What is the theory of relativity?

Einstein published special relativity in 1905. It starts with two ideas. First: the laws of physics are the same for everyone moving at a constant speed. Second: the speed of light is always 299,792 kilometres per second — no matter how fast the observer is moving.

These two ideas have a strange consequence: time passes at different rates depending on how fast you move. A clock on a fast-moving spacecraft ticks slightly more slowly than a clock sitting still on Earth. This effect is called time dilation. At everyday speeds the difference is tiny, but it becomes large near the speed of light.

What does E=mc² mean?

Special relativity also produced the most famous equation in science: E=mc². It says that energy (E) and mass (m) are two forms of the same thing. They are connected by the speed of light squared (c²). A tiny amount of mass contains an enormous amount of energy. Nuclear power plants — and the Sun itself — release energy by converting a small fraction of mass through nuclear reactions.

In 1915, Einstein extended his ideas to include acceleration and gravity. General relativity describes gravity not as a pulling force, but as a curvature in spacetime — the four-dimensional fabric of space and time.

A massive object like the Sun bends spacetime around it, the way a heavy ball bends a stretched rubber sheet. Other objects — planets, light, even signals — follow those curves. This is why Earth orbits the Sun. It is also why light bends near the Sun — confirmed by Eddington's 1919 solar eclipse observations.

How is gravity related to time?

General relativity also predicts that clocks in stronger gravitational fields tick more slowly. A clock on the ground ticks fractionally slower than a clock high in orbit, where gravity is weaker. This effect — gravitational time dilation — sits alongside the velocity effect from special relativity. Both are real, measurable, and together they affect every GPS satellite in orbit.

The clearest everyday proof of both theories is the GPS network. GPS satellites orbit Earth at about 14,000 kilometres per hour. That speed causes their clocks to tick roughly 7 microseconds slow per day, as predicted by special relativity. Those satellites sit in weaker gravity than clocks on the ground. General relativity predicts their clocks tick about 45 microseconds fast per day.

The net result is that GPS clocks gain approximately 38 microseconds per day relative to clocks on Earth's surface. That sounds tiny, but at the speed of light, 38 microseconds corresponds to about 10 kilometres of position error — every single day. Engineers understood this before the first GPS satellites launched. They programmed the satellite clocks to run slightly slow before launch so that, once in orbit, they tick at the correct rate.

Without these relativistic corrections, GPS navigation would drift by 10 kilometres per day and become useless within hours. Every time you use turn-by-turn navigation, you are relying on Einstein's theory of relativity. GPS.gov explains this in more detail.

The theory of relativity can seem intimidating, but its core ideas are accessible once you build up the right foundations. Start with speed, distance, and time — the concepts are rooted in everyday experience, even if the conclusions are surprising.

Understanding relativity unlocks the most exciting questions in physics — black holes, gravitational waves, and the nature of time itself. It also connects to practical technology — from nuclear energy to GPS — making it one of the most applicable discoveries in the history of physics.

Epivo's how-the-universe-works course covers special relativity, general relativity, and the cosmos in a conversational format. Parents curious about how AI tutoring works can find more information on our page for parents.

For the technical treatment, see Ashby (2003) in Living Reviews in Relativity. The official GPS.gov resource explains the satellite system in accessible terms.