What is gravity?

There are four fundamental forces in the universe, including electromagnetic force, strong and weak nuclear forces, and of course, gravity. The most familiar is gravity and it holds your feet down to the ground because the mass of the planet exerts a fundamental force on the mass of your body. In fact, gravity stirs up an attraction between any two objects in the universe: moons, dust motes, coyotes -- you name it. Wherever you find matter, you'll find gravity. You could never travel to a gravity-free planet, only one with greater or lesser mass resulting in greater or lesser gravity.

Gravity is what holds our feet to the ground but what is gravitational pull? In short, when you stand on a scale, you are measuring the gravitational pull of the earth on you, known as your weight.

Gravity can also affect time, in that a clock will tick more slowly near a massive object than farther away. For example, if you place one clock on a mountain and one at sea level, you will see that the one at sea level runs slower than the one on the mountain. This occurs because objects with a lot of mass create a strong gravitational field, which is a curving of space and time. Essentially, the stronger the gravity, the more spacetime curves, and in turn, the slower time moves.

On a larger scale, gravity arranges cosmic bodies into orbits and even causes drifting space particles to pull together slowly into larger and larger clumps that eventually become planets, stars and galaxies. Back in the 1600s, Isaac Newton defined gravity as a universal force acting on all matter. According to Newton's theory, the exact expression of gravity came down to mass and distance. The farther apart two particles are and the less massive they are, the less the gravitational force.

Newton's law provided a great understanding of how gravity works but we still don't know why it works. Newton's gravitational law stood unchallenged for three centuries. Then, in the 1900s, a wild-haired physicist by the name of Albert Einstein stepped in the ring and let fly with his general theory of relativity.

Einstein argued that gravity was far more than just a force; it was a curve in the fourth dimension of space and time. Given sufficient mass, an object can cause an otherwise straight beam of light to curve. Astronomers call this effect gravitational lensing, and it's one of the primary methods of detecting unobservable cosmic phenomena such as black holes. Similarly, the less gravity there is, the faster time passes, a phenomenon known as gravitational time dilation. For instance, a clock aboard an orbiting satellite advances slightly faster than a counterpart on Earth's surface.

Einstein's theory of general relativity also introduced a profound concept in the world of physics: gravitational waves. These waves are ripples in the fabric of space-time itself, and they are generated when massive celestial objects, such as neutron stars and black holes, orbit one another. It was Einstein's groundbreaking equations that first theorized the existence of these waves. Notably, gravitational waves were initially considered a theoretical concept until, in 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) achieved a groundbreaking feat by successfully detecting the exceedingly faint signals of these waves. This discovery opened up a new and exciting window to explore the universe, offering valuable insights into the dynamic interactions of celestial bodies.

While Einstein's theory brought gravity up to speed with modern science, we still don't know everything about gravity. Some scientists attribute gravity to hypothetical particles called gravitons, which -- in theory -- cause objects to be attracted to one another.

Finally, there's the field of quantum gravity, in which scientists attempt to reconcile general relativity with quantum theory. Quantum theory addresses how the universe works at the smallest subatomic levels. The field has helped scientists develop the standard model of particle physics, which details most of the inner workings of the universe -- with one notable exception. The standard model doesn't explain gravity.

So while quantum theory and relativity together explain most of the observable universe, they also contradict each other at times, such as in the study of black holes or the early universe. Not surprisingly, numerous scientists continue to work toward a unified theory.

Whatever theories we ultimately adopt, it's difficult to overstate the importance of gravity. It's the glue that holds the cosmos together, even if it still stirs up unanswered questions about the universe.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.