Relativity in five steps
The speed of light as it moves through empty space is the same for everyone, which means it would be impossible to catch up with a beam of light, no matter how fast you might, hypothetically, be travelling.
It’s also impossible to tell the difference between moving uniformly and not moving at all – this is known as the principle of relativity. Einstein’s principle was pre- empted by Galileo in the 17th Century.
From these two ideas, Einstein concluded that time and distance are not constants: moving clocks run slow, moving rulers shrink. So someone zipping around in a spaceship will age slower than someone on Earth.
Gravity also affects the passage of time. Clocks slow down under the influence of gravity so, for example, clocks tick faster at the top of Mount Everest. This is a key result of Einstein’s General Theory of Relativity.
Space and time are not fixed – they are malleable and subjective, and together they form a universal, four- dimensional fabric called space-time. This idea is central to physics.
ABOVE: Jodrell Bank’s Lovell Telescope played a part in confirming the existence of gravitational waves, and so confirming Einstein’s theories