What it is
Special relativity (1905) showed that space and time are linked and that the speed of light is the same for every observer. General relativity (1915) went further: mass and energy curve spacetime, and that curvature is what we feel as gravity.
It is not only for cosmology. Satellite navigation depends on it: clocks on GPS satellites tick at a measurably different rate from clocks on the ground, and the system corrects for it every day.
Mass curves spacetime
An illustrative model of general relativity’s central idea, with the measured GPS clock correction.
Sources: Living Reviews in Relativity · LIGO Caltech
A model to explain the idea; not drawn to scale.
Ideas to carry
- 1
Spacetime bends
Mass tells spacetime how to curve; curved spacetime tells matter how to move.
- 2
Time is local
Clocks run at different rates depending on speed and gravity. This has been measured, not just predicted.
- 3
Ripples
Accelerating masses send out gravitational waves that stretch and squeeze space as they pass.
Established What we know
- The speed of light in vacuum is defined as exactly 299,792,458 metres per second.[1]
- Without relativistic corrections, GPS satellite clocks would drift by about 38 microseconds a day relative to ground clocks, enough to ruin positioning.[2]
- The 2017 Nobel Prize in Physics recognised decisive contributions to LIGO and the observation of gravitational waves.[3]
Interpretation The older lens · Pythagoreanism
The Pythagoreans heard a “music of the spheres” in the planets’ motion; relativity hears spacetime itself ring.
Reading
- 1905On the Electrodynamics of Moving BodiesAlbert Einstein
- 1915The Field Equations of GravitationAlbert Einstein
- 1994Black Holes and Time WarpsKip Thorne
Newton: gravity is a force
Einstein: it is the floor being curvy
Connected subjects, projects and writing
- Computing and informationSubject · Intelligence
Established GPS software applies relativistic clock corrections every day.
- AstronomySubject · Cosmos
Planets, stars and galaxies, measured from Earth and from space, and the old light that carries them to us.
- Black holesSubject · Cosmos
Regions where gravity is so strong that nothing, not even light, can escape: from theory to photographs.
- Quantum mechanicsSubject · Cosmos
The physics of the very small, where energy comes in packets, outcomes are probabilities and particles can be entangled.
Sources
- CODATA value: speed of light in vacuum NIST. Checked 5 Nov 2026.
- Relativity in the Global Positioning System Living Reviews in Relativity, 2003. Checked 5 Nov 2026.
- The Nobel Prize in Physics 2017 Nobel Prize, 2017. Checked 5 Nov 2026.