I have just started a book on GM. As motivation for the idea that gravity must affect the energy of light the following example is given. A body of rest mass,M, falls in a gravitation field and bounces off of the ground after it has achieved velocity,v. At impact, it transforms completely into a light ray that rises upwards with energy approximately equal to m + 1/2mv^2. When it reaches the original drop height it again reflects and turns back into a body of mass equal to the photon's energy. If this energy is not m then it would be possible to create a perpetual motion machine. Why is that?