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Gravity: What is it?

NASA used a Lunar Landing Walking Simulator to study astronauts' ability to perform tasks whilst experiencing one-sixth of normal gravity in preparation for the Apollo moon landings.
NASA used a Lunar Landing Walking Simulator to study astronauts' ability to perform tasks whilst experiencing one-sixth of normal gravity in preparation for the Apollo moon landings. (Image credit: NASA)

Gravity is one of the universe's fundamental forces and dominates every moment of our conscious experience. It keeps us close to the ground, drags baseballs and basketballs out of the air and gives our muscles something to struggle against. Cosmically, gravity is just as consequential. 

From collapsing hydrogen clouds into stars to gluing galaxies together, gravity represents one of just a few players that determine the broad strokes of the universe's evolution. 

In some ways, the story of gravity is also the story of physics, with some of the field's biggest names finding fame by defining the force that ruled their lives. But even after more than 400 years of study, the enigmatic force still lies at the heart of some of the discipline's greatest mysteries.

Related: What is dark matter? 

Gravity: A universal force

Four fundamental forces act upon us every day. The strong force and the weak force operate only inside the centers of atoms. The electromagnetic force rules objects with excess charge (like electrons, protons, and socks shuffling over a fuzzy carpet), and gravity steers objects with mass. 

The first three forces largely escaped humanity's notice until recent centuries, but people have long speculated about gravity, which acts on everything, from raindrops to cannonballs. 

Ancient Greek and Indian philosophers observed that objects naturally moved toward the ground, but it would take a flash of insight from Isaac Newton to elevate gravity from an inscrutable tendency of objects to a measurable and predictable phenomenon. 

Newton's leap, which became public in his 1687 treatise "Philosophiae Naturalis Principia Mathematica", was to realize that every object in the universe — from a grain of sand to the largest stars — pulled on every other object. This notion unified events that appeared unrelated, from apples falling on Earth to the planets orbiting the sun. He also put numbers to the attraction: Doubling the mass of one object makes its pull twice as strong, he determined and bringing two objects twice as close quadruples their mutual tug. Newton packaged these ideas into his universal law of gravitation.  

What causes gravity?

Massive bodies warp the fabric of space and time around them, leading to nearby objects following a curved path. This artist's illustration shows gravity bending the fabric spacetime around the Earth and sun.  (Image credit: vchal via Getty Images)

Newton's description of gravity was accurate enough to detect the existence of Neptune in the mid-1800s before anyone could see it, but Newton's law isn't perfect. In the 1800s, astronomers noticed that the ellipse traced by Mercury's orbit was moving more quickly around the sun than Newton's theory predicted it should, suggesting a slight mismatch between his law and the laws of nature. The puzzle was eventually resolved by Albert Einstein's theory of general relativity, published in 1915. 

Before Einstein published his groundbreaking theory, physicists knew how to calculate a planet's gravitational pull, but their understanding of why gravity behaved in such a way had advanced little beyond that of the ancient philosophers. These scientists understood that all objects attract all others with an instantaneous and infinitely far-reaching force, as Newton had postulated, and many Einstein-era physicists were content to leave it at that. But while working on his theory of special relativity, Einstein had determined that nothing could travel instantly, and the pull of gravity should be no exception. 

For centuries, physicists treated space as an empty framework against which events played out, according to the Stanford Encylopedia of Philosophy. It was absolute, unchanging and didn't — in any physical sense — really exist. General relativity promoted space, and time as well, from a static backdrop to a substance somewhat akin to the air in a room. Einstein held that space and time together made up the fabric of the universe and that this "spacetime" material could stretch, compress, twist and turn — dragging everything in it along for the ride. 

Einstein suggested that the shape of spacetime is what gives rise to the force we experience as gravity. A concentration of mass (or energy), such as the Earth or sun, bends space around it like a rock bends the flow of a river. When other objects move nearby, they follow the curvature of space, as a leaf might follow an eddy around the rock (although this metaphor isn't perfect because, at least in the case of planets orbiting the sun, spacetime isn't "flowing"). We see planets orbit and apples fall because they're following paths through the distorted shape of the universe. In everyday situations, those trajectories match the force Newton's law predicts. 

Einstein's field equations of general relativity, a collection of formulas that illustrate how matter and energy warp spacetime, gained acceptance when they successfully predicted the changes in Mercury's orbit, as well as the bending of starlight around the sun during a 1919 solar eclipse

In Photos: Einstein's 1919 solar eclipse experiment tests general relativity 

Gravity: A tool of discovery

The modern description of gravity so accurately predicts how masses interact that it has become a guide for cosmic discoveries. 

American astronomers Vera Rubin and Kent Ford noticed in the 1960s that galaxies appear to rotate fast enough to spin off stars like a dog shakes off water droplets. But because the galaxies they studied weren't whirling apart, something appeared to be helping them stick together. Rubin and Ford's thorough observations provided strong evidence supporting Swiss astronomer Fritz Zwicky's earlier theory, proposed in the 1930s, that some invisible variety of mass was speeding up galaxies in a nearby cluster. Most physicists now suspect this mysterious "dark matter" warps spacetime enough to keep galaxies and galaxy clusters intact. Others, however, wonder if gravity itself might pull harder at galaxy-wide scales, in which case both Newton's and Einstein's equations would need adjustment. 

Tweaks to general relativity would have to be delicate indeed, as researchers recently started detecting one of the theory's most subtle predictions: The existence of gravitational waves, or ripples in spacetime, caused by the acceleration of masses in space. Since 2016, a research collaboration operating three detectors in the United States and Europe has measured multiple gravitational waves passing through Earth. More detectors are on the way, launching a new era of astronomy in which researchers study distant black holes and neutron stars — not by the light they emit, but by how they rumble the fabric of space when they collide.

Yet general relativity's string of experimental successes gloss over what many physicists see as a fatal theoretical failure: It describes a classical spacetime, but the universe ultimately appears to be quantum or made up of particles (or "quanta") such as quarks and electrons.

The classical notion of space (and gravity) as one smooth fabric clashes with the quantum picture of the universe as a collection of sharp little pieces. How to extend the reigning Standard Model of particle physics, which spans all known particles as well as the other three fundamental forces (electromagnetism, the weak force, and the strong force), to cover space and gravity at the particle level remains one of the deepest mysteries in modern physics. 

Additional resources

Watch this video explaining Newtonian gravity, from PBS Digital Media's Crash Course. Learn more about gravity and how the warps and ripples in space work, from the Australian Academy of Science. Read astrophysicist Ethan Siegel's explanation of how gravity bends (and unbends) space, generating gravitational waves on Medium.  

Bibliography

Head, Keith. "Gravity for beginners." University of British Columbia (2003).

Poisson, Eric, and Clifford M. Will. "Gravity." Cambridge University Press (2014).

Why does gravity pull us down and not up? Mario Borunda, The Conversation (2021).

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Charlie Wood
Charlie Wood

Charlie Wood is a freelance journalist covering physical sciences both on and off this pale blue dot. He contributes to Space.com and LiveScience, as well as Popular Science, Scientific American, Quanta Magazine, and others. These days he writes from New York but in previous lives he taught physics in Mozambique and science English in Japan. Find him on Twitter @walkingthedot.