Hi, I'm confused about how Doppler effect shifts the wavelength of light depending on the speed of the emitter. It makes sense from a classical point of view that the wavelength should shift. However, when I consider that light comes from photons, quantized energy, any shift in frequency corresponds to a shift in energy. This seems to mean for the Doppler effect to work, energy cannot be conserved. The best I've found on my own search for this is the following article, which says energy is not conserved in general relativity: http://blogs.discovermagazine.com/cosmicvariance/2010/02/22/energy-is-not-conserved/ Is this the reason for the Doppler effect or am I missing something simple?