What is gravity exactly?

The Road Runner cartoons take a playful approach to gravity. Wile E. Coyote, whenever he has the chance, chases after his speedy adversary off the edge of a cliff — but only falls when he realizes he’s running on air.

Of course, gravity doesn’t work like this in reality. But what’s happening when heavy objects like boulders or Acme anvils crash to the Earth? And what exactly is gravity? Let’s begin with what’s beneath your feet and work our way up — all the way to the vastness of the universe. Get comfy, because this is going to take a full page to break down.

Gravity on Earth as a Core Force

There are four key forces in the universe, which include electromagnetic force, strong and weak nuclear forces, and of course, gravity. Gravity is the most familiar to us as it keeps us grounded by pulling us toward the Earth’s center due to the planet's mass. In fact, gravity creates an attraction between any two masses in the universe: moons, dust particles, coyotes — you name it. Wherever there is matter, gravity is at work. You can't travel to a planet that’s entirely gravity-free, only one with more or less mass that affects gravity’s strength.

Exploring Gravitational Pull at Earth's Surface

Gravity is what keeps us tethered to the ground, but what exactly is gravitational pull? Simply put, when you step onto a scale, you’re measuring the force with which the Earth pulls on your body, which is your weight.

How Gravity Affects Time

Gravity can even influence the passage of time. A clock near a massive object will tick slower than one positioned further away. For example, if you place one clock on a mountain and another at sea level, the one at sea level will run slower. This happens because large objects with mass generate a strong gravitational field, causing space and time to curve. Essentially, the greater the gravity, the more space-time bends, which results in time moving more slowly.

Newton's Law of Gravity

On a grander scale, gravity organizes celestial bodies into orbits and even causes drifting space particles to slowly coalesce into larger clusters that ultimately form planets, stars, and galaxies. In the 1600s, Isaac Newton introduced gravity as a universal force acting upon all matter. According to Newton’s theory, the force of gravity is determined by the mass and distance between objects. The farther apart two particles are and the smaller their mass, the weaker the gravitational force between them.

Newton's law revolutionized our understanding of gravity, but we still don’t fully grasp why it exists. For three centuries, Newton's gravitational theory remained unchallenged. Then, in the 1900s, a brilliant physicist named Albert Einstein entered the scene and introduced his groundbreaking theory of relativity.

Einstein's Theory of Gravity

Einstein proposed that gravity is not merely a force; it’s a bend in the fabric of space and time itself. With enough mass, an object can even bend a straight path of light. This effect, called gravitational lensing, is a key tool for astronomers to detect unseen cosmic phenomena like black holes. Likewise, where gravity is weaker, time progresses more quickly, a concept known as gravitational time dilation. For example, a clock in orbit around Earth runs slightly faster than one on the planet’s surface.

Einstein's theory of general relativity introduced a revolutionary concept to physics: gravitational waves. These waves are ripples in the very fabric of space-time, generated when enormous celestial bodies like neutron stars and black holes orbit one another. It was Einstein’s groundbreaking equations that first predicted the existence of these waves. Gravitational waves were long considered purely theoretical until, in 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made history by detecting the faint signals of these waves. This discovery opened up a new avenue for exploring the universe and gaining insights into the dynamic interactions of celestial bodies.

Though Einstein's theory brought gravity into alignment with modern science, many mysteries about gravity remain unsolved. Some scientists speculate that gravity is caused by hypothetical particles called gravitons, which, in theory, are responsible for the attraction between objects.

Quantum Gravity

Lastly, we have the field of quantum gravity, where scientists are working to merge general relativity with quantum theory. Quantum theory seeks to explain how the universe functions at the smallest subatomic scales. This field has led to the development of the standard model of particle physics, which describes most of the universe’s inner workings — with one significant exception: gravity.

So, while quantum theory and general relativity together account for most of the observable universe, they sometimes clash, especially when studying black holes or the early universe. Not surprisingly, many scientists are still striving to develop a unified theory that reconciles both perspectives.

No matter what theories we ultimately embrace, it's impossible to overemphasize the significance of gravity. It's the force that binds the universe together, even though it continues to provoke deep, unresolved questions about the cosmos.