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relativity
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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light.
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Understand Albert Einstein's theory of relativity, about what is absolute and not relative
Two-panel illustration comparing classical and relativistic velocity addition: The top panel shows an archer on a moving train and a stationary archer both shooting arrows, with their speeds adding. The bottom panel shows a person on the train and one on the ground both shining light toward a target, but the speed of light is the same for both observers, illustrating the constancy of the speed of light in special relativity. Illustration showing a red train moving past a tree struck by lightning, with a stationary observer on the ground and another observer inside the train. Distance measurements labeled x and x prime represent different frames of reference, demonstrating relativity concepts. An illustration of Einstein's relativity concept shows a red train moving at four-fifths the speed of light between two trees struck by lightning, with an observer on the ground positioned equidistant from the strikes, highlighting how simultaneity depends on the observer's frame of reference. Two graphs illustrate effects of relativity as velocity approaches the speed of light: the left graph shows time dilation increasing sharply, while the right graph shows length contraction decreasing, both as functions of velocity. A grid representing spacetime is distorted by three spheres of different sizes, illustrating how massive objects curve spacetime according to general relativity. black hole in M87 experimental evidence for general relativity
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Italian-born physicist Dr. Enrico Fermi draws a diagram at a blackboard with mathematical equations. circa 1950.
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Italian physicist Guglielmo Marconi at work in the wireless room of his yacht Electra, c. 1920.
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Written by
Sidney Perkowitz
Charles Howard Candler Professor of Physics Emeritus, Emory University, Atlanta. Author of Empire of Light, Universal Foam, Hollywood Science, Slow Light, and others.
Sidney Perkowitz
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In 1925 the British philosopher Bertrand Russell, in his ABC of Relativity, suggested that Einstein’s work would lead to new philosophical concepts. Relativity has indeed had a great effect on philosophy, illuminating some issues that go back to the ancient Greeks. The idea of the ether, invoked in the late 19th century to carry light waves, harks back to Aristotle. He divided the world into earth, air, fire, and water, with the ether (aether) as the fifth element representing the pure celestial sphere. The Michelson-Morley experiment and relativity eliminated the last vestiges of this idea.

Relativity also changed the meaning of geometry as it was developed in Euclid’s Elements (c. 300 bce). Euclid’s system relied on the axiom “a straight line is the shortest distance between two points,” among others that seemed self-evidently true. Straight lines also played a special role in Euclid’s Optics as the paths followed by light rays. To philosophers such as the German Immanuel Kant, Euclid’s straight-line axiom represented a deep level of truth. But general relativity makes it possible scientifically to examine space like any other physical quantity—that is, to investigate Euclid’s premises. It is now known that space-time is curved near stars; no straight lines exist there, and light follows curved geodesics. Like Newton’s law of gravity, Euclid’s geometry correctly describes reality under certain conditions, but its axioms are not absolutely fundamental and universal, for the cosmos includes non-Euclidean geometries as well.

Considering its scientific breadth, its recasting of people’s view of reality, its ability to describe the entire universe, and its influence outside science, Einstein’s relativity stands among the most significant and influential of scientific theories.

Sidney Perkowitz