Relativity: Special and General
Einstein introduced two revolutionary ideas. They changed our view of space and time.
The special theory came first. It rests on two simple postulates. First, the laws of physics are the same in all inertial frames. Second, the speed of light in a vacuum is constant for every observer. From these ideas follow strange consequences. Time can stretch. Length can shrink. Mass can increase as speed approaches that of light. The famous equation E = mc² links mass and energy.
General relativity builds on the special case. It adds gravity to the picture. Einstein proposed that gravity is not a force. Instead, massive objects curve spacetime. Free‑falling objects follow the curved paths. This explains why apples fall and planets orbit. It also predicts that light bends near massive bodies. Clocks run slower in stronger gravitational fields. These effects have been confirmed by experiment.
Both theories have practical impact. GPS satellites rely on corrections from relativity. Particle accelerators need the mass‑energy relation. Black holes are a direct prediction of general relativity. The universe itself expands, a fact that follows from Einstein’s equations.
In summary, special relativity describes how motion affects measurements. General relativity describes how mass shapes spacetime. Together they form the foundation of modern physics. Their insights continue to guide new discoveries.