this page honors Albert Einstein (1879-1955)

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In Einstein's view:

The non-mathematician is seized by a mysterious shuddering when he hears of "four-dimensional" things, by a feeling not unlike that awakened by thoughts of the occult. And yet there is no more common-place statement than that the world in which we live is a four-dimensional space-time continuum.

Since that's the case, it's vital to consider the dimension of time as well as the three-dimensional nature of space in order to get an accurate view of how things work.

Einstein's theory encompasses two kinds of relativity. The principle of special relativity is based on "the principle of the physical relativity of all uniform motion."

In contrast, according to the general theory of relativity all bodies in motion are equivalent whatever may be their state of motion. In his popular book about the theory of relativity, the reader is asked to visualize a passenger walking along the corridor of a moving train, and in "order to have a complete description of the motion, we must specify how the [passenger's] body alters its position with time; i.e. for every point on the trajectory it must be stated at what time the body is situated there."

To read more:

Relativity, the special and the general theory: a popular exposition, by Albert Einstein. Bonanza Books, ©MCMLXI (1961).

The illustrated a brief history of time, by Stephen Hawking. Updated and expanded edition. Bantam Books, 1996.