Products You May Like
Legend has it that Isaac Newton had the moment of inspiration that would lead to his theory of gravity when, on a warm afternoon, he saw an apple fall from a tree and wondered why it should fall down instead of up. (Some versions of the story have the apple falling and hitting poor Newton on the head.) He named his new hypothesis after the Latin gravitas, for “weight.” The tale of Newton’s bruised noggin may be apocryphal, but his interest in what makes things move—and especially what makes them fall—was very real. Newton had come home to stay when his university closed down due to a bout of bubonic plague, and he was looking for something to occupy his mind. In 1665, he found it, apple or no apple.
It took Newton, one of the world’s greatest mathematical minds and, by all accounts, an irascible jerk, 20 years to articulate his thoughts on gravity to his satisfaction. His biggest problem? The question of whether the Earth’s gravitational influence could extend all the way to the Moon.
Two hundred years later, the calculations that took humanity to the Moon were based on Newton’s mechanics. But how does gravity work in the first place? What did Newton understand that was such a revolution?
Gravity: The Basics
Can someone just… explain gravity to me?
Let’s start with a definition. Gravity, or gravitational attraction, is the tendency of mass to gather toward itself, drifting together even across great distances due to curvature in spacetime. This tendency allows the formation of stars, planets, galaxies, and black holes.
Standing on Earth’s surface, the planet’s mass creates a gravitational force sufficient to accelerate any object downward (toward the core of the planet, or perpendicular to the planet’s surface) at 9.8 m/s²—that is, an additional 9.8 meters per second, each second. Everything experiences the same amount of gravity, but if you take a hammer in one hand and a feather in the other and drop them at the same time, the hammer will hit the ground first. Why? Drag (air resistance) opposes acceleration due to gravity. In a vacuum, such as on the Moon, both objects will hit the ground at the same time.
How does gravity work?
Gravity applies an effective force of mutual attraction to things with inertial mass, including physical matter and photons. The force of gravity is transmitted through spacetime at the speed of light, which creates wavefronts we can detect with special equipment like the LIGO gravitational wave detectors.
In classical (or “Newtonian”) mechanics, which describes the motion of macroscopic objects (i.e., things larger than an atom, such as planets), gravity is sometimes called a central force. A central force is directed towards or away from a point called the center of force. Gravity, electrical charge, and magnetism are three examples of central forces. Centers of electromagnetic energy radiating inward or outward are known as poles.
Newton used a mathematical approach to gravity not unlike Coulomb had done with electrostatics, with a field that falls off as the inverse square of the distance between two objects. Looking at it this way, the gravitational force at a point can be expressed as a vector, with magnitude and direction.
How is gravity transmitted?
Christian Huygens, a contemporary of Isaac Newton, discovered that light carries energy. This suggests a force-carrying graviton as an obvious theoretical parallel to a photon. But where photons are the force carrier of the electromagnetic field, relativity frames gravity as an emergent consequence of the way inertial mass warps spacetime. Instead of requiring a force carrier, according to general relativity, what we think of as gravity is more like the idea of “downhill.”
Massive or high-energy objects warp the mesh of spacetime, dragging it in toward themselves and creating a gravitational field of influence or “gravity well” from which it can be difficult to escape.
Central Forces
When matter is collected in one place, it forms a center of mass from which its inward-pointing field of gravitational influence extends. The force of attraction between two objects falls off as distance increases from the center of mass.
Objects under the influence of a gravitational field will move toward the field’s center of gravity. Sometimes, as with the Sun and Jupiter, their mutual center of gravity or barycenter lies slightly outside one of the bodies; Jupiter is large enough that it drags the Sun in a little circle, centered slightly outside of the Sun’s radius, as Jupiter makes each orbit.
On Earth, we have to contend with our own gravity well when we launch rockets and spacecraft; if a rocket isn’t powerful enough to escape its gravity well, it will fall back to Earth. As matter becomes more and more dense, that effect becomes more pronounced. Black holes create a gravity well so deep that there’s a threshold around black holes called an event horizon, a boundary in space marking the point of no return. Nothing inside the event horizon can escape from a black hole. Indeed, it’s thought that the only thing that can ever escape a black hole’s gravity is a frisson of virtual particles called Hawking radiation, thrown off every so often when subatomic symmetries align.
Sacred Geometry
Astronomers in ancient Greece noticed that the planets sometimes seem to move in retrograde across the sky, backward with respect to their normal orbits. This offended some astronomers’ sense of cosmos, the orderliness of the universe. In a universe perfectly ordered by the hands of their gods, there was little room for irrational numbers or eccentric orbits.
In their attempt to reconcile their geocentric models with their empirical observations, they proposed the idea of epicycles: complex orbits that were neither circular nor elliptical, with planets dancing around the Earth in paths that look like geometric lace.
Geocentric models resorted to convoluted orbits to explain the apparent motion of planets through the sky.
Credit: Public domain
Geocentrism reigned unchallenged for more than two thousand years. Despite the repeated proposal of a heliocentric solar system over the millennia by scholars as respected as Leonardo da Vinci, heliocentrism wasn’t taken seriously until the medieval era. However, the scientific consensus began to change in the 1500s. Nicolaus Copernicus developed a heliocentric model, backing up his argument with astronomical observations—and predictions that would confirm his model as correct or invalidate it. Galileo Galilei, using the newly invented refracting telescope, made and published observations showing that the planet Venus went through phases just like the Moon, and that Jupiter was orbited by its own moons.
Music of the Spheres
Then, Johannes Kepler put forth a solution to the problem of retrograde planets that would have satisfied even the strictest Pythagorean. Even with their orbits taking the “imperfect” shape of an ellipse, Kepler showed, a planet swept out an equal geometric area of its orbit over the same length of time, no matter where in its elliptical orbit it might be, nor how eccentric that ellipse. Kepler was a big believer in musica universalis, the music of the spheres; the idea that an inaudible mathematical harmony existed between the orbits of the planets was central to his Mysterium Cosmographicum.
Johannes Kepler’s nesting Platonic solids, as depicted
Credit: Johannes Kepler. From Kepler’s “Mysterium Cosmographicum”, Tübingen 1596, Tabula III: Orbium planetarum dimensiones, et distantias per quinque regularia corpora geometrica exhibens.
In 1687, Isaac Newton published his opus, Philosophiæ Naturalis Principia Mathematica (Latin for Mathematical Principles of Natural Philosophy, but many affectionately call it just Principia for short), which combined his laws of motion with a new mathematical analysis—calculus!—that could replicate Kepler’s empirical observations of the planets and their moons. In Principia, Newton proposed a law of universal gravitation that now bears his name. Newton’s law of universal gravitation holds that any two bodies, no matter how far they may be separated in space, are attracted by a force proportional to their mass and inversely proportional to the square of the distance between them.
‘Spooky Action at a Distance’
Yet the question remained: How could one planet affect another at such a great distance? Newton considered action at a distance to be, in his own words, “so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it.” Familiar as he was with electrostatics, Newton’s theory of gravitation didn’t require what he viewed as an exotic, unnecessary transmission mechanism when the inverse-square law modeled gravitational attraction entirely well enough.
Newton was by no means in the scientific minority on the topic of action at a distance. Albert Einstein operated under some assumptions of aether theory when developing his theory of relativity. Einstein would eventually dismiss the notion of quantum entanglement between two particles as spukhafte Fernwirkungen (translated as “spooky action at a distance”). Likewise, Newton and others of his day believed there must be a transmission medium, such as the luminiferous aether, through which electromagnetic or gravitational forces could exert a force on bodies separated in space.
Quantum Gravity
What Einstein knew that Newton didn’t is that the universe is permeated not by an aetheric substance made of molecules of some kind but by an invisible warp and weft of field lines along which forces such as gravity are transmitted. No aether is required to produce the effects described in Maxwell’s laws of electromagnetism or to produce gravity as we understand it. Today, it looks like the graviton will go the way of the aether.
On the deepest level, our cosmos is governed by four fundamental forces or fundamental interactions: electromagnetism and gravity, whose reach appears unlimited, and the weak and strong nuclear forces, which constrain themselves to the smallest scale, the inner workings of the atom. We call them fundamental forces because when we try to answer how spacetime works under the hood, these four forces appear not to be reducible to simpler interactions.
The human understanding of gravity still has some problems. Chief among them is the difficulty of applying current models to the subatomic scale or to extremely high-energy environments such as black holes or the very early universe.
As technology advances, scientists’ understanding of what the laws of physics will permit stays in a state of flux. Once fusion, which must surmount the Coulomb barrier, was an exotic fiction; now, it’s an engineering problem. It may be the same with gravity and other phenomena on the quantum scale. Until then, we can all enjoy the view from where we stand—on the shoulders of giants.