Gravity, or gravitation, is the fundamental interaction that pulls together all things that have mass or energy. It is the weakest of the four fundamental interactions of physics. It is many orders of magnitude weaker than electromagnetism, the strong interaction and the weak interaction. Even so, it shapes the universe at large scales because it has unlimited range and is always attractive between ordinary masses. Gravity holds planets in orbit, binds stars into galaxies, drives the formation of stars and black holes, and governs how the cosmos expands. Two theories describe it. Isaac Newton's law of universal gravitation is an excellent approximation in most everyday and astronomical situations. Albert Einstein's general theory of relativity is the more complete account and treats gravity as the curvature of spacetime.
Early ideas
In ancient Greece, Aristotle taught that heavy bodies move toward the centre of the universe because that is their natural place. He also held that heavier bodies fall faster than lighter ones, a view that went largely unchallenged for centuries. During the Scientific Revolution, Galileo Galilei used experiments with inclined planes to show that, if air resistance is ignored, falling bodies speed up at the same uniform rate whatever their mass. Around the same time, Johannes Kepler worked out three empirical laws of planetary motion from astronomical observations. These included the finding that planets move in elliptical orbits with the Sun at one focus.
Newtonian gravitation
In his Philosophiæ Naturalis Principia Mathematica (1687), Newton argued that the force pulling an apple to the ground is the same force that keeps the Moon in orbit around the Earth. His law of universal gravitation states that every pair of point masses attracts each other. The force is proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G m₁m₂ / r². Combined with his laws of motion and the methods of calculus, this law accounted for Kepler's laws and explained tides in the ocean. It also predicted the paths of comets. In 1846 it led to the discovery of Neptune, whose position was calculated from irregularities in the orbit of Uranus. Newtonian gravity remains central to classical mechanics, spaceflight navigation and much of astrophysics.
The proportionality factor G is the gravitational constant. Henry Cavendish first measured the gravitational attraction between laboratory masses in 1797–98 using a torsion balance, which allowed him to infer Earth's density. G is still one of the least precisely known fundamental constants, partly because precise experiments disagree with one another. The CODATA 2022 recommended value is 6.67430(15) × 10⁻¹¹ m³ kg⁻¹ s⁻², with a relative uncertainty of about 22 parts per million.
Newton himself was uneasy that his theory described action at a distance without explaining how it worked. The theory also has a known observational problem. The perihelion of Mercury's orbit advances by about 43 arcseconds per century more than Newtonian mechanics predicts once the pull of the other planets is taken into account.
General relativity
Einstein completed the general theory of relativity in 1915. It rests on the equivalence principle: locally, the effects of gravity cannot be told apart from those of acceleration. In this theory, mass and energy curve spacetime, and freely falling bodies follow the straightest possible paths (geodesics) through that curved geometry. The Einstein field equations link the curvature of spacetime to the distribution of matter and energy. When fields are weak and speeds are low, the theory reduces to Newton's law.
General relativity accounted for Mercury's anomalous perihelion shift. It also predicted that gravity bends light, which Arthur Eddington's expedition confirmed during the solar eclipse of 1919. Further predictions include gravitational redshift and gravitational time dilation. Clocks run more slowly deeper in a gravitational field, and the satellites of the Global Positioning System must correct for this effect. Gravitational lensing, in which massive objects bend the light of more distant sources, is now a standard tool in astronomy.
Black holes and gravitational waves
The theory predicts black holes: regions where spacetime is curved so strongly that nothing, not even light, can escape from inside the event horizon. In 2019 the Event Horizon Telescope published the first image of the shadow of a black hole, the one at the centre of the galaxy M87.
General relativity also predicts gravitational waves, ripples in spacetime that travel at the speed of light. On 14 September 2015, the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Hanford, Washington, and Livingston, Louisiana, recorded the signal GW150914. The LIGO and Virgo collaborations announced it on 11 February 2016. The waves came from two black holes of roughly 29 and 36 solar masses merging. It was the first direct detection of gravitational waves, and it started the field of gravitational-wave astronomy. Rainer Weiss, Kip Thorne and Barry Barish received the 2017 Nobel Prize in Physics for the discovery.
Gravity in cosmology
Gravity is the main force behind large-scale structure in cosmology. Applied to the universe as a whole, general relativity gives models of an expanding universe, which underlie the Big Bang model. Two observations do not fit a universe made only of visible matter. Galaxies rotate faster than their visible matter can explain, and galaxy clusters bend light more strongly than expected. Most researchers attribute these effects to dark matter, while some propose modified theories of gravity instead. Since 1998, observations have shown that the expansion of the universe is speeding up. This is usually attributed to dark energy, which can be represented by a cosmological constant in Einstein's equations.
Open problems
General relativity does not fit into the framework of quantum mechanics. The Standard Model of particle physics successfully describes the other three interactions as quantum fields, but gravity is left out. Attempts at a theory of quantum gravity include string theory and loop quantum gravity. Some such theories propose a hypothetical carrier particle called the graviton. No experiment has yet confirmed any of these approaches. The problem matters most for understanding singularities inside black holes and the earliest moments of the universe, where both quantum effects and strong gravity are significant. Other active research includes tighter measurements of G, tests of the equivalence principle, and searches for deviations from the inverse-square law at very short distances.
References
- Search for New Physics with Atoms and Moleculesarxiv.org
- All of the CODATA recommended values of G.researchgate.net
- Precision measurement of the Newtonian gravitational constantacademic.oup.com
- Gravitational Wave Astronomy: Delivering on the Promisesarxiv.org
- When spacetime vibrates: An introduction to gravitational wavesarxiv.org
- First observation of gravitational wavesen.wikipedia.org
- GW150914 Press Releaseligo.caltech.edu