Two different questions
“What is time?” actually hides two separate questions. The first is practical: how do we measure it? The second is conceptual: why does time seem to flow in one direction, from past to future, when most basic laws of physics look much the same run forward or backward? The first has a well-developed answer. The second is better understood than it once was, but parts of it remain open.
How time is measured
Every clock works the same way in principle: it counts something that repeats at a steady rate. Early timekeepers used the repeating motion of the sun across the sky, which gave sundials and the idea of the day. Water clocks and candle clocks counted steady flows and burning. Mechanical clocks counted the swings of a pendulum or the vibrations of a balance wheel.
Modern precision comes from counting vibrations of atoms. Quartz watches count the vibrations of a small crystal that oscillates at a very stable frequency when electricity is applied. Atomic clocks go further, using the natural frequency of radiation linked to changes in energy states inside atoms. Since atoms of a given kind are essentially identical, this gives an extremely steady reference, and the international definition of the second is based on such an atomic frequency.
Time keeping also involves conventions. Time zones, leap years and occasional adjustments called leap seconds exist because Earth’s rotation is not perfectly regular, as the slow wobbles and long-term changes in its spin show. Satellite navigation depends on very precise timing, a point that comes up again in discussions of precise timing services, since small timing errors translate into position errors.
Time is not the same for everyone
Einstein’s theories of relativity changed the picture. Two clocks that move relative to one another, or sit at different heights in a gravitational field, do not tick at exactly the same rate. Clocks closer to a massive body run slightly slower than those farther away. These effects are tiny in everyday life, but they are measurable with precise atomic clocks, and they must be accounted for in satellite systems. This means there is no single universal “now” that all observers agree on, a point that also appears in debates about the history of time and about whether time travel could be possible in any form.
Why does time seem to run one way?
The laws that describe how individual particles move mostly do not distinguish forward from backward. A film of two billiard balls colliding looks plausible in reverse. Yet a film of an egg breaking looks absurd when run backward. Something besides those basic laws must explain the difference.
The leading idea involves entropy, roughly a measure of how many microscopic arrangements correspond to the way things look at a larger scale. There are vastly more ways for an egg to be scrambled than to be whole, so random jostling almost always carries a system from an orderly state to a disordered one, rarely the other way. The second law of thermodynamics states this tendency: in an isolated system, entropy typically increases.
This also explains why heat flows from hot to cold, why ice melts in a warm room, and why a drop of ink spreads in water. Nothing forbids the reverse in principle. It is just overwhelmingly improbable.
Where the arrow starts
If entropy tends to increase toward the future, then it must have been low in the past. Many physicists tie this to the early universe, which appears to have begun in a highly ordered state. Why it began that way is an open question in cosmology, so the arrow of time is partly a puzzle about the beginning of everything. Claims that this is fully settled should be treated with caution.
Time and the mind
Our experience of time involves more than physics. We remember the past but not the future, and memory formation itself is a process that increases entropy somewhere, such as in the brain’s chemistry and the heat it gives off. That gives a plausible link between the physical arrow and the psychological one. Separately, our sense of how fast time passes depends on attention, emotion and age, a topic that psychologists study under time perception. That sense can drift away from clock time without any change in the clock.
What remains unsettled
Physicists still debate whether time is a basic ingredient of reality or something that emerges from more fundamental ideas, especially in attempts to combine gravity with quantum physics. Some approaches treat time as essential, others suggest it might be an approximate description. None of these proposals has been confirmed.
What can be said confidently is this: time can be measured with remarkable precision, it depends on motion and gravity, and its apparent one-way direction is closely tied to the tendency of systems to move toward more probable, more disordered arrangements.