Hi.
What’s a Graviton? No one really knows.

To answer this question, i’m gonna have to talk a little about quantum field theory (ooooooohhhh) and a little about general relativity.

Vector  Bosons

wave particle duality

HI. So, in the quantum mechanics you might be kind of familiar with, you are aware that the concepts of “waves” and “particles” do a very good job of explaining the physical world. It seems that an object we might think is a particle (like a baseball, or an electron), will move around like a wave.

The same holds true… A wave, when you treat it quantum mechanically, can behave like a particle.

Field Theory

Remember how, in high school, there was something called a field? a gravitational field, for instance, is the “field” emitted by the earth which causes objects with mass to be attracted towards it. There is also an electromagnetic field. This shouldn’t be news to you, and it’s not like i’m asking you to do math.

In classical physics the field is the thing which mediates the force between two objects.

Quantum Field Theory

When You treat the field using quantum mechanics, suddenly all of the “waves” in the field turn into particles; and so instead of the classical field mediating the force between two object, we end up with little excited particles in the field mediating the interactions by going back and forth between the two objects.

This paints a vastly different picture than the one told by the fields. The story for why a proton and an electron are attracted to each other is different.

In quantum electrodynamics, for instance, the interaction between a proton (+ charge) and an electron (-charge) goes like this: the proton shoots out a “photon” which ends up hitting the electron. It’s like a little messenger, and it says “get closer to the proton”.  meanwhile the electron has shot out a pHoton of its own, which asks the pRoton to get closer. and so they get closer to each other. Then, the electron shoots more photons, and so does the pRoton, and the closer the two charged particles get, the more often pHotons messengers arrive; and so the proton, and the electron are pulled together with an effective 1/r^2 force law.

Imagine two ships shooting cannon balls at each other. no wait. that would be repulsive. imagine two ships shooting ICE CREAM balls at each other. the closer they get, the more ice cream hits, and the more they are drawn to each other.

Okay, so the rule in Quantum Field Theory is that any *force* law is, at the quantum level, just ships shooting Ice Cream balls (to attract) or Cannon balls (To Repel) each other. The Ammunition mediating the electromagnetic force is called the Photon (as we’ve said). The ammunition mediating the Weak force are the W and Z bosons; and the ammunition mediating the Strong force are gluons (pronounced “glue on”). We call this ammunition “Vector Bosons”.

Here’s the kicker. these particles are just quantum-excited waves in what we would (in classical physics) call the field.  So, Waves->Quantum Waves-> Cannon-balls mediating fundamental forces.

What about gravity?

Well, classically, gravity is a field. And so, we would expect that in quantum field theory, the gravitational force between two objects would be “mediated” by an ammunition called something a “graviton”. So, two objects with mass shoot balls of ice cream (called gravitons) at each other, causing the familiar gravitational attraction.

there’s 3 problems with this picture.

1) Gravity Can’t be Quantized. Well, when we try to use the tools of quantum mechanics to understand gravity, we end up with terrible contradictions. The crux of the matter is that everything in a gravitational field will end up with infinite masses. That’s no good. No one knows how to properly approach the problem of “quantizing gravity” and constructing a model for a graviton. This is the theory of “quantum gravity” that everyone’s waiting for.

2) The Theory of general relativity describes gravity in terms of curvature of space and time, rather than as a force between two objects. So, what would a graviton be? a gravitational wave emitted by one object and absorbed by another? Perhaps. But this walks a complicated line: general relativity is a non-linear theory of gravity. The effect of this, blatantly put, is that the “graviton” picture equates small (linear) perturbations of geometry, to the large scale structure of the geometry as a whole. Tricky business, and walking a tightrope. To further the issues, the fundamental assumptions of General Relativity and Quantum mechanics don’t really agree.

1) Gravity is too weak to measure and test in the particle physics regime. So we can’t do tests on how two atoms will interact gravitationally. So, the theory is working in the dark here.

So when you hear someone say “graviton” it could mean a whole host of things: Small gravitational waves; quantum particles mediating a gravitational force in a way intuitive to quantum mechanics (but based on a new quantum theory of gravity). Physicists are using the word as a placeholder in the book of physics for “where gravity finally gets quantized”.

good luck everyone!