# What do gravitons do?

1. Nov 6, 2013

### copernicus1

If general relativity states that objects move along geodesics in spacetime, why is there a need for gravitons? What do they do? If they are passed back and forth between energetic objects to create the gravitational interaction, this seems to completely disregard GR, but if objects move along curved paths in spacetime as Einstein believed, I have a hard time understanding what need there is for gravitons.

2. Nov 6, 2013

### Simon Bridge

GR may be wrong ... or some of the assumptions may be wrong.
Gravitons form part of what a likely alternative to GR could look like.

We do know there are problems with GR in relation to quantum mechanics.

3. Nov 7, 2013

### haael

Think of it as a purely mathematical tool.

I'm speaking informally here:

Spacetime curvature may be described as a function. Say, a polynomial. Virtual gravitons are like elementary functions that can be composed into more sophisticated functions.

You can imagine virtual gravitons as sinusoids of different period, that you can add and get any space curvature you like.

Let me say it again, it is only an analogy. In reality, things are quite different.

However, you can see that you can describe particle motion using spacetime curvature or virtual gravitons and these descriptions will be equivalent.

Elementary gravitons are of interest of quantum mechanics, because they should be quantized. That means, non-commutative. When you multiply two functions describing spacetime curvature, the result will depend on the multiplication order. How exactly, this can be deduced from gravitons. In the case of elementary gravitons, the commutation relation should take particulary simple form.
Note however, that there is no known quantization of full GR today. Parts of it have been however quantized. There are also "quantizations" of GR that are not mathematically consistent.

It may even turn out that full GR can not be quantized at all and decomposition of spacetime curvature into gravitons is meaningless.

4. Nov 7, 2013

### dauto

GR also predicts the existence of gravitational waves. According to QM, all waves are quantized (yes, all). If both of those tenets are correct than there exist a quanta of gravitational waves. Let's call it a graviton.

5. Nov 7, 2013

### copernicus1

Thanks! Can you elaborate and/or provide references regarding the parts of GR that have been quantized?