relativity: Quick Facts

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Nov. 11, 2026

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Photos and Videos

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

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.

curvature and parallel motion

Understand Albert Einstein's theory of relativity, about what is absolute and not relative

How are gravitational waves applied in science and in everyday life?

Know the different kinds of dimensions and how they are different from one another

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Topics

hypertime Galilean relativity Galilean invariance gravitational red shift kinematic relativity time dilation twin paradox special relativity general relativity quantum field theory radiation time light electromagnetism gravity radioactivity chemical reaction phase temperature entropy

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Key People

Portrait of Albert Einstein in a suit and tie, looking slightly to the side with a neutral expression.

Albert Einstein

German-American physicist

Henri Poincaré, 1909.

Henri Poincaré

French mathematician

Bernhard Riemann, lithograph after a portrait, artist unknown, 1863.

Bernhard Riemann

German mathematician

Arthur Stanley Eddington.

Arthur Eddington

British scientist

Hermann Weyl

Hermann Weyl

German-American mathematician

An artist's representation of Penrose tiling

Roger Penrose

British mathematician

Otto Heckmann at the European Southern Observatory's (ESO's) one-metre Schmidt telescope at La Silla, Chile, 1971.

Otto Heckmann

German astronomer

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Tullio Levi-Civita

Italian mathematician

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Hermann Minkowski

German mathematician

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Vladimir Aleksandrovich Fock

Russian mathematical physicist

Italian physicist Guglielmo Marconi at work in the wireless room of his yacht Electra, c. 1920.

All About Physics Quiz

Italian-born physicist Dr. Enrico Fermi draws a diagram at a blackboard with mathematical equations. circa 1950.

Physics and Natural Law