What is Gravity?

Gravity holds the Earth together and drives the nuclear fusion that makes the stars shine. It keeps you from flying off into space as you spin around Earth's axis at up to 1,000 miles per hour and whirl around the Sun at 66,000 miles per hour. It tethers you to the core of the Milky Way galaxy and even to the center of the Virgo supercluster of galaxies, 60 million light years away.

But what is gravity? For a truly fundamental answer to that question, we'll have to get back to you. Scientists have been working on it at least since the days of Galileo, and they're not quite there yet.

Aristotle was the first on record to weigh in. His explanation for why things fall was simply that it is the nature of Earthly things to seek the Earth, just as it is the nature of celestial things to circle the Earth.

A couple of millennia later, Isaac Newton had the profound insight that what causes an apple to fall is exactly the same thing that causes the Moon to orbit Earth instead of fly away from it. Gravity, he said, is an attractive force which all material things exert. The laws he formulated to describe it have worked well enough to enable us, three centuries later, to send explorers to other worlds.

But Albert Einstein pushed gravity into another revolution. He fused space and time into spacetime and said that everything in the universe causes the spacetime around it to curve, a phenomenon we perceive as gravity. Apples and the Moon merely move along the contours of a giant 4-dimensional dimple in spacetime induced by Earth's presence. Many experiments have searched for flaws in this General Theory of Relativity, and it has survived them all with flying colors.

However, General Relativity's equations break down at the scales required to describe the center of a black hole or the beginning of the Big Bang. And it is incompatible with an equally spectacularly successful theory in the realm of quantum physics, called the Standard Model, which covers subatomic particles and all known forces except gravity. So physicists are working feverishly to develop a new theory that can encompass them both: a quantum theory of gravity. String theory and Loop Quantum Gravity are two current contenders, but neither has produced any hard evidence as yet.

While you're waiting for the ultimate answer about the fundamental nature of gravity, we invite you to consider some of the mind-boggling things we know about it so far.

10 Shocking Things You Didn't Know about Gravity

1. Gravity slows time.

The stronger the gravitational field, the more slowly time progresses. The seconds tick by more sluggishly in your car than at the distance of a satellite — not by much, but enough to require your GPS device to take the effect into account when calculating where on Earth you are.

2. Gravity is weak.

Strange to think of the force that shapes the cosmos as weak. But you can easily win a tug of war over a paperclip against the entire Earth, when you don't stand a chance against a relatively modest magnet. Why is gravity so much weaker than magnetism and the other forces? We don't know, but one theory says that gravity might be concentrated in an unseen fourth spatial dimension, and what we experience is merely what spills over into our familiar three dimensions. Another theory says gravity may be leaking from our three into many other dimensions we can't yet detect.

3. Gravity is twisted.

According to General Relativity, a rotating object not only bends spacetime, it twists spacetime around with it like a spinning bowling ball in a vat of molasses. NASA's Gravity Probe B demonstrated this phenomenon as it orbited Earth. What's more, really large objects like stars and galaxies warp spacetime so much, it can act as a lens which magnifies and distorts the images of more distant galaxies.

4. Astronauts floating in space are not in zero gravity.

Zero gravity is a misnomer. So is microgravity. There's plenty of gravity in outer space. Otherwise, the Moon wouldn't orbit the Earth and Earth wouldn't circle the Sun. Free-floating astronauts are actually in freefall, just like skydivers before their parachutes open (but without the wind resistance of course). In fact, gravity is the only force acting on them. Yes, it's true that even NASA uses "microgravity" as a shorthand way of saying that a spacecraft is in freefall, but now you know what's really going on.

5. If the Sun were magically to disappear, we would continue orbiting where it had been for about 8 minutes.

Gravity travels at the speed of light. If the Sun were magically to cease to exist, it would be about 8 minutes before Earth would stop being bathed in sunlight, and the same amount of time before the planet would stop feeling the gravitational attraction of the Sun and would fly off in a straight line tangential to its orbit.

6. Gravity can make virtual things real.

Quantum mechanics tells us — and experiments confirm — that space is seething with virtual particle-antiparticle pairs that pop into existence for the briefest of moments before they are absorbed back into spacetime. When this happens near a black hole, one particle can pop into existence inside the event horizon (the point of no return) while its mate pops into existence outside. In that case, one virtual particle falls into the black hole while the other flies off into space as a permanent real particle. Even stranger, the infalling particle adds negative energy to the black hole, making it slightly less massive by exactly the same amount of mass as the escaping particle carries away. Eventually, the black hole can evaporate. This realization is what made Stephen Hawking famous among astrophysicists.

7. There is too much gravity.

Something was wrong with galaxies. The stars in the outer parts were orbiting the galactic centers just as quickly as the stars further in, in flagrant violation of Newton's well-established laws of gravitation. It meant that either our understanding of gravity was wrong or something other than the stars, gas and dust known to comprise galaxies was providing extra gravity — lots of it. For a number of reasons, most astronomers think the second explanation is more likely. In fact, scientists have calculated that, to account for the gravitational effects observed in galaxies and galactic clusters, there must be five times as much of this mysterious "dark matter" — detectable only by its gravitational pull — as there is of the ordinary matter which was the only matter we were aware of before this discovery.

What's more, cosmologists have calculated that ordinary and dark matter combined provide only about 30 percent of the gravity needed to explain the observed "flatness" of the universe. Where does the remaining 70 percent come from? The even more mysterious "dark energy" which drives the accelerating expansion of the universe. And that leads us to...

8. Gravity can push things apart.

That's what's happening to the universe as a whole. Dark Energy, which pervades all of space, induces gravity, as does everything else in the universe including gravity itself. But because Dark Energy has negative pressure — analogous to a stretched rubber band which pulls inward instead of pushing outward — the gravity it generates repels instead of attracts. (It's not intuitive, but the math works.) And because there's so much of it compared to the more-familiar attractive gravity provided by the rest of the contents of the universe, it is pushing the universe to expand faster and faster. As Caltech cosmologist Sean Carroll put it, "Some people like to refer to this as antigravity, but that's misleading. It's really just good old ordinary gravity with a funny kind of source."

Eventually, many billions of years from now, it all may end in a "Big Rip," an end-of-times counterpart to the Big Bang, in which the expansion of space tears everything apart, even atoms. Alternatively, it's possible that gravity behaves differently over very large distances or in different parts of the universe, and that's why it appears that expansion is accelerating. Experiments are underway to find out.

9. Gravity can make icy moons habitable.

The moons of Jupiter, Saturn, Uranus and Neptune are much too distant for the Sun's warmth to keep water liquefied, and liquid water is considered necessary for life as we know it. Nevertheless, Jupiter's moon Europa is one of the most interesting places in the Solar System to look for life, thanks to gravity.

The gravity of Europa's neighboring moons have pulled it into an elliptical orbit around Jupiter, and the giant planet's gravity is stronger at the closest part of Europa's orbit than at the farthest part. When Europa is closest, Jupiter's gravity stretches the moon by pulling harder on the side facing the planet than on the opposite side — the same phenomenon that causes ocean tides on Earth. At farther distances, the moon gets to relax back into its spherical shape. The alternating stretching and relaxing produces internal friction, which heats the ice beneath the frozen crust enough to make it liquid or at least slushy. That's probably good enough to support some of the microbes that make their home on Earth, though we don't yet know whether there are Europan counterparts swimming around.

A similar effect has been detected within Saturn's moon, Titan, making it another likely candidate for a subsurface ocean. And gravity's manhandling of Io, another of Jupiter's moons, produces so much heat that it has become the most volcanically active world in the Solar System, constantly recoating its surface in lava.

10. In quantum gravity, the entire universe can have materialized out of nothing.

We saved the most mind-bending item for last. We don't yet have a working theory of quantum gravity, but scientists are able to determine what some of the implications would be. One is that the universe could have burst into existence from nothing at all. Astrophysicist Lawrence Krauss wrote, "In quantum gravity, universes can, and indeed always will, spontaneously appear from nothing. Such universes need not be empty, but can have matter and radiation in them, as long as the total energy, including the negative energy associated with gravity, is zero." And that's what the energy of our universe adds up to.

If you feel the room spinning at this point, we suggest that you sit down, lower your head to your knees, and let gravity return the blood to your brain.