144 The Poetry of Physics and The Physics of Poetry
with a fixed periodicity. The frequency of light that an atom emits is
related to the periodicity of the electron’s orbits. Since two identical
atoms will radiate the same discreet frequencies or spectral lines if they
are in the same frame of reference, a change in the frequency of the
radiation of an atom can indicate a change of the time in the frame of
reference in which it is situated. The first detection of gravitational time
dilation was made by observing the slowing down of the ‘atomic clocks’
on the surface of extremely dense white-dwarf stars. The expected shift
of the spectral lines towards the red was observed. A similar effect was
observed for the spectral lines emitted from the surface of the Sun.
Pound in a laboratory setting at Harvard University made a more precise
observation of the gravitational time dilation. Pound compared the
spectral lines of two atomic clocks separated by a mere 20 meters in the
Earth’s gravitational field. He found that, the ‘atomic clock’ sitting closer
to the Earth and hence, more strongly influenced by its gravitational
field, slowed down more than its counterpart 20 meters above it.
Einstein’s General Theory of Relativity describes the effect on space
and time measurements of an observer, either imbedded in a gravitational
field or else undergoing non-uniform motion. The General Theory of
Relativity also provides a theory of gravitation, which competes with
Newton’s theory of gravity. In addition to providing relativistic
corrections to Newton’s theory, Einstein’s theory attempts to explain
how the gravitational interaction arises. Newton’s principle of action at a
distance is replaced by a field concept. According to Einstein, matter
warps the space around it. The warped space, in turn, affects the motion
of the matter contained in it. The Sun, for example, warps the space in
which the solar system is embedded, creating grooves in which the
planets move. Determining the gravitational interaction of matter
becomes a matter of geometry.
Before turning to the details of Einstein’s model of gravity, we must
first discuss the four-dimensional space-time continuum of relativity,
which replaces Newton’s concept of three-dimensional space and
absolute time. In the Special Theory of Relativity, we discovered an
intimate relationship between space and time. Time is no longer
independent of the position or motion of an observer. Shortly
after Einstein’s Special Theory appeared in 1905, Minkowski proposed
a four-dimensional space-time continuum with time playing the role of
the fourth dimension. In this space, every event is described by four co-
ordinates or numbers. Three of these co-ordinates, x, y, and z, describe