 With all of these caveats in place, we are almost ready to understand Baierlein’s derivation of Einstein’s equation. Because Baierlein’s derivation involves analyzing a physical interaction in which an object emits light, i.e., electromagnetic radiation, we need first to state two dynamical properties of electromagnetic radiation. First, considered as an electromagnetic wave, like all waves, light carries energy. This is quite familiar to us today at an everyday level, especially because of all the “solar” devices we use. Second, light also carries momentum. As Baierlein reports, in the late nineteenth century, James Clerk Maxwell had already determined that a burst of light of energy \(E\) had momentum \(E/c\). So, for example, if a laser beam strikes a freely-floating mirror in outer space, the collision of light against mirror will impart a finite, non-zero momentum to the mirror. One can calculate exactly how the mirror will have its state of motion changed as a result of this interaction by using the principle of conservation of momentum. 