Presume there are two massive bodies in open space, having zero relative velocity, and having the same space ambient temperature. That is, the incoming and out-flowing black body radiation is balanced and the temperature is unchanging. Presume these bodies become gravitationally attracted, and fall together. On impact the temperature will rise, and then the excess thermal energy will be radiated away, returning to the ambient state. By both Newtonian and Relativistic mechanics, the total rest mass is equal to the sum of the rest mass of the two combined bodies. (Black Hole theory depends on it.)
The question is, from whence did the radiated energy come?
The question is, from whence did the radiated energy come?