Gravity is a force that pulls objects toward each other. Earth's gravity keeps you on the ground and makes things fall. Gravity can only pull, never push; gravity is why things fall down when you drop them or throw them, and it draws objects toward the center of a planet or other body. A dropped object falls faster and faster as it falls; near Earth's surface, a falling object speeds up by about 9.8 meters per second every second. With no air, heavy objects and light objects fall at the same rate. Galileo explored falling bodies by dropping different weights or sliding them down inclined planes. Legend has it that Galileo dropped heavy and light balls from the leaning tower of Pisa. On the Moon, Apollo 15 commander David Scott released a hammer and a feather together; with no air resistance, the feather fell at the same rate as the hammer. Anything that has mass also has gravity, and objects with more mass have more gravity; gravity gets weaker with distance, so the closer two objects are, the stronger their pull.

Newton's theory of universal gravitation was published in 1687; the Royal Society planned to publish Newton's Principia but was short of funds, so Edmond Halley agreed to pay the expenses himself. Newton's law of universal gravitation states that the force of attraction between any two massive bodies is proportional to their masses and inversely proportional to the square of the distance between their centers. If you double the distance between two masses, the gravitational attraction between them falls to one-fourth of its former value; if you double a mass, you double the attraction. The law has a number inside it: the gravitational constant G. Before Henry Cavendish, scientists could work out only the product of G and the mass of Earth; there was no practical way to separate G from the mass of Earth; as of November 2026, the 2022 CODATA value of the Newtonian constant of gravitation is 6.674 30 x 10^-11 m^3 kg^-1 s^-2, with a standard uncertainty of 0.000 15 x 10^-11. The determination of an exact value of the gravitational constant has eluded physicists for more than 300 years. Brian Greene says Newton never told us how gravity exerts its influence.

A falling apple is said to have inspired Isaac Newton to connect it with the motion of the Moon. Newton predicted that, if gravity weakens with the square of distance, the Moon falls toward Earth in one second 1/3600th as far as an apple falls in its first second. To stay in orbit the Moon must fall about 1.4 millimeters toward Earth each second; an object in motion stays in motion unless something pushes or pulls on it. An orbiting object is always falling toward the planet, but it moves sideways fast enough that it never hits it. Astronauts on the space station float because they and the station are all falling together around Earth. Gravity does exist in space: it keeps spacecraft in orbit around Earth and Earth in orbit around the Sun.

Mass is a quantity of matter, set by the numbers of atoms and molecules an object contains, so it does not change from place to place; weight is the gravitational force acting on an object, so it varies with gravity; a person at low elevation in New Orleans weighs slightly more than one in Denver with the same mass. A 1.0-kilogram mass weighs 9.8 newtons on Earth and only about 1.6 newtons on the Moon; gravity is also slightly stronger over places with more mass underground than over places with less. From 2002 to 2017 NASA's two GRACE spacecraft measured variations in Earth's gravity, and GRACE-FO is its successor. GRACE watched changes in sea level and could detect changes in Earth's crust brought on by earthquakes.

Albert Einstein described gravity as a curve in space that wraps around objects such as stars and planets; in general relativity, matter tells spacetime how to curve, and curved spacetime tells matter how to move. Einstein argued that the more strongly you feel gravity, the slower time passes for you; time dilation from gravity is tiny, so it would not ordinarily be noticed by humans, though it has been measured in extremely sensitive experiments. In 2010 NIST physicist David Wineland and colleagues ran two optical clocks about a foot apart in height, and the clocks ticked at slightly different rates because of the slight difference in gravity. Atomic clocks on GPS satellites run 38 microseconds per day faster than clocks on Earth: orbital speed slows them 7 microseconds, and weaker gravity speeds them up 45. Gravity Probe B was designed to measure two key predictions of general relativity by monitoring the orientations of ultra-sensitive gyroscopes relative to a distant guide star.

Newton's gravity could not explain one observation: Mercury's elliptical orbit gradually moves around the Sun, which is called precession; gravity also pulls on light, so a massive object can bend light. In 1911 Einstein challenged astronomers to watch total solar eclipses and compare star positions with and without possible warping by the Sun; in 1917 the Joint Permanent Eclipse Committee chose two stations to watch the 1919 eclipse: Sobral in Brazil and the island of Principe. In 1919 the first measurements made against the Newtonian theory were taken, and they arguably aligned with Einstein's general relativity. Massive objects bend light from more distant objects, an effect called gravitational lensing. Lensing also magnifies: it lets telescopes see objects that would otherwise be too far away and too faint to be seen; in 2020 ESA/Hubble released an image showing the largest ring-shaped lensed image of a galaxy, an Einstein ring, discovered up to that time.

A black hole is so dense that gravity just beneath its surface, the event horizon, is strong enough that nothing, not even light, can escape. Black holes do not suck in other matter; from far enough away, their gravity acts just like that of any other object of the same mass. One type of black hole is born when massive stars run out of fuel and explode in supernovae. Astronomers tracked the orbits of several stars near the center of the Milky Way to prove it houses a supermassive black hole, a discovery that won the 2020 Nobel Prize. As of November 2026, it is about 4 million times the Sun's mass.

A gravitational wave is an invisible ripple in space; scientists first directly detected gravitational waves in 2015, using LIGO. LIGO scientists estimate the first merging black holes were about 29 and 36 times the mass of the Sun, 1.3 billion years away. LIGO can detect a change in the length of its arms smaller than one-ten-thousandth the diameter of a proton, about 10^-19 meter. LIGO catches only the last few orbits of a pair: by the time its frequency is detectable, the objects are just seconds away from colliding. The Livingston detector recorded the first event 7 milliseconds before the detector at Hanford, which showed that the source was in the Southern Hemisphere; by the time the first waves reached Earth, the space-time wobbling they made was 10,000 times smaller than the nucleus of an atom. As of November 2026, ESA's LISA will be the first space-based observatory dedicated to studying gravitational waves.

In 1933 Fritz Zwicky found that galaxies in the Coma Cluster moved too quickly for the gravity of their observed ordinary matter, and he suggested an invisible form of matter; in the 1970s Vera Rubin studied stars at the outer edges of spiral galaxies, which moved as if held in place by a large amount of unseen matter. As of November 2026, scientists estimate that ordinary matter is only about 5% of the universe, dark matter about 27%, and the rest is thought to be dark energy; as of November 2026, scientists do not yet know what dark matter is. ESA's Euclid mission studies how the Universe has evolved over the past 10 billion years, from which astronomers can infer the properties of dark energy, dark matter and gravity. General relativity has passed all experimental tests so far, but singularities inside black holes and at the Big Bang show the need for a theory beyond Einstein. The prediction of a singularity may signal the limits of relativity, where quantum effects not included in the theory become important in a more complete description of gravity.

Words to know

• precession: orbit slowly turning
• spacetime: space with time
• singularities: points relativity fails
• lensing: light bending