In 1905, Albert Einstein published his Special Theory of Relativity. Its two core postulates are: (1) the laws of physics are the same in all inertial (non-accelerating) reference frames, and (2) the speed of light in a vacuum is constant for all observers, regardless of their motion or the motion of the light source.
From these seemingly simple postulates flow extraordinary consequences. Time dilation: a clock on a fast-moving spaceship ticks slower relative to a stationary observer. Length contraction: objects moving at high speed are shorter in the direction of motion. Mass-energy equivalence: E=mc², perhaps the most famous equation in science, tells us that mass and energy are two forms of the same thing.
These effects are not illusions — they have been confirmed countless times. GPS satellites must account for both special relativistic time dilation (fast motion slows clocks) and general relativistic effects (weaker gravity speeds them up). Without these corrections, GPS would accumulate errors of kilometres per day.
In 1915, Einstein extended his work to accelerating frames and published General Relativity (GR). Its central insight: gravity is not a force transmitted through space, but a curvature of spacetime caused by mass and energy. Matter tells spacetime how to curve; curved spacetime tells matter how to move.
A useful (if imperfect) analogy: imagine a bowling ball placed on a stretched rubber sheet. The sheet curves around the ball, and a marble placed nearby will roll towards it — not because of a force, but because it follows the curved surface. In GR, planets orbit the Sun because they follow the straightest possible paths (geodesics) through curved spacetime.
GR predicted the bending of light around massive objects (gravitational lensing), the precession of Mercury's orbit, gravitational time dilation (clocks run slower in stronger gravity), and the existence of gravitational waves.
One of GR's most dramatic predictions was gravitational waves — ripples in the fabric of spacetime propagating outward from accelerating masses, like ripples on a pond. Einstein himself doubted they'd ever be detected; they are fantastically weak.
On 14 September 2015, LIGO (Laser Interferometer Gravitational-Wave Observatory) detected gravitational waves for the first time — from the merger of two black holes 1.3 billion light-years away. The waves stretched and squeezed the 4-kilometre detector arms by less than 1/10,000th the diameter of a proton. This was the most precise measurement in the history of science.
Since then, LIGO and its sister detector Virgo have detected dozens of black hole mergers, neutron star mergers (including one simultaneously observed in light, gamma rays, X-rays, and gravitational waves — inaugurating multi-messenger astronomy), and more. A new observational window on the universe has opened.
Test what you've just learned.
1.What does E=mc² tell us?
2.In General Relativity, what is gravity?
3.What did LIGO detect on 14 September 2015?
4.How does General Relativity affect GPS satellites?