Chapter 9: Einstein and Relativity Theory (319 KB) - D Cassidy Books

3637_CassidyTX_09 6/14/02 12:09 PM Page 441
9.12 BREAKING WITH THE PAST 441
9.12 BREAKING WITH THE PAST
Although Einstein’s theory of special relativity did not represent a major
break with classical physics, it did break with the mechanical world view.
Our understanding of nature provided by special relativity, together with
subsequent advances in quantum mechanics, general relativity theory, and
other innovations, will slowly shape the new world view that is emerging.
Special relativity introduced an important break with the mechanical
world view concerning the notion of absolute rest and absolute motion,
which ceased to exist as a result of Einstein’s work. Until that time, most
physicists defined absolute rest and motion in terms of the so-called ether,
the stuff that filled all of the space and transmitted light and electric and
magnetic forces. As noted earlier, Einstein simply ignored the ether as “superfluous,”
since only relative motions were used in his theory. At the same
time, and even before, a large number of careful experiments of different
sorts to detect the ether had failed completely. One of these, the most famous
one, was a series of experiments, during the 1880s, in which the American
scientists Albert A. Michelson and Edward Morley attempted to detect
the “wind” of ether experienced by the Earth as it moved through the
supposed stationary ether on its orbit around the Sun. If such an ether existed,
scientists believed, it should cause an “ether wind” over the surface
of the Earth along the direction of motion. Since light was believed to be
a wave moving through the ether, somewhat like sound waves through the
air, it should be affected by this wind. In particular, a light wave traveling
into the wind and back should take longer to make a round trip than a wave
traveling the exact same distance at a right angle, that is, across the wind
and back. (See the calculation in the Student Guide.) Comparing two such
waves, Michelson and Morley could find no difference in their times of
travel, within the limits of precision of their experiment. Within a few years
of Einstein’s theory, most physicists had abandoned the notion of an ether.
If it could not be detected, why keep it
v
Ether wind
FIGURE 9.17 Earth moving through the stationary
ether, according to nineteenth-century
concepts.