Gravity: A simple introduction (2024)

by Chris Woodford. Last updated: January 6, 2022.

What goes up, must come down. That's one way of looking at theweird phenomenon we call gravity, but it's far from the wholestory. Some things—space probes and satellites spinning over our heads—never come down. And the idea of gravity as a simple up-down force happening purely on Earth is very wrong too.

Gravity is like invisible elastic stretched through the whole universe,holding the stars and planets together and pulling them toward one another.Much more significantly, it's a fundamental force between every bitof matter in the universe and every other bit: just like Earth andthe Sun, you have your own gravity, and so do I. From Aristotle toKepler and from Newton to Einstein, understanding gravity haschallenged some of the best scientific minds in history. Thanks totheir efforts in getting a grip on this tricky topic, we can do allkinds of neat things, from figuring out where we are with GPS satellites to making sure bullets hit the spot.But that still leaves an important question: just what is this thingwe call gravity and how does it work? Let's take a closer look!

Photo: Gravity in all its beauty! Gravity's pull varies from place to place, both on Earth and elsewhere. This is a map showing how the strength of gravity varies across the Mare Orientaleon the moon. Image courtesy of NASA/JPL-Caltech.

What is gravity?

Gravity is a pulling force (always a force of attraction) betweenevery object in the universe (every bit of matter, everything thathas some mass) and every other object. It's a bit like an invisiblemagnetic pull, but there's no magnetism involved. Some people liketo call this force gravitation and reserve the word gravityfor the special kind of gravitation ("what goes up must come down")that we experience here on Earth. To my mind, that's unnecessaryand wrong, and I'll explain why when we talk about Isaac Newton ina moment. In this article, I'm going to use the word gravity foreverything (both gravity and gravitation).

If you've heard physicists talk about the four fundamentalforces (or four fundamental interactions) that control everythingthat happens around us, you'll know that gravity is one ofthem—along with the electromagnetic force and the two nuclearforces that work on very small scales inside atoms (known asthe strong and weak forces). Gravity is very different from theseother forces, however. Most of us can remember playing with magnetsin school and one of the first things you learn by doing that is thattwo magnets can attract or repel. Gravity can certainly attract, butit never repels. While magnetism can be an incredibly strong forceover very short distances, gravity is generally a much weaker force,though it works over infinitely long distances. The gravity exertedby your body, right now, is pulling the Sun toward you—just a tinybit—across a distance of something like 150 million kilometers!

Artwork: Gravity keeps the planets in orbit around the Sun, even at immense,astronomical distances. Spacecraft, like Mariner 10 shown here, sometimes use what's called a "gravity assist" to help them achieve a new orbit or a different velocity. Artwork courtesy of NASA

Gravity on Earth

Cars, trucks, airplanes, mosquitos, your body and everythingaround you—it's all stuck to Earth by the force of gravity. Ifwe've just said gravity is a weak force, how is that possible? Howcan such a weak force pull something like a huge Jumbo jet downtoward the ground?

Gravity might be weak, but Earth has a lot of itbecause our planet is so big and we're relatively close to it(compared to our distance from the Sun, anyway). Often it helps tothink of Earth's gravity originating at a single point at thecenter of the planet's core. The amount of gravity you feel at anyplace on Earth depends how far you are from this point, so it'sslightly more at sea level and slightly less when you're up amountain. Now Earth isn't a perfect sphere: technically, it'swhat's called an oblate spheroid—it's flattened at the polesand bulges at the equator. That means gravity also lessens withlatitude (it's slightly less at the equator than at the poles).Finally, because Earth is spinning around, and people at the polesare moving less quickly (relative to space) than those at theequator, that also slightly reduces the effects of gravity. Add allthese things together and you get a variation in gravity between theequator and the poles of much less than 1 percent, so for mosteveryday purposes, we can say that gravity at sea level is the sameright across Earth.[1]

Artwork: The force of gravity is slightly lower at the equator than at the poles.Please note that this figure is not drawn to scale and Earth's bulge is hugely exaggerated!

Why might it matter that gravity varies on Earth? First, andperhaps least importantly, it affects how much you weigh. If gravityis more at sea level, you weigh more there! The quickest way to loseweight is to climb an especially high mountain—not because all theeffort makes you lose any body mass, but because the force of gravityis weaker the further you go from the center of the Earth. Second,gravity pulls everything toward Earth (the invisible, gassyatmosphere around us as well as everything else), and that's why wehave air pressure (on land) and water pressure (in the oceans). Thesethings vary with altitude (above Earth's surface) and depth (belowsea level).

If gravity is a force tugging us toward a point in the center ofthe planet, why don't we keep on being pulled in? Why doesn'tgravity tug you through the floor of your house and the rocks belowto suffer a really rather unpleasant death in the heart of Earth'sfiery core, deep beneath your feet? If you're sitting still in achair right now, it means all the forces on your body are balanced.So the downward pull of gravity must be balanced exactly by another,upward-pushing force. As gravity tries to pull you down, the atoms inthe chair push back upward—you can't squash atoms that easily—andcounteract with what is essentially an electromagnetic force. Whensomeone stands on the floor and goes nowhere, the ground iseffectively pushing back up again and saying "I will not besquashed." We call the upward-pushing force from the ground thatbalances downward-pulling gravity the normal force.

Gravity in space

Photo: Astronauts train for space in a "vomit comet": It simulates weightlessness by making deep dives toward Earth.Photo courtesy of NASA on The Commons.

Scientists used to think Earth sat at the center of the Universe:theories of astronomy were geocentric, which meansEarth-centered. Until the 16th century, most people thought the Sunrotated around Earth, rather than (as we now know) the other wayaround. There was tremendous religious opposition to the idea thatEarth spun around the Sun, which is called the helicentric(Sun-centered theory). That idea was first put forward by the ancientGreek thinker Aristarchus (c.310–250 BCE), revived by Polishastronomer Nicolaus Copernicus (1473–1543), and championed byGalileo Galilei (1564–1642). When it comes to gravity, we nowaccept that Earth isn't at the center of things: it isn't specialand it's no different from anywhere else. That's one reason whyit makes no sense to talk about gravity (Earth's "special"gravitation) and gravitation (other kinds of gravitation, in othersituations or elsewhere). But you'll still see both of those twowords used widely. (Isaac Newton formulated a law of "gravitation,"as we'll discover in a moment.)

Photo: Galileo Galilei, an Italian astronomer, investigatedhow gravity accelerates things on Earth and championed theidea that Earth orbits the Sun.Picture from Carol M. Highsmith's America Project in the Carol M. Highsmith Archive,courtesy of US Library of Congress

Just like Earth, every planet (or moon) has a different amount ofgravity; bigger planets (or moons) have more gravity than smallerones. So our own Moon has gravity, but it's about one sixth as muchas Earth's (because the Moon has less mass and it's muchsmaller). That's why astronauts weigh one sixth as much on the Moonand why they can jump about four times higher in the air when they'rethere. Jupiter has gravity too and because it's bigger and moremassive than Earth, you'd weigh almost three times more there andstruggle to jump very far at all.

Gravity also explains why the universe looks and behaves the wayit does. If you've ever wondered why planets are nice round shapes(roughly spherical) and not square boxes, gravity is the answer. Whenthe planets were busily forming from fizzing atoms and swirlingatomic dust billions of years ago, gravity was the force that tuggedthem together. If lots of matter is pulled toward a central pointfrom many different directions, a sphere is what you end up with—justlike you end up with a snow ball if you pat snow together hardfrom all sides. Invisible "strings" of gravity also explain whythe planets dance around one another in the strange cosmic patternswe call orbits. Although we often think of orbits as circular,they're actually ellipses, which are stretched-out, oval relativesof circles (a circle is a special kind of ellipse).

How does gravity work?

Early scientific ideas about gravity were based on watching howthings naturally fell toward the ground. Aristotle, the ancient Greekphilosopher, who lived about 2350 years ago, famously believed thatheavier things fall faster than light ones, so if you drop a stoneand a feather at the same time, the stone wins the race and hits theground first.

Meanwhile, the whole question of how planets moved inspace was considered an entirely different matter. In Aristotle'smind, Sun, moon, planets, and stars all marched in circular orbitsround Earth. Astronomers such as Ptolemy (Claudius Ptolemaeus,100–170 CE) built on this model, but didn't really connect motionin space with what was happening back on Earth. Like Aristotle,Ptolemy was confident that the Sun and planets spun in circles roundEarth. Even though his ideas were wrong, his book of astronomy, TheAlmagest, was accepted as scientific truth for over 1400 years(until Nicolaus Copernicus came along) because no-one else had anybetter ideas. "Almagest" actually means "The Greatest":Ptolemy's really was the greatest scientific explanation of theworld people had at that time.

Artwork: Before people understood gravity, they had to devise ingenious explanations forwhy the planets moved. In this 14th-century illuminated manuscript, angels make the planets rotate bycranking giant handles! For more about the development of these ideas, seethe fascinating Wikipedia articleDynamics of the celestial spheres.Illustration attributed to the atelier of the Catalan Master of St Mark, Spain, 14th century,courtesy of the British Library and Wikimedia Commons.

Aristotle was correct in one sense (a stone beats a feather in arace to the ground), but we now know that everything falls at exactlythe same rate and the feather only loses because air resistance(drag) pushes up against it, slowing it down. The person who figuredthis out, toward the end of the 16th century, was Italian astronomerGalileo Galilei (1564–1642). According to some historians, Galileoexperimented with metal balls on a tilted ramp, quickly concludingthat the force of gravity accelerates every object—feathers justlike stones—at exactly the same rate, which we now call theacceleration due to gravity (or g). Other science historians claimGalileo figured out his ideas by dropping balls from the Leaning Towerof Pisa, while a third theory is that all these were "thoughtexperiments" that he carried out in his own mind. Either way, wenow know that all masses are accelerated in the same way; for mostpractical purposes, g is a constant value everywhere on Earth(although, as we saw up above, it does vary slightly due to altitude,latitude, and so on).

Photo: Isaac Newton—the man behind our modern understanding of gravity.Picture courtesy of US Library of Congress.

While Galileo was musing over gravity on Earth, other astronomershad been coming up with more detailed accounts of how planets movedin space. Nicolaus Copernicus (1473–1543) and Galileo advanced theidea that Earth moved around the Sun. The German Johannes Kepler(1571–1630) believed this too—and realized exactly how it mightwork. He took a treasure trove of very accurate and detailedobservations compiled by another astronomer, Tycho Brahe (1546–1601),and used it to figure out three deceptively simple mathematical lawsthat seemed to sum everything up. Kepler realized that the planetsmove in ellipses around the Sun, not circles as had long beensupposed. He found that they "sweep out equal areas in equaltimes," which essentially means they move faster when they'renearer the Sun and slower when they're further out—but in a verypredictable way. Finally, he showed how the size of a planet'sorbit was related to the time it took for the planet to make acomplete circuit around the Sun. It's pretty obvious that if a planethas a bigger orbit, it will take longer to go around it, but Keplerfigured out the precise relationship between the two things.(Specifically, he showed that the time a planet takes to orbit,squared, is proportional to its distance from the Sun, cubed.)

But the real stroke of genius in understanding gravity came fromEnglish scientist Isaac Newton (1642–1727). He realized that theforce of gravity that makes things fall to Earth is exactlythe same as the force of gravitation that keeps the planetsspinning around in space, which is why I prefer to use the same wordfor both phenomena. According to the popular myth, Newton figuredthis out when he saw an apple falling in his garden. Whether thatreally happened, no-one knows—but it was an incredibly impressiveinsight that changed science forever. Building on Kepler's work,Newton calculated that a falling apple experienced the same gravityas the Moon would experience being pulled toward Earth. That led himto his groundbreaking law of universal gravitation, published in1687.

Artwork: Three men who revolutionized astronomy: Copernicus (left), Galileo (right), and Kepler (far right) developed our modern view of the universe with the Sun at its center. Illustration by W. Marshall from a book cover c.1640, about a decadeafter Kepler's death. Artwork courtesy of US Library of Congress.

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Einstein meets gravity

The Universal Law of Gravitation was an astounding success, butthere was one strange thing it couldn't explain: a slight oddity inthe motion of the planet Mercury, known as its perihelion precession.When theories don't explain everything, we know they can't bequite right. And indeed, at the beginning of the 20th century, thebrilliant German-born physicist Albert Einstein (1879–1955) realizedthat the "classical", Newtonian picture of gravity wasn't quiteright either.

Photo: Albert Einstein's General Relativity is currentlyour most comprehensive theory of gravity. Photo courtesy ofUS Library of Congress.

Einstein had already revolutionized physics with a remarkable newtheory called relativity. Newton had shown that there was nothingspecial about Earth's gravity; it was the same as gravity anywhere.But it didn't really explain what caused gravity or how it passedfrom one side of the universe to the other through all the thingsthat sat in between. In September 1905, Einstein's original idea,known as the Special Theory of Relativity, set out a new way oflooking at three-dimensional space and time, blending them togetherto make four-dimensional space-time (or the space-time continuum).Einstein also had new ways of looking at light, which traveled at aconstant speed and set an effective speed limit for the universe.Putting these ideas together suddenly made the sober science ofphysics look like a painting by Salvador Dali. If you traveled close to the speed of light, someone watching yourprogress as you passed by would see you shrink and your time slow down!

All very strange, but where did gravity fit in? In 1915, Einsteinextended his ideas to make what he called a new, General Theory ofRelativity. One key aspect of this theory is called the principle ofequivalence, and it says that the force produced by gravity isexactly the same as the force produced by acceleration. In otherwords, gravity and acceleration are exactly the same thing. Thatmeans, for example, that if you wake up to find yourself inside aspace rocket and you feel a force very much like weight, you have noway of knowing whether it's caused by gravity (because the rocketis sitting on the launchpad on Earth) or acceleration (because it'shurtling through space at ever-increasing speed, producing a forceidentical to what you'd normally think of as gravity).

According to Einstein's new model of gravity, big masses (like planets) bend thevery fabric of space time, like a heavy ball sitting on a huge rubbermat. That makes other masses, moving nearby, curve in towardthem—giving a pulling force or attraction that looks identical tothe thing we've always called gravity. In other words, gravity isthe curvature of spacetime around mass (or energy, which is the sameas mass). This was a weird and revolutionary idea and few peopleunderstood it or believed it, to begin with, but it could explain theodd motion of mercury—and much more. In 1919, scientists observinga solar eclipse found the Sun bent light around it exactly asEinstein's theory predicted. (Extending this idea a little bit, wecan see that if light was bent in just the right way, it would giveoptical effects just like a lens. This concept, known as agravitational lens, was confirmed experimentally in 1979 and 1988.)

Artwork: According to Albert Einstein's General Theory of Relativity, gravityis the curvature of space time around mass or energy. Imagine if the Sun were a heavy metal ball sitting ona rubber mat. If you rolled a marble in a straight line nearby, it would curve inward because the mat is distortedslightly by the heavy ball. This is roughly how a big mass like the Sun curves space-time around it, pulling things like Earth toward it with the force we call gravity.

How does gravity travel?

Einstein's General Theory completely explains Newton'sUniversal Law of Gravitation, so you could say that it makes itunnecessary. However, Newton's law is much simpler and works inmost everyday situations, so it's still widely used in physics.(The same goes for Newton's "classical" laws of motion andEinstein's Special Theory of Relativity. Newton's laws areperfectly adequate for most everyday situations, so we still thoseall the time.) And just as Newton's law turned out not to be acomplete explanation of gravity, so there are things that Einstein'slaw doesn't explain. In other words, Einstein's theoryisn't complete either.

Photo: Detectors used to help find gravitational waves. Photo courtesy of NASA/JPL-Caltech.

The current way of explaining forces—how they act "invisibly"on things at a distance removed—is to think about particles beingexchanged, but this turns out not to be compatible with Einstein'stheories. Exactly how gravity works is still not understood. We don'tknow why it's so weak (compared to other forces) or how it"travels" from one side of the universe to the other. One idea isthat it involves the exchange of hypothetical particles known asgravitons, but no-one has ever seen one of those. Despite itsincompleteness, we know that much of Einstein's theory is correct,because the predictions it makes have been borne out by experimentalobservations, like those described up above. Another key predictionfrom Einstein's theory is that, when large masses are disturbed(for example, when stars explode or black holes swallow one another),they send out rippling waves (gravitational waves) through space-timeat the speed of light. That was fully confirmed in September 2015 byscientists at LIGO (Laser Interferometer Gravitational-waveObservatory in the United States—and the discovery earned threescientists the Nobel Prize in Physics in 2017. Einstein's GeneralTheory also tells us that the color of light can be shifted towardred by the pull of gravity—another prediction confirmed in reality.

From Aristotle to Einstein, we've made great progress, but whenwill we have a complete theory of gravity? Perhaps tomorrow, perhapsnever. Science is a never-ending quest to understand—and that'swhat makes it so fascinating. Long may its mysteries continue toinspire us!

Artwork: Gravity constantly throws up exciting new discoveries. How aboutthis "black hole tango," in which a supermassive black hole is forming through the merging of smallerblack holes drawn together by gravity. Artwork courtesy of NASA.

A brief history of gravity

• 350 BCE: Babylonians could predict the movements of planets and the comings and goings of eclipses.
• 340 BCE: Aristotle (384–322 BCE) publishes On the Heavens, which describes how the sun and planets move in circular orbits around a central Earth. Aristotle's idea of gravity on Earth is based on the idea that heavy things seem to fall faster.
• 300 BCE: Aristarchus (c.310–250 BCE) suggests things might work the other way. Perhaps the Earth really spins around the Sun? This is the first attempt at a heliocentric (Sun-centered) theory.
• 150 CE: Ptolemy (100–170 CE) publishes the Almagest, a detailed geocentric (Earth-centered) theory of the universe. It remains the most influential book of astronomy for the next 1400 years.
• 1543: Rejecting Ptolemy's ideas, Nicolaus Copernicus (1473–1543) decides to revive the heliocentric theory, in a major challenge to the church.
• 1589: Galileo Galilei (1564–1642) demolishes more of Aristotle's ideas. His new theory is that gravity makes objects fall at the same rate, however light or heavy they are. Like Copernicus, he supports a heliocentric model that brings him into conflict with the church.
• 1500s: Tycho Brahe (1546–1601) makes detailed observations of planetary movements and tries to unite Ptolemy's ideas with those of Copernicus. In his model, the Sun and moon spin around Earth (geocentric), but everything else spins around the Sun (heliocentric).
• 1609: Johannes Kepler (1571–1630) uses Brahe's observations to work out three mathematical laws describing how planets move in ellipses.
• 1687: Building on Kepler's work, Isaac Newton (1642–1727) sums up gravity in the universal law of gravitation.
• 1798: Henry Cavendish (1731–1810) comes up with an accurate figure for the gravitational constant, G, in an experiment to measure the weight (density) of the world.
• 1915: Albert Einstein (1879–1955) publishes his General Theory of Relativity, which extends his earlier Special Theory to include gravity.
• 1919: Frank Watson Dyson and Arthur Eddington confirm the General Theory by observing distorted light rays during a solar eclipse.
• 2015: Gravitational waves are detected directly for the first time.
• 2017: The discovery of gravitational waves earns a Nobel Prize in Physics for Rainer Weiss, Kip Thorne, and Barry Barish.

1. ↑In Geophysics: A Very Short Introduction, p.72, William Lowrie quotes a common estimate of 0.5 percent.

Find out more

On this website

• Bullets: An understanding of gravity is essential if you want to know how to fire a bullet accurately.
• Energy: Energy powers forces like gravity.
• Motion: How Isaac Newton revolutionized our understanding of force and motion.
• Science: A full list of our science articles.
• Science of sport: In many ways, sport is a battle against gravity.

Books

For younger readers

These are good for ages 9–12:

• A Crash Course in Forces and Motion by Emily Sohn. Capstone, 2019. A graphic/comic introduction to forces.
• Can you Feel the Force? by Richard Hammond. New York/London: Dorling Kindersley, 2007/2015. A simple, fun introduction to physics for ages 8–10.
• Fatal Forces by Nick Arnold. Scholastic, 2014. A more wordy introduction from the Horrible Science series. For ages 10–12, 128 pages.
• Gravity: Scientific Pathways by Chris Woodford. Rosen, 2013. My own little introduction to the history of gravity, from ancient science to Einstein and modern gravity. For ages 9–12, 64 pages.

For older readers

• Six Easy Pieces by Richard Feynman. Basic Books/Penguin, 2011. Chapter 5, The Theory of Gravitation, is a short overview that covers much the same ground as this article, but with a bit more detail about Kepler and planetary motion.
• Newtonian Mechanics by A.P. French. W. W. Norton, 1971. A classic introduction for undergraduates and bright high-school students.
• Physics: Algebra/Trig by Eugene Hecht. Thomson-Brooks/Cole, 2003.