Theory of Relativity and Time — Einstein Explained Simply

Special Relativity (1905)

Einstein proposed two postulates: (1) the laws of physics are the same in all inertial frames, and (2) the speed of light in a vacuum is constant for all observers regardless of relative motion. This second postulate forces time to be relative — two observers moving differently must disagree about how much time has passed between events.

General Relativity (1915)

Ten years later, Einstein extended his theory to include gravity. General Relativity describes gravity not as a force but as the curvature of spacetime caused by mass and energy. A massive object like Earth or a black hole curves spacetime around it, and clocks deep in a gravitational well tick slower than clocks far away.

Why c is the speed limit

As an object with mass accelerates toward the speed of light, its relativistic mass increases, requiring ever more energy to accelerate further. At c, infinite energy would be required. Photons — which have no rest mass — travel at exactly c. Nothing with mass can reach or exceed it. This is a hard constraint built into the structure of spacetime.

Try it in the Relativity Simulator

The Cosmic Dashboard Relativity Simulator lets you accelerate a virtual spacecraft to 0.999999c. Drag the ENGINE POWER slider and watch the starfield compress into a forward cone, trails rainbow-shift past 0.98c, and the two clocks — Earth time and ship time — drift apart at a rate set by the Lorentz factor. At extreme speeds a warp bloom and flashing event-horizon warning visualise the hard limit of c.

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