A satellite overhead carries a clock that runs fast by thirty-eight millionths of a second a day, and the map in your pocket corrects for it. The speed of light is the same for everyone, so time and length are not; gravity is not a force but the shape of spacetime. Run on the page: frames and light; simultaneity, time dilation and the Lorentz transformation; spacetime diagrams and four-vectors; E = mc² and the view from a fast ship; curved space and Einstein's equations; orbits, lensing and clocks; black holes and neutron stars; gravitational waves and the expanding universe. Boost a diagram, send the twins on a trip, bend light round the Sun and photograph a black hole — every clock reading and every image computed by the page.
Every diagram, clock reading, image and number on these pages is computed by the page or cited with its source; your progress stays in this browser. The course teaches tested physics and names open questions as open; nuclear energy is treated as physics only. Measured data are cited and dated, and where none could be used the data are synthetic and say so. The course names no product. Not affiliated with or endorsed by any organisation, observatory, publisher or author mentioned here.