The Speed of Light

    Michael Fowler, UVa

Early Ideas about Light Propagation

As we shall soon see, attempts to measure the speed of light played an important part in the development of the theory of special relativity, and, indeed, the speed of light is central to the theory. 

The first recorded discussion of the speed of light (I think) is in Aristotle, where he quotes Empedocles as saying the light from the sun must take some time to reach the earth, but Aristotle himself apparently disagrees, and even Descartes thought that light traveled instantaneously.  Galileo, unfairly as usual, in Two New Sciences (page 42) has Simplicio stating the Aristotelian position,

SIMP.  Everyday experience shows that the propagation of light is instantaneous; for when we see a piece of artillery fired at great distance, the flash reaches our eyes without lapse of time; but the sound reaches the ear only after a noticeable interval. 

Of course, Galileo points out that in fact nothing about the speed of light can be deduced from this observation, except that light moves faster than sound.  He then goes on to suggest a possible way to measure the speed of light.  The idea is to have two people far away from each other, with covered lanterns.  One uncovers his lantern, then the other immediately uncovers his on seeing the light from the first.  This routine is to be practised with the two close together, so they will get used to the reaction times involved, then they are to do it two or three miles apart, or even further using telescopes, to see if the time interval is perceptibly lengthened.  Galileo claims he actually tried the experiment at distances less than a mile, and couldn’t detect a time lag.  From this one can certainly deduce that light travels at least ten times faster than sound.