For a test particle which is all by itself and is at rest, the total energy of the particle is the rest mass. But what if let say, this test particle is a proton. This proton is at rest and is not in the vicinity of any other particles. It is all by itself. Considering that proton has charge, from QED, we know that this proton will emit virtual photons with certain probability. Would this change the energy of the proton, and hence the rest mass of the proton? From gravitational force, and that the proton has a mass, the proton will exert a gravitational field that spreads to the whole space. If there is another massive particle nearby, then, there will be an attraction potential energy, and hence, it will change the energy of the proton. But what if there are no particles nearby? The proton still exerts a field. Will the proton's energy (rest mass) be changed due to its own field?