Albert Einstein's **theory of relativity** is a set of two theories in physics: special relativity and general relativity. These theories were conceived in order to explain the fact that electromagnetic waves do not conform to the Newtonian laws for motion. Electromagnetic waves were shown to move at a constant speed, independent of the motion of an observer. The core idea of both theories is that two observers who move relative to each other will measure different time and space intervals for the same events, but the content of physical law will be observed the same by both.

## Special relativity

Einstein's 1905 paper, "On the Electrodynamics of Moving Bodies", introduced the special theory of relativity. Special relativity considers that observers in inertial reference frames which are in uniform motion relative to one another cannot perform any experiment to determine which one of them is in "absolute motion". The theory postulates that the speed of light in vacuum will be the same for these observers (i.e. an observer invariant speed).

One of the strengths of special relativity is that it can be derived from only a few premises:

- The speed of light in vacuum is a constant and is equal to 299,792,458 metres per second.
- The laws of physics are the same for all observers in inertial frames.

## 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). However, German mathematician David Hilbert wrote and made public the covariant equations before Einstein. This resulted in not a few accusations of plagiarism against Einstein, but it is probably closer to reality that they both were co-creators of general relativity. 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" spacetime, and this curvature affects the path of free particles (and even the path of light).

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