The image is popular and general: A young man sits under an apple tree. An apple falls, strikes him and sets him on a course to receive those epiphanies, eureka moments, in which he deduces how the physical world works. The young man was Isaac Newton and the consequent laws are known as Newtonian physics. Move ahead two hundred years and the second image, though less well known, is of another young man with unruly hair, sitting on a hillside in Germany not far from the patent office where he worked. There Albert Einstein received his own epiphany, followed by several eureka moments, from which he deduced those laws of physics, now termed the Theory of Relativity, a theory that superseded Newton’s 200 year old theories of mechanics, and, in no small part, ushered in the Modern Era. The Theory of Relativity, is actually comprised of two theories: special relativity and general relativity. The concepts introduced in these two theories are three-fold: (1) The measurement of certain quantities is dependent upon the speed of the observer; (2) Space and time (“spacetime”) should be considered in relation to one another; and (3) The speed of light is, nonetheless, an absolute constant, invariant and the same for all observers. In any construct that allows for the relative nature of relationships, the observer must seek out the sole constant on which all relationships depend for their accurate expression. Einstein found it in the speed of light, and from that constant looked anew at the behavior of the smallest elements of matter (Special Relativity, 1905), as well as the projected behavior of an infinite cosmos (General Relativity, 1916). (Summary by Michael Hogan)

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