Imagine that gravity stopped. This is only a thought experiment, and the real story of how gravity was described, from Newton to Einstein, is what makes the question worth asking. Newton's law of universal gravitation says every point mass attracts every other point mass by a force along the line between them, and things with more mass pull harder. Gravity gets weaker the farther apart two things are: the pull between two objects grows with the product of their masses and shrinks with the square of the distance between their centers. The size of that pull is set by the gravitational constant G, which is about 6.674 times 10 to the minus 11 cubic meters per kilogram per second squared, as of November 2026.
The story of how the law was found is told in a famous scene. Newton's helper wrote that the idea came to him in a garden as he watched an apple fall: gravity might reach as far as the Moon. A test was possible. The Moon is about 60 Earth radii away, so by the inverse-square law Earth's pull there should be 60 squared, or 3,600 times weaker than at the surface. Measured, the Moon accelerates toward Earth at about 0.00272 meters per second squared, against 9.8 for a falling apple, which fits the 3,600 times weaker prediction. The inverse-square law comes from geometry: gravity spreads thinner and thinner, over an area that grows as the square of the distance from the source.
Newton also pictured how a curved path comes about. A moving object keeps moving in a straight line unless something pushes or pulls it, so anything riding on the spinning Earth is held to a curved path by gravity. Newton imagined a cannon on a very tall mountain: fire the ball faster and it lands farther away, and at the right speed it falls around Earth forever, which is an orbit. Without gravity to pull on the ball, none of this would bend the ball's path into a curve. Everything in orbit is falling around its planet: it moves sideways so fast that it keeps missing the ground, and the International Space Station, in low Earth orbit, circles the planet once in about 90 minutes.
Einstein changed the picture. Einstein's general theory of relativity treats gravity as the curving of space and time by mass and energy. Einstein described gravity as a curve in space that wraps around a star or a planet; nearby objects are pulled into the curve. Because mass and energy are what curve space-time, wherever there is mass or energy there is curvature, and no known way to shut it off. That matches what we see in daily life: no technology we have can switch gravity off or block it. Gravity works the same everywhere in the cosmos; how strongly we feel it depends on how much mass is around. There is no such thing as antigravity: no machine can neutralize gravity, though a magnet can balance its pull on one object.
Curved space affects light as well as things with weight. Gravity pulls on light too: a flashlight beam shone upward grows imperceptibly redder, a change scientists can measure. At the far end of that scale is the black hole. A black hole packs so much mass into so little space that its gravity does not even let light escape. Einstein also predicted that gravity could ripple like a wave, and that prediction was later tested. Einstein predicted gravitational waves in 1916 in his general theory of relativity: ripples in space-time from massive accelerating objects such as orbiting black holes. Gravitational waves travel at the speed of light, 186,000 miles per second, and squeeze and stretch anything in their path. On September 14, 2015, LIGO sensed gravitational waves from two black holes colliding 1.3 billion light-years away, the first direct detection. Each LIGO observatory has two arms more than 4 kilometers long; a passing wave changes their lengths slightly, and lasers and mirrors detect it.
Words to know
- constant: fixed number
- curvature: amount of bend
- orbit: curved path around
- detection: first noticing
