Reading passage
Few conclusions in science offend intuition as thoroughly as the discovery that time is not universal. In everyday experience, a second is a second, and two clocks that are synchronised remain so forever. Einstein's theories of relativity demolished this assumption a century ago, showing that the rate at which time passes depends on how fast an observer moves and how deep they sit in a gravitational field. Far from being a cosmic illusion, this effect is measured daily and shapes technologies that millions of people use without realising it.
Special relativity, published in 1905, introduced the first of these phenomena, known as time dilation. The theory rests on a single stubborn fact: the speed of light in a vacuum is the same for all observers, regardless of their own motion. For this to hold true, moving clocks must tick more slowly than stationary ones. The effect is negligible at everyday speeds but becomes dramatic as velocity approaches that of light; a traveller moving at 99 percent of light speed would age only one year for every seven that passed on Earth.
Einstein's general theory of 1915 added a second twist. Gravity, he argued, is the curvature of space-time produced by mass, and clocks run slower where the field is stronger. A clock at sea level ticks more slowly than an identical clock on a mountain, and a clock on the surface of a neutron star would crawl compared with one drifting in empty space. Time, in other words, is stitched into the fabric of the universe and stretches with it.
The first direct confirmations came from particles rather than people. Muons, unstable particles created when cosmic rays strike the upper atmosphere, decay within about two microseconds and should therefore never reach the ground. Yet detectors at sea level record them in abundance, because at near-light speeds their internal clocks run so slowly, from our perspective, that they survive the journey. In 1971, the Hafele-Keating experiment carried atomic clocks around the world on commercial aircraft and found exactly the discrepancies relativity predicted.
Today, the most consequential application is the global positioning system. Satellites orbit at about 20,000 kilometres, where weaker gravity speeds their clocks by 45 microseconds a day, while their orbital motion slows them by seven microseconds. The net gain of 38 microseconds must be corrected continuously; uncorrected, positional errors would accumulate at roughly ten kilometres per day, rendering the system useless within hours. Every smartphone navigation fix is, in effect, a practical lesson in relativity.
Modern atomic clocks have pushed these tests to extremes. Instruments based on strontium atoms are now so precise that they register the gravitational slowing of time across a height difference of less than a metre, and pairs of such clocks disagree measurably when one is raised by a single centimetre. Physicists speak of using networks of clocks to map the Earth's gravitational field directly, a technique called chronometric geodesy, which could monitor underground water reserves or volcanic magma movements.
Relativity also imposes strange orders on events. Because simultaneity depends on the observer's frame of reference, two events that appear simultaneous to one observer occur in sequence to another, and for widely separated events even the order can reverse. This does not permit causality to run backwards, since no signal can travel faster than light, but it does mean that the notion of a universal present moment, a cosmic now shared by the whole universe, has no physical meaning.
Philosophers and physicists still debate what these findings imply about the nature of time itself. Some argue that relativity supports a block universe, in which past, present and future all exist equally and the flow of time is an artefact of human perception. Others insist that emerging theories of quantum gravity will restore a privileged role to the present. What is not disputed is the empirical record: every precision test so far conducted has confirmed Einstein's predictions, often to better than one part in a billion.