Suppose you have two particles of mass 'm'. Their combined rest energy will be: E_rest=2mc^2 Its said that when they interact (gravitationaly), they're total energy will decrease due to the negative gravitational potential energy. The rest of the energy is stored in the field (they say). E_total=E_rest - |V| , where: V= - Gmm/r And (they say) we assign: E_total = m' c^2 And with this equation we argue that the new mass of the system is smaller than the sum of the constituents: m' < 2m. But i noticed this: If i place the two masses so close to each other that |V| becomes greater than E_rest, then E_total < 0 !! And if we assign again: E_total=m'c^2, then we get the beautiful result: m'<0.. What's that? Antigravity?