**Theory of relativity**

From Wikipedia, the free encyclopedia.

^{2}

Albert Einstein's **theory of relativity** is a set of two theories in physics: special relativity and general relativity. The core idea of both theories is that two observers who move relative to each other will often measure different 'time' and 'space' intervals for the same events, but the content of physical law will be the same for both.

## Special relativity

**Special relativity**, developed in 1905, only considers observers in inertial reference frames which are in uniform motion with respect to each other. Einstein's paper that year was called "*On the Electrodynamics of Moving Bodies*". While developing this theory, Einstein wrote to Mileva (his wife) about "our work on relative motion". This paper introduced the special theory of relativity, a theory of time, distance, mass and energy. The theory postulates that the speed of light in vacuum will be the same for these observers. Special relativity solved the puzzle that had been apparent since the Michelson-Morley experiment, which had failed to show that light waves were travelling through any medium (other known waves travelled through media - such as water or air). It had been suggested that light waves actually did not travel through any medium: the speed of light was thus fixed, and not relative to the movement of the observer. This was impossible under Newtonian classical mechanics however, and Einstein provided a new system which allowed for this.

This leads to redefinitions of such fundamental notions as time, distance, mass, energy and momentum with wide ranging consequences. Moving objects appear heavier and compressed in the direction they are moving, while moving clocks appear to run slower. Light has momentum. The speed of light emerges as an upper limit for the speed of matter and information. Mass and energy are seen as equivalent. Two events judged to be simultaneous by one observer may be seen as non-simultaneous by other observers which are in motion with respect to the first one. The theory does not account for gravitational effects.

The mathematical basis of special relativity is provided by the Lorentz transformation, which had been introduced earlier by the Dutch physicist Hendrik Lorentz, but Einstein showed a different way to understand these concepts. His explanation arose from two axioms: one was Galileo's old idea that the laws of nature should be the same for all observers that move with constant speed relative to each other; and the other was that the speed of light is the same for every observer. Special relativity had several striking consequences because the absolute concepts of time and size are rejected. The theory came to be called the "special theory of relativity" to distinguish it from his later theory of general relativity, which considers all observers to be equivalent.

## General relativity

**General relativity** was published by Einstein in 1916 (submitted as a series of lectures before the Prussian Academy of Sciences [ November 25 1915 ]). The theory gave an introduction of an equation that replaced Newton's law of gravity. It uses the mathematics of differential geometry and tensors in order to describe gravity.

This theory considered all observers to be equivalent, not only those moving at a uniform speed. The laws of general relativity are the same for all observers, even if they are accelerated with respect to each other. In general relativity, gravity is no longer a force (as it was in Newton's law of gravity) but is a consequence of the curvature of space-time. General relativity is a geometrical theory which postulates that the presence of mass and energy "curves" space, and this curvature affects the path of free particles (and even the path of light), an effect we interpret as a gravitational force. The theory can be used to build models of the evolution of the universe and is hence a crucial tool in cosmology. The theory provided the foundation for the study of the standard model of cosmology and gives scientists the tools understanding features of the universe that were not discovered until well after Einstein's death. General relativity becomes a method of perceiving all of physics.