String theory is based upon our understanding of matter and the other forces of nature, in terms of quantum mechanics, including Newton's law of gravity.

Sir Isaac Newton developed his theory of gravity in the late 1600s. This amazing theory involved bringing together an understanding of astronomy and the principles of motion (known as *mechanics* or *kinematics*) into one comprehensive framework that also required the invention of a new form of mathematics: calculus. In Newton’s gravitational theory, objects are drawn together by a physical force that spans vast distances of space.

The key is that gravity binds all objects together (much like the Force in *Star Wars*). The apple falling from a tree and the moon’s motion around Earth are two manifestations of the exact same fundamental force.

The relationship that Sir Isaac Newton discovered was a mathematical relationship (he did, after all, have to invent calculus to get it all to work out), just like relativity, quantum mechanics, and string theory.

In Newton’s gravitational theory, the force between two objects is based on the product of their masses, divided by the square of the distance between them. In other words, the heavier the two objects are, the more force there is between them, assuming the distance between them stays the same.

The fact that the force is divided by distance squared means that if the same two objects are closer to each other, the power of gravity increases. If the distance gets wider, the force drops. The inverse square relationship means that if the distance doubles, the force drops to one-fourth of its original intensity. If the distance is halved, the force increases by four times.

If the objects are very far away, the effect of gravity becomes very small. The reason gravity has any impact on the universe is because there’s *a lot *of it. Gravity itself is very weak, as forces go.

The opposite is true, as well, and if two objects get extremely close to each other — and I’m talking *extremely *close here — then gravity can become incredibly powerful, even among objects that don’t have much mass, like the fundamental particles of physics.

This isn’t the only reason gravity is observed so much. Gravity’s strength in the universe also comes from the fact that it’s always attracting objects together. The electromagnetic force sometimes attracts objects and sometimes repulses them, so on the scale of the universe at large, it tends to counteract itself.

Finally, gravity interacts at very large distances, as opposed to some other forces (the nuclear forces) that only work at distances smaller than an atom.

Despite the success of Newton’s theory, he had a few nagging problems in the back of his mind. First and foremost among those was the fact that though he had a model for gravity, he didn’t know *why* gravity worked.

The gravity that he described was an almost mystical force (like the Force!), acting across great distances with no real physical connection required. It would take two centuries and Albert Einstein to resolve this problem.