Gravitons: Explained for New Learners

[LeeJeffries]

I am new to all this "stuff", so please forgive me

If GR describes gravity as a straightline movement through a curved space-time, why does there need to be a "carrier" at the quantum level? What would it be carried between?

When people say at the quantum level GR stops working, is there a simple way of explaining why that is? I know its something in the maths compared with what is observed, but is there any way for me to understand what exactly "isn't working"?

Thanks
Lee

 

[Mentz114]

GR is not a quantum theory, as you'll be aware, so it has no need of gravitons as it stands.

The quantum field theories we know are based on a Lagrangian that has terms for (say) 2 particles and the interaction between them. If the particles exchange energy and momentum, it is via the interaction term. In a quantum theory the energy exchange is quantized and those interaction quanta are the virtual particles, or carriers.

The problem with quantizing Newtonian gravity is that it is unrenormalizable because of the gravitational self energy.

The nearest thing to a Lagrangian in GR is the Einstein-Hilbert action, which seems to have no interaction term and describes a kinematical ( as opposed to dynamic) system. So the recipe for making a quantum theory doesn't work in the first place.

When people talk about gravitons they are assuming that a 'standard' quantum field theory for gravity will be found where the interaction term in the Lagrangian will be the product of two rank-2 tensors, something like A^mnT_mn where A is a tensor potential and T is the energy momentum tensor of the source. This will give a carrier particle with spin-2 for attraction and one with spin-0 for repulsion.

[caution - I'm not an expert so don't take this as a complete or 100% correct answer]

 

[bcrowell]

We have three forces in nature: strong, electroweak, and gravitational. The first two are quantized. If you try to couple a quantized field to a non-quantized one, you get a theory that is not self-consistent. E.g., this is what Bohr and his followers wanted to do in the early 20th century: quantize the atom while keeping the electromagnetic field classical. It doesn't work. For example, if a classical EM wave impinges on some quantized atoms, ionizing some of them via the photoelectric effect, there is no way to to maintain conservation of energy.