gravity is a form of energy. But now we notice a curious fact. Imagine trying to pluck the Earth out of its orbit around the sun. You would have to do work—that is, expend energy—to draw it away against the sun’s gravitational pull. So the gravitational energy binding the Earth to the sun is negative (it requires work to sever the bond). If the gravitational field has negative energy, it must also have negative mass and must be subtracted from the positive mass-energy of the sun and planets.
