental equations do not prefer a direction, the assumption that science simply confirms forward-flowing time is already shaky, and the real explanation has to come from somewhere else.

How precisely time is measured

Time is defined using the most stable repeating process scientists have found: the cesium-133 atom. The National Institute of Standards and Technology defines one second as 9,192,631,770 oscillations of the microwave radiation that a cesium-133 atom absorbs and emits between two specific energy states. Counting those oscillations gives a reference that does not drift the way older, astronomical definitions of the second could, and atomic clocks built on that transition are what satellite navigation, power grids and physics laboratories actually synchronize against.

Time is relative, not absolute

Einstein's relativity adds a complication: there is no single, universal clock that everyone shares. How fast a clock ticks depends on how fast it moves and how strong the gravity around it is, and this has been measured directly, not only predicted. GPS satellites carry atomic clocks that feel both effects at once. According to NIST, their orbital speed should make them run about 7 microseconds a day slow, while the weaker gravity at their altitude should make them run about 45 microseconds a day fast, for a net gain of about 38 microseconds every day.

The same kind of effect shows up at ordinary heights on Earth. In 2019, comparing an optical atomic clock raised up Tokyo's Skytree tower with one at ground level showed the elevated clock running about four nanoseconds a day faster, matching Einstein's prediction that gravity slows time near a large mass. NASA notes that without correcting for these effects, small timing differences would add up quickly and throw off GPS position calculations, which is why the relativistic correction is built into the system rather than treated as a negligible detail.

Why time has a direction at all

None of this explains why time has a direction, since relativity's equations, like Newton's, run the same in either direction. The accepted explanation is thermodynamic. The second law of thermodynamics holds that the entropy, or disorder, of an isolated system never decreases. Physicist Sean Carroll puts the everyday version plainly: an egg can be scrambled but not unscrambled, and milk can be stirred into coffee but not unstirred. Because disordered arrangements vastly outnumber orderly ones, systems drift toward disorder by simple probability, and that steady drift is what gets experienced as time moving forward.

Recent physics adds a twist to that explanation. Some researchers working in quantum information theory argue that rising entropy traces back to quantum entanglement rather than to probability alone: as a system becomes entangled with its surroundings, precise information about its exact state becomes practically unrecoverable, which looks, from the outside, like the same one-way drift toward disorder. The nineteenth-century statistical account and this newer quantum account are both still being compared and refined.

What is still debated

The entropy explanation only pushes the real question back a step. If disorder always increases, the universe must have started in an extraordinarily ordered, low-entropy state for there to be room left to get more disordered over billions of years. Philosophers call this the Past Hypothesis. Sean Carroll describes the puzzle as unsolved: current cosmological theories, including inflation, do not yet explain why the universe began that orderly rather than in an already-disordered, high-entropy state.

Philosophers add another layer of uncertainty. The Stanford Encyclopedia of Philosophy frames an open dispute between thinkers who hold that time's passage is an objective, mind-independent phenomenon, and thinkers who argue that only relations of earlier and later are real, with the sense that time flows being, on that view, just a feature of consciousness. Physics has not settled which side is correct, and may not be equipped to.

Questions people ask

Verdict

Real or Legend has filed the mechanics of time as confirmed and its ultimate direction as unsolved. How time is measured, and how it stretches and compresses with speed and gravity, are established facts, verified with atomic clocks flown on satellites and mounted on towers. Why time moves forward at all traces back to the second law of thermodynamics, but that explanation depends on the universe having started in a low-entropy state that nobody has yet explained. That gap, not the mechanics, is the mystery still open in the file.