The Problem of Time
Time is among the most fundamental and the most elusive concepts in human thought. We experience it constantly. We measure it precisely. We build our lives around it. And yet, when asked what time actually is — what it consists of, where it comes from, whether it would exist if nothing were happening — we struggle to answer.
Aristotle defined time as the measure of motion. Newton described it as absolute and universal, flowing at the same rate everywhere in the universe regardless of what was happening within it. Einstein showed that Newton was wrong: time is not absolute. It flows at different rates depending on velocity and gravity. A clock on a mountain runs slightly faster than a clock in a valley. A clock on a fast-moving spacecraft runs slower than one at rest. These are not metaphors or approximations. They are measured facts.
Relativity
An astronaut who spent a year travelling at 99 per cent of the speed of light would return to find that seven years had passed on Earth.
Einstein's special theory of relativity, published in 1905, established that time and space are not separate entities but aspects of a single four-dimensional structure: spacetime. The speed of light is constant for all observers, regardless of their motion — and this seemingly simple fact has enormous consequences. If the speed of light cannot change, then time and space must change instead.
Time dilation is the slowing of time experienced by a moving observer relative to a stationary one. The faster an object moves, the slower time passes for it. At everyday speeds, the effect is immeasurably small. At speeds approaching the speed of light, it becomes significant. An astronaut who spent a year travelling at 99 per cent of the speed of light would return to find that seven years had passed on Earth.
Gravitational time dilation is a consequence of general relativity. Clocks in stronger gravitational fields run slower. GPS satellites, orbiting at high altitude where gravity is weaker, run faster than clocks on Earth's surface, and must be corrected for this effect or navigation errors would accumulate at a rate of several kilometres per day.
The Arrow of Time
The laws of physics are almost entirely symmetric with respect to time — they work equally well whether time runs forward or backward. A film of two billiard balls colliding looks physically plausible whether played forwards or in reverse. And yet, at the macroscopic level, time clearly has a direction. Eggs break and do not reassemble. Coffee cools and does not reheat spontaneously. The past is different from the future.
The explanation for this asymmetry lies in entropy — the measure of disorder in a system. The second law of thermodynamics states that the entropy of a closed system tends to increase over time. Disorder increases. Things fall apart. This is the arrow of time: not a law of physics in the fundamental sense, but a statistical tendency so overwhelming that it functions as one.
Why entropy was low in the past — why the universe began in a highly ordered state — is one of the deepest unsolved problems in physics. The arrow of time is, ultimately, a question about the initial conditions of the universe.
The Beginning of Time
According to the best current cosmological models, time began approximately 13.8 billion years ago at the Big Bang — the moment at which the universe came into existence in an extremely hot, dense state and began expanding. Before the Big Bang, the question of what happened is not merely unanswered but possibly unanswerable: 'before' presupposes time, and time, according to these models, did not yet exist.
Stephen Hawking and James Hartle proposed the no-boundary proposal: the idea that the universe has no beginning in time in the same way that a sphere has no edge. On this model, asking what happened before the Big Bang is like asking what is south of the South Pole. The question does not malform — it dissolves.
Measuring Time
The second is the SI unit of time and is defined, since 1967, as the duration of exactly 9,192,631,770 oscillations of the radiation emitted by a caesium-133 atom transitioning between two specific energy states. This definition makes the second the most precisely defined of all base units — atomic clocks based on it are accurate to within one second in 300 million years.
The measurement of time has driven technological progress for centuries. The invention of accurate marine chronometers in the eighteenth century made longitude calculable and transformed navigation. The synchronisation of railway timetables in the nineteenth century made standard time zones necessary. The development of atomic clocks in the twentieth century made GPS, mobile communications, and the synchronisation of the global internet possible.