String theory isn't the only thing that has gravitons, so does Quantum Theory. A graviton is a particle that carries gravitational force. Once again, that doesn't tell us what a graviton is, or what gravitational force is. We are still defining everything in terms of what it does. That's science. All it can tell us is how things interact.

Bill Gill

C is not the speed of light in a vacuum.
C is the universal speed limit.