The Physics of "Relativity"

This song deals with Einstein's theory of special relativity, published in his "miraculous year" of 1905. (Special relativity deals only with objects moving at constant velocity. General relativity deals with accelerating objects, and the interaction between gravity and spacetime.) The special theory yields many of the most famous results, including the ideas that moving objects become shorter ("Fitzgerald contraction", mentioned in the third verse of the song), that moving clocks tick slowly (time dilation), and that events which appear simultaneous in a "stationary" reference frame aren't simultaneous if viewed from a "moving" reference frame. These three effects combine to eliminate all the apparent paradoxes. (For example, if I see a moving person, her watch appears to me to tick slowly, but from her point of view it's my watch that ticks slowly!) If you're feeling ambitious, you might read my handouts about these effects; you should be able to understand them if you know basic algebra: Part 1 .. Part 2.

The special theory of relativity also yields the most famous equation, E = mc2, which says that the mass m of an object can equivalently be thought of as energy E. The factor c is the speed of light (about 3 x 108 m/s), so that a modest amount of mass corresponds to an enormous amount of energy. This equation is given in the first verse of the song, and referred to in the last verse; in a nuclear fission bomb, a portion of the relatively innocuous "rest mass" of the uranium or plutonium is transformed into the much more destructive energy of a thermonuclear explosion.

One of the most interesting aspects of this equation is that it tells us that what we normally think of as energy can behave like mass, in the sense of exerting a gravitational pull on other objects. This even works for potential energy!

--WFS 8/30/05



Albert Einstein
"When the Special Theory of Relativity began to germinate in me, I was visited by all sorts of nervous conflicts... I used to go away for weeks in a state of confusion."

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Background image: covers from some of Tom Lehrer's albums