What is Gravity?

Of the universe’s fundamental forces, only one dominates every moment of our conscious experience: gravity.
It keeps us close to the ground, drags balls 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.

Gravity as an artist sees it

Gravity as a universal force

Today, scientists know of four forces — things that attract (or repel) one object to (or from) another:

  1. the strong,
  2. the weak,
  3. the electromagnetic and
  4. the gravitational force.

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. 

Isaac Newton (1643-1727)

Newton’s leap, which became public in his 1687 treatise Philosophiæ 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 totally unrelated, from apples falling to Earth (although it probably didn’t inspire his breakthrough, Newton did work near an apple tree) 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. 

Gravity as the geometry of space

Newton’s description of gravity was accurate enough to detect the existence of Neptune1 in the mid-1800s, before anyone could see it, but Newton’s law isn’t perfect.
In the 1800’s, 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. According to NewtonMercury’s wobble was caused by the gravitational pull of some other planet. Enter Vulcan – the so-called “other” planet – first observed in 1859; confirmed by the greatest astronomer of the day, Urbain Le Verrier; and hailed by The New York Times as one of the great discoveries of the century. Trouble was, it didn’t exist2.

Urbain Le Verrier (1811-1877)

The puzzle was eventually resolved by Albert Einstein’s theory of general relativity, published in 1915. The story goes as follows: Einstein is preparing the third of four papers he delivers to the Prussian academy, and he does a calculation to see what his new theory predicts for the orbit of Mercury. His new theory correctly provides what astronomers call the table for Mercury, accurately describing how it moves around the Sun. Einstein tells friends about this in the most extraordinary language. He said he felt actual palpitations of his heart and that he was so excited he couldn’t work for three days, he was so overcome with joy. So, this grand theory that would explain the universe in a new way rested on this almost forgotten issue of what causes Mercury to wobble.

Albert Einstein (1879-1955)

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. 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.

Gravity as 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.

Vera Rubin (1928-2016)

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.

1 September 23, 1846. The planet Neptune – now considered by most astronomers to be the outermost major planet in our solar system – was discovered on this date, using mathematics. Johann Gottfried GalleUrbain Jean Joseph Le Verrier, and John Couch Adams all worked independently to help discover this world in 1846. Their separate work to find Neptune led to an international dispute as to whom to attribute the discovery of the farthest planet of our solar system.
Neptune cannot be seen without a telescope. Its discovery didn’t come solely through the use of a telescope, though. It came from astronomers’ analysis of data related to Uranus’ orbit. Astronomer noticed discrepancies in Uranus’ observed position in contrast to its predicted position; the planet was not quite where it was mathematically predicted to be.

2 The first sighting of Vulcan was by amateur French astronomer Edmond Modeste Lescarbault and put us inside his stone barn in the village of Orgeres-En-Beauce on March 26, 1859.
Edmond Lescarbault, a young doctor in love with astronomy, had built an observatory in a stone barn with a little dome on top of it. On that day in 1859, he sees a patient, then goes across the backyard to his stone barn, climbs up and looks through his telescope. As he trains his telescope on the sun, he sees a round object on the face of the Sun. He times it as it moves steadily across the sun, records the data, then another patient arrives, so he checks out that patient, then comes back to the barn. This round dot is still crossing the Sun. He tracks it continuously, taking notes on its path until it finally goes over the other edge of the Sun. He sits on that result and says nothing.
At this point he doesn’t know anything about Le Verrier’s work on Mercury. Finally, he learns about this in a popular account Le Verrier had published and sends a letter to Le Verrier.
Le Verrier is at a New Year’s Eve party when he gets the letter and treks out to Lescarbault’s house, which involves a train ride and then a 12-mile walk, to interrogate him. Le Verrier becomes convinced that Lescarbault really did see what he claims to have seen and that the proper interpretation is that this is a transit of a planet.
It’s not clear who first named it, but it quickly became known as Vulcan.

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