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O
ur everyday experiences and observations have to do with objects that move at
speeds much less than the speed of light. Newtonian mechanics was formulated by
observing and describing the motion of such objects, and this formalism is very
successful in describing a wide range of phenomena that occur at low speeds.
However, it fails to describe properly the motion of objects whose speeds approach
that of light.
Experimentally, the predictions of Newtonian theory can be tested at high speeds
by accelerating electrons or other charged particles through a large electric potential
difference. For example, it is possible to accelerate an electron to a speed of 0.99c
(where c is the speed of light) by using a potential difference of several million volts.
According to Newtonian mechanics, if the potential difference is increased by a factor
of 4, the electron’s kinetic energy is four times greater and its speed should double to
1.98c. However, experiments show that the speed of the electron—as well as the speed
of any other object in the Universe—always remains less than the speed of light,
regardless of the size of the accelerating voltage. Because it places no upper limit on
speed, Newtonian mechanics is contrary to modern experimental results and is clearly
a limited theory.
In 1905, at the age of only 26, Einstein published his special theory of relativity.
Regarding the theory, Einstein wrote:
The relativity theory arose from necessity, from serious and deep contradictions
in the old theory from which there seemed no escape. The strength of the new
theory lies in the consistency and simplicity with which it solves all these
difficulties . . . .
1
Although Einstein made many other important contributions to science, the special
theory of relativity alone represents one of the greatest intellectual achievements of all
time. With this theory, experimental observations can be correctly predicted over the
range of speeds from v ! 0 to speeds approaching the speed of light. At low speeds,
Einstein’s theory reduces to Newtonian mechanics as a limiting situation. It is important
to recognize that Einstein was working on electromagnetism when he developed the
special theory of relativity. He was convinced that Maxwell’s equations were correct, and in
order to reconcile them with one of his postulates, he was forced into the revolutionary
notion of assuming that space and time are not absolute.
This chapter gives an introduction to the special theory of relativity, with emphasis
on some of its consequences. The special theory covers phenomena such as the
slowing down of moving clocks and the contraction of moving lengths. We also discuss
the relativistic forms of momentum and energy.
In addition to its well-known and essential role in theoretical physics, the special
theory of relativity has practical applications, including the design of nuclear power
plants and modern global positioning system (GPS) units. These devices do not work if
designed in accordance with nonrelativistic principles.
1
A. Einstein and L. Infeld, The Evolution of Physics, New York, Simon and Schuster, 1961.