Properties and behavior
 

Speed of electromagnetic radiation and the Doppler effect
 

Electromagnetic radiation, or in modern terminology the photons h, always travel in free space with the universal speed c--i.e., the speed of light. This is actually a very puzzling situation which was first experimentally verified by Michelson and Edward Williams Morley, another American scientist, in 1887 and which is the basic axiom of Albert Einstein's theory of relativity. Although there is no doubt that it is true, the situation is puzzling because it is so different from the behaviour of normal particles; that is to say, for little or not so little pieces of matter. When one chases behind a normal particle (e.g., an airplane) or moves in the opposite direction toward it, one certainly will measure very different speeds of the airplane relative to oneself. One would detect a very small relative speed in the first case and a very large one in the second. Moreover, a bullet shot forward from the airplane and another toward the back would appear to be moving with different speeds relative to oneself. This would not at all be the case when one measures the speed of electromagnetic radiation: irrespective of one's motion or that of the source of the electromagnetic radiation, any measurement by a moving observer will result in the universal speed of light. This must be accepted as a fact of nature.
 

What happens to pitch or frequency when the source is moving toward the observer or away from him? It has been established from sound waves that the frequency is higher when a sound source is moving toward the observer and lower when it is moving away from him. This is the Doppler effect, named after the Austrian physicist Christian Doppler, who first described the phenomenon in 1842. Doppler predicted that the effect also occurs with electromagnetic radiation and suggested that it be used for measuring the relative speeds of stars. This means that a characteristic blue light emitted, for example, by an excited helium atom as it changes from a higher to a lower internal energy state would no longer appear blue when one looks at this light coming from helium atoms that move very rapidly away from the Earth with, say, a galaxy. When the speed of such a galaxy away from the Earth is large, the light may appear yellow; if the speed is still larger, it may appear red or even infrared. This is actually what happens, and the speed of galaxies as well as of stars relative to the Earth is measured from the Doppler shift of characteristic atomic radiation energies h^v.