Hello everybody. Imagine a box in which two spheres are separated by some distance. nothing is moving inside. Einsteins E=m*c^2 must be always valid. Since nothing moves the energy of box is E=m°*c^2 where m° is rest mass. Since the spheres exert gravitational force on each other they will be ruching towards each other after some time. just before the collison the energy of box is E=m*c^2 and it is higher than previous energy since the motion os spheres has increased their mass and made it m. Now you can see that law of conservation of energy is violated although no energy is given/take out of system. Thinking of this experiment I could not find the answer to the question, JUST WHERE DOES THE POTENTIAL ENERGY FIT IN THE E=MC^2 EQUATION ? OR DOES IT AT ALL ?