Most people, scientifically oriented or otherwise, have at least a vague idea that some quantity or concept called "gravity" is what keeps objects, including themselves, tethered to Earth. They understand that this is a blessing in general, but less so in certain situations – say, when perched on a tree branch and a little unsure of how to get back to the ground unscathed, or when trying to set a new personal record in an event like the high jump or the pole vault.
It is perhaps difficult to appreciate the notion of gravity itself until seeing what happens when its influence is lessened or obliterated, such as when watching footage of astronauts on a space station orbiting the planet far from the Earth's surface. And in truth, physicists have little idea of what ultimately "causes" gravity, any more than they can tell any of us why the universe exists in the first place. Physicists have, however, produced equations that describe what gravity does exceptionally well, not just on Earth but throughout the cosmos.
A Brief History of Gravity
Over 2,000 years ago, the ancient Greek thinkers came up with a lot of ideas that have largely withstood the test of time and survived to modernity. They discerned that faraway objects such as planets and stars (the true distances from Earth of which, of course, the observers had no way of knowing) were, in effect, physically bound to one another despite presumably having nothing like cables or ropes connecting them together. Absent other theories, the Greeks proposed that the movements of the sun, the moon, the stars and the planets were dictated by the whims of gods. (In fact, all of the planets knows in those days were named after gods.) While this theory was neat and decisive, it was not testable, and was therefore no more than a stand-in for a more satisfying and scientifically rigorous explanation.
It was not until about 300 to 400 years ago that astronomers such as Tycho Brahe and Galileo Galilei recognized that, contrary to biblical teachings then close to 15 centuries old, the Earth and the planets revolved around the sun, rather than the Earth being at the center of the universe. This paved the way for explorations of gravity as it is currently understood.
Theories of Gravity
One way to think of the gravitational attraction between objects, expressed by the late theoretical physicist Jacob Bekenstein in an essay for CalTech, is as "long range forces that electrically neutral bodies exert on one another because of their matter content." That is, while objects may experience a force as a result of differences in electrostatic charge, gravity instead results in a force owing to sheer mass. Technically, you and the computer, phone or tablet you're reading this on exert gravitational forces on each other, but you and and your Internet-enabled device are so small that this force is virtually undetectable. Obviously, for objects on the scale of planets, stars, whole galaxies and even clusters of galaxies, it is a different story.
Isaac Newton (1642-1727), credited with being one of the most brilliant mathematical minds in history and one of the co-inventors of the field of calculus, proposed that the force of gravity between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. This takes the form of the equation:
where Fgrav is the gravitational force in newtons, m1 and m2 are the masses of the objects in kilograms, r is the distance separating the objects in meters and the value of the proportionality constant G is 6.67 × 10-11 (N ⋅ m2)/kg2.
While this equation works superbly for everyday purposes, its value is diminished when the objects in question are relativistic, that is, described by masses and speeds well outside of typical human experience. This is where Einstein's theory of gravity comes in.
Einstein's General Theory of Relativity
In 1905, Albert Einstein, whose name is perhaps the most recognizable in the history of science and the most synonymous with genius-level feats, published his special theory of relativity. Among other effects this had on the existing body of physics knowledge, it called into question the assumption built into Newton's concept of gravity, which is that gravity in effect operated instantaneously between objects regardless of the vastness of their separation. After Einstein's calculations established that the speed of light, 3 × 108 m/s or about 186,000 miles per second, placed an upper bound on how quickly anything could be propagated through space, Newton's ideas suddenly looked vulnerable, at least in certain instances. In other words, while Newtonian gravitational theory continued to perform admirably in almost all imaginable contexts, it was clearly not a universally true description of gravity.
Einstein spent the next 10 years formulating another theory, one that would reconcile Newton's basic gravitational framework with the upper bound the speed of light imposed, or appeared to impose, on all processes in the universe. The result, which Einstein introduced in 1915, was the general theory of relativity. The triumph of this theory, which forms the basis of all gravitational theories to the present day, is that it framed the concept of gravitation as a manifestation of the curvature of space-time, not as a force per se. This idea was not altogether new; the mathematician Georg Bernhard Riemann had produced related ideas in 1854. But Einstein had thus transformed gravitational theory from something rooted purely in physical forces into a more geometry-based theory: It proposed a de facto fourth dimension, time, to accompany the three spatial dimensions that were already familiar.
The Gravity of Earth and Beyond
One of the implications of Einstein's general theory of relativity is that gravity operated independently of the mass or the physical composition of objects. This means that, among other things, a cannonball and a marble dropped from the top of a skyscraper will fall toward the ground at the same speed, accelerated to precisely the same extent by the force of gravity despite one being far more massive than the other. (It is important to note for completeness' sake that this is technically true only in a vacuum, where air resistance is not an issue. A feather clearly falls more slowly than a shot put does, but in a vacuum, this would not be the case.) This aspect of Einstein's idea was testable enough. But what about relativistic situations?
In July 2018, an international team of astronomers concluded a study of a triple-star system 4,200 light-years from Earth. A light-year being the distance light travels in one year (about six trillion miles), this means that the astronomers here on Earth were observing light-revealing phenomena that actually occurred in about 2,200 B.C. This unusual system consists of two tiny, dense stars – one a "pulsar" spinning on its axis 366 times per second, and the other a white dwarf – orbiting each other with a remarkably short period of 1.6 days. This pair in turn orbits a more distant white dwarf star every 327 days. In short, the only description of gravity that could account for the mutual frenetic movements of the three stars in this highly unusual system was Einstein's general theory of relativity – and the equations, in fact, fit the situation perfectly.