Glossary

LIGHT CLOCK

A type of clock where light bounces between a pair of mirrors. These provide a useful way to think about Einstein’s Special Theory of Relativity, which says a moving clock will run slower than a stationary one.

TWIN PARADOX

The puzzle that two identical twins should age at different rates depending on how they move. There’s actually no paradox – Einstein’s Special Theory of Relativity explains why this is true.

NEUTRON STAR

These astonishingly dense dead stars have a mass roughly equal to the Sun, but squeezed into the size of a city. Spinning neutron stars emit pulses of radio waves, which can be used by astronomers to test Einstein’s theory of gravity.

PRINCIPLE OF RELATIVITY

The idea that there is no way to define ‘at rest’ in any absolute sense. In other words, all motion is relative.

SPACE-TIME

Modern physics combines the three dimensions of space and the one dimension of time into this single, four- dimensional entity.

GRAVITATIONAL WAVE

A moving ripple in the fabric of space-time that causes lengths to change and clocks to tick at different rates as it passes by.