⏱ Clocks disagree

Reason 1 — speed

Einstein's starting point (1905): the speed of light is the same for every observer, however they move. Picture a light clock — a pulse bouncing between two mirrors. Seen from outside, a moving light clock's pulse travels a longer, diagonal path at the same speed, so each tick takes longer. The slowing factor is called γ (gamma): at 87% of light speed it's 2.

It isn't a quirk of light clocks. If some other clock — an atom, a heartbeat — slowed differently, you could compare them and detect your "absolute" motion, which relativity rules out. Every process slows alike. Established

Each of two passing observers sees the other's clock run slow. That's not a contradiction: they only compare directly if one turns round to meet the other — the twin paradox.

Reason 2 — gravity

General relativity (1915) rests on the equivalence principle: in a small enough region, gravity can't be told apart from acceleration. Follow that through and light climbing out of gravity loses energy — and clocks lower down tick slower. Near Earth's surface a clock one metre higher runs faster by about one part in 10¹⁶. Established

In orbit the two effects compete: higher up, gravity speeds a clock up; the orbital speed slows it down. On the ISS speed wins; for GPS satellites gravity wins.

At cosmic scale

Light from distant galaxies is stretched by the expansion of space (redshift), and everything it carries is stretched with it: a supernova at redshift 1 appears to unfold twice as slowly. That's expansion rather than speed or gravity in the everyday sense, but it's measured and it fits. Established

How we know

• 1941 — Muons made by cosmic rays high in the atmosphere reach the ground in numbers they shouldn't, given their short lives: their clocks run slow (Rossi & Hall).
• 1959–60 — Light sent up a 22.5 m tower at Harvard shifts in frequency by the amount gravity predicts (Pound & Rebka).
• 1971 — Atomic clocks flown round the world east and west disagree with clocks left at home, as predicted (Hafele & Keating).
• 1977 — Muons circling a storage ring at CERN at 99.94% of light speed live about 29 times longer (Bailey et al.).
• 2010 — Optical clocks lifted just 33 cm run measurably faster (Chou et al., NIST).
• 2022 — The effect measured across a single millimetre-sized cloud of atoms (Bothwell et al., JILA).
• Every day — GPS satellite clocks are corrected by about 38 microseconds a day; without it positions would drift kilometres.

How sure?

Established Both effects have been measured many ways, to high precision, for over eighty years. What's still open is deeper: why time behaves this way at all, and what time is when quantum mechanics and gravity meet (hole H1).