Imagine that gravity stopped. This is only a thought experiment, and it cannot happen: no technology we have can switch gravity off or block it. Yet the question is worth asking, because asking what would stop happening without gravity is a way of seeing what gravity does. Gravity is the invisible pull that makes things fall down and keeps you on the ground, and everything that has mass has gravity, with things that have more mass pulling harder. It is the weakest of the four forces of nature, since a child's magnet can lift a steel paper clip against the pull of the whole Earth, yet it feels dominant in daily life because it acts at infinite range and every speck of mass adds to it.
Newton described gravity as a force pulling any two masses toward each other in his 1687 book, the Principia. His law of universal gravitation says every point mass attracts every other point mass by a force along the line between them, and the pull between two objects grows with the product of their masses and shrinks with the square of the distance between their centers. 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'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. Newton's insight about the inverse-square law came from thinking about how the Earth pulls on the Moon, and he built a general law of universal gravitation from it.
That idea could be tested against the Moon, whose average distance from Earth is about 384,400 kilometers, which is about 60 Earth radii, 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 Moon takes about 27 Earth days to circle Earth, and 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, while 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. Everything in orbit is falling around its planet: it moves sideways so fast that it keeps missing the ground.
Einstein described the same pull in a very different way, and the difference is in what he took gravity to be. Newton described gravity as a force between masses, while Einstein described it as curved space-time, and Einstein's general theory of relativity treats gravity as the curving of space and time by mass and energy, a curve in space that wraps around a star or a planet, so that nearby objects are pulled into the curve. Gravity pulls on light too: a flashlight beam shone upward grows imperceptibly redder, a change scientists can measure. The gravitational constant G is about 6.674 times 10 to the minus 11 cubic meters per kilogram per second squared, as of November 2026. A black hole packs so much mass into so little space that its gravity does not even let light escape. 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. The first proof that gravitational waves exist came in 1974, when Hulse and Taylor found a binary pulsar whose orbit was shrinking at just the rate general relativity predicted. Taylor's team tracked the pulsar for four years and found the rate predicted by general relativity matched the observed rate to within one half of one percent; Hulse and Taylor won the 1993 Nobel Prize in Physics. 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. By the time the first waves arrived, the space-time wobble was 10,000 times smaller than the nucleus of an atom.
Gravity pulls together the material that makes stars and planets, and it is also what holds the Sun together. The Sun is a giant ball of hydrogen gas, and gravity holds all of that gas together. Inside the Sun, the inward pull of gravity is exactly balanced at each point by the outward push of gas pressure, called hydrostatic equilibrium, and that balance is self-correcting: if pressure falls short of the weight above, the star contracts and builds pressure; if it exceeds it, the star expands and releases pressure. The Sun, like most stars, is stable: it is neither expanding nor contracting, because all the forces within it are balanced. The balance lets scientists read the inside of the Sun without ever going there. To hold up the Sun against gravity, gas at its center must be about 15 million kelvins; just from the fact the Sun is not shrinking we can tell it is hot enough for fusion. The Sun's modeled central pressure is about 2.477 times 10 to the 11 bar, with a central temperature of about 15.7 million kelvins, what it takes to hold up its weight. The Sun turns about 4,260 million kilograms of mass into energy every second through fusion.
Escaping a pull takes speed, and the speeds differ widely from world to world. Earth's escape velocity, the speed needed to leave it for good, is about 11.2 kilometers per second, while the Moon's, at 2.38 kilometers per second, is much lower because its gravity is weaker. The Sun's escape velocity is 617.6 kilometers per second, 55 times Earth's, and its surface gravity is 28 times Earth's. Earth's surface at the equator moves about 465 meters per second as Earth spins, far slower than the roughly 11,186 meters per second needed to escape Earth. Mars, with less mass and weaker gravity, lost most of its original atmosphere, and even on Earth, some fast-moving molecules in the upper air escape the pull of gravity, over time scales of billions of years.
Earth's air and oceans are small next to the planet itself. Earth's mass is about 5.9722 times 10 to the 24 kilograms, and Earth's atmosphere has a total mass of about 5.1 times 10 to the 18 kilograms, while the oceans hold about 1.4 times 10 to the 21 kilograms. Earth's atmosphere is only about 1/1,200,000 the mass of Earth, a thin skin held on by gravity. The Moon's gravity pulls on the seas, and that makes the ocean tides go up and down; tides are very long waves that move through the ocean in response to the forces from the Moon and Sun, and they rise and fall regularly along the shore.
The feeling of weight is a separate matter from the pull: your weight is how hard gravity pulls on you, while a kilogram measures mass, which stays the same anywhere, and a pound measures the force of a planet's gravity tugging on you. What we call weight in daily life is the feeling of being supported by the ground or a seat pushing back on us. Near Earth's surface, gravity alone makes things speed up at 9.8 meters per second every second, called 1g. Standard gravity on Earth is defined as exactly 9.80665 meters per second squared; at the equator it is a little less (9.780) and at the poles a little more (9.832).
Take away the push from below and the feeling changes: anything in free fall, like a dropped apple or a falling spacecraft, feels weightless because nothing is pushing back on it, and on a roller coaster, free fall gives you a brief feeling of weightlessness at the top of a drop. To feel weightless on Earth, people ride airplanes that fly in big arcs, so everyone inside floats during the fall; gravity is never switched off. NASA Glenn's Zero Gravity Research Facility is a shaft about 500 feet deep where test packages free fall in a vacuum for just over 5 seconds, giving near weightlessness.
Astronauts on the space station look like they float because everything inside is falling together. Gravity does not stop in space; astronauts float because they are in free fall, not because there is no gravity. At about 250 miles up, where the space station flies, gravity is 88.8 percent as strong as on the ground, and things on the station travel at the orbital speed of the station, 17,500 miles per hour (28,000 kilometers per hour). Scientists call the feeling on the station microgravity, about one millionth of normal gravity, rather than zero gravity.
Gravity can be read from orbit, and the readings tell stories about the planet below. Gravity is slightly stronger over places with more mass underground, and NASA's GRACE spacecraft map those differences. GRACE detects tiny changes in gravity over time, which have revealed details of sea-level change and of changes in Earth's crust from earthquakes. Gravity works the same everywhere in the cosmos; how strongly we feel it depends on how much mass is around, and gravity never quite reaches zero, since far out in deep space, away from planets and stars, its pull is very faint. Before gravitational waves, nearly everything we knew about the universe came from studying waves of light; now we also study waves of gravity.
Words to know
- equilibrium: a balance
- gravitational: of gravity
- pulsar: spinning dead star
- microgravity: tiny gravity
- orbital: of an orbit
