Gravity

The force that keeps you in your chair, the Moon in the sky, and physicists awake at night.

By Greg Goodwin · Published October 24, 2026

Cheat Sheet

Standard gravity at Earth's surface is defined as 9.80665 meters per second squared.
The CODATA 2022 value of the gravitational constant G is 6.674 30(15) x 10^-11 m^3 kg^-1 s^-2, unchanged from 2018.
Gravity at the Moon's surface is about 1.62 meters per second squared, roughly one-sixth of Earth's.
Gravity is the weakest of the four fundamental forces, alongside electromagnetism and the strong and weak nuclear forces.
A 1919 solar eclipse expedition led by Arthur Eddington reported starlight bending near the Sun, supporting general relativity.
The 2017 Nobel Prize in Physics went to Rainer Weiss, Barry Barish, and Kip Thorne for LIGO.

The 60-Second Version

Gravity is the attraction between anything that has mass or energy. It keeps your feet on the ground, holds the Moon in orbit around Earth, steers the planets around the Sun, and gathers gas into stars and galaxies. Isaac Newton described it in 1687 as a force that grows with mass and weakens with the square of the distance, a law accurate enough to guide spacecraft across the solar system. In 1915 Albert Einstein's general relativity recast gravity as the curvature of spacetime caused by mass and energy: matter tells spacetime how to curve, and curved spacetime tells matter how to move. General relativity has passed test after test, including the 2015 detection of gravitational waves by LIGO. But it does not mesh with quantum mechanics, and no workable quantum theory of gravity exists, which leaves gravity the odd one out among nature's forces.

The Long Version

Newton's Universal Law

Britannica credits Isaac Newton with formulating the law of universal gravitation in 1687, in his Principia. It says any two objects attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. Double the distance, and the pull falls to one-fourth. The strength of the force is set by the gravitational constant G, which CODATA's 2022 adjustment puts at 6.674 30(15) x 10^-11 in SI units, unchanged from 2018 because no new measurements were competitive. G is among the least precisely known fundamental constants, and measurements keep disagreeing slightly. Newton's law remains valid for objects moving slowly compared with light, which is why it still describes the solar system very well.

Einstein's Curved Spacetime

Newton's theory could not be reconciled with special relativity, which led Einstein to general relativity, completed in 1915. Its central idea is that gravity is not a force pulling through space but the geometry of spacetime, bent by mass and energy. It explained a small unexplained wobble in Mercury's orbit, about 43 arcseconds per century, and predicted that light bends when passing a massive object. In 1919, expeditions led by Arthur Eddington measured starlight shifting near the Sun during a total solar eclipse, results that made Einstein famous. Global positioning satellites must account for relativistic effects on their clocks to stay accurate.

Gravity in Everyday Life

Standard gravity at Earth's surface is defined as 9.80665 meters per second squared, though the real value varies slightly with latitude and altitude. Without air resistance, all objects fall at the same rate, which Apollo 15 astronaut David Scott demonstrated on the Moon in 1971 by dropping a hammer and a feather together. The Moon's surface gravity is roughly 1.62 meters per second squared, about one-sixth of Earth's. Astronauts on the International Space Station float not because gravity is absent, since it is close to 90 percent of its surface strength at that altitude, but because they and the station are in continuous free fall around Earth.

Ripples and Black Holes

General relativity predicts that accelerating masses make ripples in spacetime called gravitational waves. On 14 September 2015, the LIGO detectors recorded the first direct observation of them, produced by two black holes of roughly 36 and 29 solar masses merging about 1.3 billion light years away. The result was announced on 11 February 2016, and Rainer Weiss, Barry Barish, and Kip Thorne received the 2017 Nobel Prize in Physics for their roles. Gravitational waves let astronomers observe events, such as black hole mergers, that emit little or no light.

What Is Still Unsolved

General relativity is a classical theory, and Britannica notes that no workable quantum theory of gravity has been developed. That gap matters for understanding the centers of black holes and the first instants of the universe. Gravity is also the weakest of the four fundamental forces, yet it dominates at large scales because it only attracts and adds up. On galactic and cosmic scales, observations of rotating galaxies and the accelerating expansion of the universe, discovered in 1998, are explained by dark matter and dark energy, though some researchers explore modifications to gravity instead.

Why People Care

Gravity shapes everything from the tides to the orbits that carry satellites and GPS. It is also where our best theories collide, so every new result, from gravitational waves to measurements of G, bears on the deepest unsolved problem in physics.

Glossary

Mass versus weight: Mass is how much matter an object has; weight is the gravitational pull on that mass, which changes from planet to planet.
Spacetime: The four-dimensional fabric of three space dimensions plus time, whose curvature in general relativity we experience as gravity.
Free fall: Motion under gravity alone, which is why astronauts in orbit feel weightless even though gravity still acts on them.
Gravitational waves: Ripples in spacetime produced by accelerating masses, such as merging black holes.
Escape velocity: The speed needed to break free from a body's gravity without further propulsion; about 11.2 kilometers per second for Earth.

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