Gravity behaves how Sir Isaac Newton described it: You jump up and gravity brings you back down to the ground. You reach the brow of a hill and gravity accelerates you down the other side. Simply put, gravity is a force that affects and changes the motion of things.
Or at least that’s what it appeared to be like until that pesky Albert Einstein showed up.
Through his general theory of relativity, Einstein elaborated a mathematical formula for gravity, where this force is understood as the “unavoidable warping” of space-time. But how does said warping occur?
Well, whenever anything (be it you, me, and even light) tries to travel through the Universe in a straight line, said thing follows a trajectory that is curved by any form of mass and energy in its vicinity. Thus, what we think of as gravity is just the curvature of the universal fabric.
Pretty simple, huh? So it seems, but problems arise once we apply this theory to another one that is hugely important in the realm of physics: quantum mechanics.
But quantising gravity can’t be that hard. Can it? I mean, we’ve got all the other fundamental forces (electromagnetism, strong nuclear and weak nuclear) quantised like it’s nobody’s business. Therefore, gravity, the feeblest and wimpiest of these forces, should be a piece of cake. Right?
Terribly wrong, in fact.
For starters, consider the following example to comprehend why this is the case:
An electron meets a photon. What happens next? Specifically, what happens in a fully quantum description of the event? Do both particles just bounce off each other? Can photons even “bounce”? How are energy and/or momentum exchanged? Will these particles fall in love or hate each other at first glance?
You get the idea.
Now, there are a couple of things that make this scenario complicated. For instance, photons can be created or destroyed at will. Flip a switch and trillions of photons start streaming out of the light bulb. They briefly experience all the joys and freedoms that life can offer, only to be snuffed out of existence as soon as they hit a wall and get absorbed by its atoms. So, given…