QFT's remarkable success lies in its ability to yield precise predictions through e.g. scattering amplitudes, capturing the essence of running couplings in a gauge-invariant manner. However, the introduction of gravity disrupts this straightforward notion of momentum dependence, posing a significant challenge to our theoretical framework.
Enter form factors—an ingenious concept that extends the idea of momentum dependence to the curved spacetime of quantum gravity. They form the central ingredient when discussing quantum gravity in a QFT language. Furthermore, within an effective action, form factors offer a unifying language, allowing us to compare and contrast different quantum gravity approaches apples-to-apples.
I will give an overview of the ideas underlying form factors, their application to scattering problems, and their potential to put different approaches on the same footing.