In physics, the problem of how physical laws and measurements change when considered by observers in various states of motion. Thus, relativity is concerned with measurements made by different observers moving relative to one another. In classical physics it was assumed that all observers anywhere in the universe, whether moving or not, obtained identical measurements of space and time intervals. According to relativity theory, this is not so, but their results depend on their relative motions.

There are actually two distinct theories of relativity known in physics, one called the special theory of relativity, the other the general theory of relativity. Albert Einstein proposed the first in 1905, the second in 1916. Whereas the special theory of relativity is concerned primarily with electric and magnetic phenomena and with their propagation in space and time, the general theory of relativity was developed primarily in order to deal with gravitation. Both theories centre on new approaches to space and time, approaches that differ profoundly from those useful in everyday life; but relativistic notions of space and time are inextricably woven into any contemporary interpretation of physical phenomena ranging from the atom to the universe as a whole.

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