# How Does Gravity Work?

How Does Gravity Work?

We’ve all heard the schoolroom description of the discovery of gravity — that Sir Isaac Newton was hit on the head with an apple and instantly postulated the concept of gravity. If only this were true!

We know a few things about gravity — we know that it causes two objects to be drawn to one another, such as when you flip a coin and it hits the floor.

We also know that gravity had some hand in forming the planets, stars, and other bodies that make up the universe. We learn that gravity keeps the moon in orbit around the Earth. Wee see gravity being used in industrial applications as well — gravity powers some engines and other technical features. But what we don’t know about gravity is far more interesting (and expansive) than what we do know.

Science Behind Gravity

Sir Isaac Newton was the first to define gravity as a force that attracts all things to all other things. Sounds vague, doesn’t it?

We know on the other hand that Albert Einstein said gravity is “a result of the curvature of space-time” — whatever that means.

These two theories are considered the most correct — and they have a lot to do with one another. Without Isaac Newton and his apple we wouldn’t have Einstein and his theory of relativity.

The basic science behind gravity suggests that (if there are no other forces at work like wind or density) an object will always travel in the straightest line possible. That’s basic mathematics and physics.

By that same token, any two objects moving along a similar paths will always remain in that similar path, meaning they will never meet. Unfortunately for science, we don’t live in a world without external forces, and it is common for particles that start off on similar paths to end up on a collision course.

Isaac Newton’s theory says this happens because of gravity, which he described as a force that attracts objects to one another.

Albert Einstein also says this phenomenon is due to gravity — but in his theory, gravity is not a force but a curve in space-time.

Let’s take a closer look at both theories.

Two Theories of Gravity

Sir Isaac Newton’s law of universal gravitation spells out the theory that all particles in the universe posess some kind of gravitational force. According to Newton, gravitational force was everywhere, from that infamous apple falling from a tree to the moon holding its place in orbit though a kind of mutual attraction with the Earth.

Newton publicized this theory in the 17th century. His theory holds that gravity is a predictable force that acts on all matter in the whole universe. According to Newton, this force is a function of both mass and distance. The Newtonian theory of gravity says that every bit of matter attracts every other bit of matter with a force that is proportional to the product of their masses, also affected by the distance between them. You can do a basic Newtonian gravity test at home. Stretch a bedsheet and place two coins anywhere on the stretched surface. You’ll see “gravity” pulling the coins toward or away from one another based on their position and their weight.

Einstein begged to differe, holding that those two objects are still constantly travelling along the straightest possible line (as the laws of physics says they must), but because of a distortion in space and time, the straightest possible line changes into a spherical one. This means that two objects that were moving along a flat plane alongside one another are now moving along a spherical plane. This is what causes their collision.

We see gravity at work all the time. Drop a pen and it will hit the floor. The idea of gravity doesn’t need much reinforcement, but the idea of how it works is much more complex. Two of humanity’s greatest thinkers — Albert Einstein and Isaac Newton — struggled with the idea of gravity. It may be up to some future scientist to completely pin down this mysterious force.

This is part of a series of blog posts we’re publishing about How Stuff Works. The other posts in this series include: