113Chapter 13Prelude to Relativity
With Maxwell’s description of light as electromagnetic radiation, the
physicists of the nineteenth century felt that they had a complete
description of the physical universe. Perhaps there were certain details to
be studied but they believed that all of the basics were understood. In
actuality, they were on the eve of a revolution in physics that would
shake the foundations of their thought as much as the Copernican
revolution had shook the thought of their predecessors some three
hundred years earlier. Their entire notion of space and time would
change as a result of this revolution, which Einstein’s Theory of
Relativity would bring. But before discussing the nature of this
revolution, let us first consider the experimental observations that lead
to the theory of relativity.
Maxwell had shown through his equations describing the electric and
magnetic fields that light was an electromagnetic wave, which traveled at
the velocity c = 2.998 × 10^8 m/sec (or approximately 300,000 kilometers
per sec). The first measurement of the velocity of light was made by
Roemer in 1676 making use of astronomical observations of the eclipses
of the Jovian moons by Jupiter itself. The satellite Io orbits Jupiter every
42 hours. Roemer noticed that the time interval between successive re-
appearances of Io varied depending on the position of the Earth. When
the Earth travels along its orbit about the sun from position A to B the
time interval between successive reappearances of Io increases. This is
due to the fact that the Earth is farther from Jupiter after each successive
reappearance and an extra amount of time is required for the light
reflected from the moon to reach the Earth. As the Earth travels back
from position B to A, the time intervals for successive reappearances
of Io decreases. The overall time lag as the Earth travels from position A
to B is 22 minutes, which is the time required for the light to travel the
diameter of the Earth’s orbit.