Relativity Theory
Relativity is a theory of space, time and gravity developed by Albert Einstein in the early twentieth century. Special relativity, published in 1905, rests on two postulates: the laws of physics are the same for all observers moving at constant velocity, and the speed of light in a vacuum is the same for every observer regardless of how the source or observer moves. From these simple ideas follow consequences that contradict everyday intuition.
Among these consequences are time dilation, in which a moving clock runs slow as measured by a stationary observer, and length contraction, in which a moving object is measured as shorter along its direction of motion. Simultaneity becomes relative, so events that appear simultaneous to one observer may not be to another. Special relativity also leads to the equivalence of mass and energy, expressing that mass can be converted into energy and energy into mass.
General relativity, completed in 1915, extends the theory to include gravity and accelerated motion. It reinterprets gravity not as a force but as the curvature of spacetime caused by mass and energy: massive objects bend the spacetime around them, and other objects follow the resulting curved paths. General relativity predicts effects such as the bending of light by gravity and the slowing of clocks in strong gravitational fields, predictions that have been confirmed by precise observations.
Frequently asked questions
- What is the main idea of special relativity?
- That the speed of light is the same for all observers and the laws of physics hold in every uniformly moving frame, leading to time dilation and length contraction.
- What is time dilation?
- Time dilation is the effect by which a clock moving relative to an observer is measured to run more slowly than one at rest with respect to that observer.
- How does general relativity describe gravity?
- It describes gravity as the curvature of spacetime produced by mass and energy. Objects move along the curved paths created by that distortion rather than feeling a pulling force.
