““Just as with sound waves, light waves emitted by a moving source show the Doppler effect. What we observe instead of the sound pitch is the color of the light: Red light has a lower frequency than blue light. If the light source is moving toward the observer, the light appears bluer; as it moves away, it will appear red. Likewise, an observer moving inside the blackbody volume will register the radiation coming from the direction opposite to his motion as being blue, that which comes from behind as red. The difference between the two frequencies can tell him his velocity of motion with respect to the blackbody radiation. This difference, however, will decrease as the temperature is lowered; it will vanish altogether at absolute zero. Regardless of an observer's velocity of motion, the radiation meeting him at zero temperature is the same from every direction. The observer therefore has no way of finding out from the radiation alone in which direction he is moving, or whether he is moving at all. Once we accept this scenario, we have already fixed the spectrum (that is, the amount of radiation as a function of its frequency) within some constant factor. However, the result obtained in this way does not appear to make sense: It implies that a blackbody at zero temperature has an infinite supply of energy in the form of zero temperature radiation. The same astonishing result can be derived by means of the quantum theory of electromagnetic radiation, which we call quantum electrodynamics. This theory, the implications of which have been verified in many instances with remarkable precision, tells us that the true vacuum at zero temperature still has an infinite supply of radiation energy. As we proceed, we will see that electromagnetic radiation is in fact only one component, albeit infinite in quantity, of the unfathomable energy supply of the vacuum.””
