We Have No Idea: A Guide to the Unknown Universe

by Jorge Cham

Quantum mechanics fails to describe gravity for two reasons. First, fitting gravity into the Standard Model requires a particle that transmits the force of gravity. Physicists have creatively called this hypothetical particle the “graviton.” If it exists, it would mean that, as you are sitting (or standing) there being pulled down by gravity, all the particles in your body are constantly throwing and receiving tiny little quantum balls with all the other particles of the Earth beneath you. And as the Earth goes around the Sun, there’s a constant stream of gravitons being exchanged between all the particles on the Earth and all the particles in the Sun. The problem is, nobody has ever seen a graviton, so this theory could be completely wrong. The other reason physicists have trouble incorporating gravity into quantum mechanics is that we already have a great theory of gravity, one that Einstein came up with in 1915. It’s called general relativity, and it works pretty well on its own. It looks at gravity in a totally different way: rather than thinking of gravity as a force between two objects, Einstein looked at gravity as a distortion of space itself. What does that mean? Einstein realized that gravity becomes simple if you stop thinking about space as an abstract concept, the invisible backdrop for all matter, and instead think of it like a dynamic fluid or flexible sheet. The presence of matter (or energy) bends space around it, changing the path of objects. In Einstein’...