Gravity is in the unique position of directly affecting everything. No other field (unless I'm mistaken) does this. Most people say there is a gravitational f

Gravity is in the unique position of directly affecting everything. No other field (unless I'm mistaken) does this. Most people say there is a gravitational force between two objects that have mass. Mass (rest-mass/invariant mass) is just another form of energy. Photons don't have rest mass, yet they are affected by gravity. Every type of thing we've ever observed is affected by, and contributes to the gravitational field, because everything we've ever observed involves energy. A gravitational field that has energy even interacts with itself. My personal opinion is that we'll eventually find that gravity is something completely different than the other three fundamental interactions, and can't be lumped in with an analogous theory. It has a unique relationship with spacetime.