content: Given E=mc^2, you can increase and decrease energy by doing the same to mass.  Or, to look at it another way, since c^2 is constant, if you were to take energy out of the system (whatever body has m), mass should decrease. But keep in mind that this equation is really only meant to enumerate how much energy is holding together a given piece of matter.  In other words, if the atom were broken apart, how much energy would be liberated in the process. So the question is, how do you take energy out of matter?  Can the gravitational vibrations of atoms cancel each other out?  While the energy/matter is still "there" the effect would be that no gravitation is experienced. @JodyOnTheBlock