Of the four fundamental forces of physics, gravity is the one you know in your bones. Gravity owns you. Try to cross a downhill street slick with ice and you slide helplessly; whatever is in control, it's not you. First you appreciate friction, then you understand the full omnipotence and omnipresence of gravity.
Gravity pulled the first matter in the earliest universe into the largest structures and inside them, spun up galaxies in which stars coalesced. Gravity regulates stars' lives and when some die, it compacts them into neutron stars or black holes. Gravity sets the orbits of planets around their stars. On Earth, it drags down mountains, moves glaciers, creates the tides, and drives all convecting systems, from the earth's fluid mantle to the weather to a pot of soup. Physicists understand gravity in great detail and with great accuracy, but they suspect they’re missing something—something big enough to change or even unify our most comprehensive theories of the universe.
Gravity was the first force to be studied quantitatively, says David Kaiser, historian of science at MIT, most notably by Isaac Newton, who was trying to understand motion. The force that drives a moving body equals the body's mass times its acceleration, Newton said: The larger it is and the faster it accelerates, the bigger the force. Newton put that equation together with the laws proposed by Johannes Kepler—in particular, the law that says the planets' distances from the sun are related to the time they take to orbit it. And then, astoundingly—"It's famously difficult to figure out what led Newton from A to B in his head," says Kaiser—Newton proposed a law that describes the force between his putative, famously falling apple and the Earth. He then extended that law to the force between any two celestial bodies. Newton called this universal gravitation, the revolutionary idea that, whether you’re an apple or a planet, whether you’re falling from a tree or orbiting in space, you obey the same rules.
Using Newton's equations, scientists could for the first time measure masses that are otherwise immeasurable. "What amazes me," says Gabriela Gonzalez, physicist at Louisiana State University, "is that we can weigh the sun that way." Universal gravitation allows us to “weigh” planets, binary stars, black holes, and even the invisible dark matter which floats otherwise undetectably through the universe and about whose nature, no one has a clue.
Newton's law had a flaw: It did not explain how one thing could act on another instantly, across any distance, with nothing in between. Nobody liked this “action at a distance,” including Newton. "It sounded occult," says Kaiser, "like alchemy."
Einstein didn't like it either, and found an alternative. In his general theory of relativity, he proposed that gravity is the result of the nature of space-time. Space-time can be thought of as a continuous three-dimensional fabric which a body warps according to its mass; the more massive the body, the deeper the warp. A smaller body is not attracted to a larger one; it's just rolling into the deeper valley.
But Einstein's theory of gravity contains a flaw, or maybe just a puzzle. Gravity doesn't fit in with the universe's other three fundamental forces: the electromagnetic, the weak, and the strong. The other three can all be described by quantum mechanics, which explains the three forces as fields created and carried by waves which are also particles. To date, gravitational waves remain undetected and gravitational particles called gravitons are probably undetectable. So at bottom this force that's so familiar, whose quantification you read every day on your bathroom scales, is—what?
This is where gravity becomes odd. If gravity is, as physicists say, mass telling space how to curve and space telling mass how to move, then space is inextricably related to mass. And mass, says Einstein's E = mc2, always implies energy. So space must have energy too. And it does: In quantum theory, even empty space—a vacuum—has energy. The amount of energy in the vacuum, say quantum theorists, is so enormous that space should be curved so tightly that the universe would fit into a proton.
You could be forgiven for thinking this last is the ravings of theoretical physicists. But vacuum energy also crops up in another problem. For the last 14 years, astronomers measuring the universe's expansion have found that the universe is not, as they'd expected, being slowed by the pull of its own gravity. Instead, the expansion is accelerating, speeding up; some push is countering gravity's pull. A physicist with a sense of poetry, Michael Turner at the University of Chicago, called the push "dark energy." And Turner and other physicists say that the simplest, most elegant explanation for dark energy is the energy in the vacuum.
Except the universe isn't curled up inside a proton: Simple and elegant or not, vacuum energy doesn't make the dark energy problem go away. "The mystery of dark energy," says Leonard Susskind, physicist at Stanford University, is that compared to the calculated amount of vacuum energy, "there's so little of it." Maybe the calculations are wrong. Maybe dark energy won't be understood until the excruciatingly complex supersymmetric and/or string theories get worked out. By this point, however, the reasoning is so mathematical—"Oh boy, is it mathematical!" says Susskind—that it's hard even for physicists to follow.
An ordinary human hardly knows what to make of it. First, you can believe that Newton and Einstein between them described gravity exquisitely. Second, theorists don't have the last word, and experimentalists are looking for gravitational waves. Gravitational waves are created when accelerating bodies distort space-time, says Gonzalez, "though the distortions are very, very small." But something like two neutron stars coalescing into a black hole should create intense waves that should be detectable.
The most sensitive experiment is the Laser Interferometer Gravitational-Wave Observatory, or LIGO, which can measure distortions smaller than 10-18 meters. LIGO hasn't yet found gravitational waves, but it's being upgraded to be able to detect waves over a larger volume of sky.
In fact, a number of different experiments, proposed and operating, stationed all over the world and out in space, are trying in differing ways to find gravitational waves of differing wavelengths coming from astronomical objects that range from binary white dwarf stars in our own galaxy, to the echo of the big bang itself. None of these experiments have found gravitational waves either. But if they do, the waves will carry new kinds of information from the hearts of the universe's most turbulent creatures.
So what's the matter with gravity? It may or may not be related to dark energy and it doesn't fit in with the other forces. If it's not a particle or a wave, then what else it might be is unclear. "And that's where it sits now," Turner says. "But wouldn't you rather have no answer than the wrong answer?"
Editor's picks for further reading
About.com: Introduction to Newton’s Law of Gravity
Astronomical Review: Why Gravity is So Weak
In this essay, Martin Rees compares the strength of gravity to the strength of the other fundamental physical forces.
FQXi: The Myth of Gravity
An article on a new model in which gravity is not a fundamental force.
NOVA: Relativity and the Cosmos
In this essay, Alan Lightman explores the history and meaning of general relativity.