MadSci Network: Physics |
The photon is always emitted at the same energy in the rest frame of the original atom. However, if you are moving with respect to that atom, you will measure the energies of both the atom and the emitted photon differently. Exactly how you measure them requires relativity to analyze, but the effect is there nonrelativistically as well. For example, if I am in a train moving away from you, and I throw a ball out the window towards you, I would measure it as having a certain amount of energy in my own rest frame. You would measure it as having less energy, because you would measure its speed as being less than I would. And if the train is moving towards you, the opposite happens - you see the ball as having more energy than I see it as having. So if the atom is moving away from you, the photon will appear to you to have less energy; if it is moving toward you it will appear to have more. The speed of the photon doesn't change, of course, but its frequency (and therefore energy and "color") do. In either case, though, energy is conserved during the total reaction according to both observers (you and the one on the star); you just measure the energies of the individual objects involved differently. Here's another way to think about this problem: Suppose you were initially at rest with respect to the atoms which are emitting photons of a single energy. Now you start moving away from them. What has happened to the energy of the photons? Nothing. But something has happened to you, and you now measure the energies of the photons differently. Is the energy of the photons conserved? Yes (although you need to take your own motion into account to determine this, if your motion is changing while you are measuring the photons). A reference that I like for special relativity is Spacetime Physics, by Taylor and Wheeler. (Unfortunately, I don't know whether it is still in print.)