148 The Poetry of Physics and The Physics of Poetry
exerted on it by the Sun, but rather because this world line is the shortest
possible path it can find in the curved space-time continuum surrounding
the mass of Sun. The space around the Sun is curved or non-Euclidean
because of its gravitational mass.
Einstein’s model of the gravitational interaction is able to account for
something, which Newton’s model could never explain. Mercury, the
closest planet to the Sun, orbits the Sun every 88 days in an elliptical
orbit. The entire orbit of Mercury rotates at an extremely slow rate about
the Sun. The distance of closest approach, the perihelion, of Mercury,
advances 43 seconds of arc per century. Since there are 360 degrees in a
circle, 60 minutes in a degree and 60 seconds a minute, it would
take a little more than 3,000,000 years for the perihelion of Mercury to
make one complete rotation about the Sun. This extremely small effect,
nevertheless, cannot be explained in terms of Newton’s theory of gravity.
Astronomers tried to explain this effect in terms of the interaction of the
other planets on Mercury. In fact, the existence of a planet lying between
Mercury and the Sun, called Vulcan, was postulated to explain the
advance of the perihelion of Mercury. This planet was never discovered
and the anomalous behaviour of Mercury remained a mystery until
Einstein’s General Theory of Relativity, which was able to explain this
effect in terms of the curvature of space.
Dicke has since shown that the advance for the perihelion of Mercury
could also be explained if the shape of the Sun were not spherical. Since
the shape of the Sun is not known, this is also a possible explanation. The
experimental evidence to support the General Theory of Relativity is
not as conclusive as the evidence supporting the Special Theory of
Relativity. Experimental confirmation of Einstein’s Special Theory
occurs every day in a laboratory of the elementary particle physicists
who explore the properties of elementary particles using high-energy
accelerators. Experimental confirmation of general relativity has only
been obtained in three instances:
- The bending of light by the Sun.
- The shift of spectra from the surface of a white-dwarf star, from the
surface of the Sun and from different elevations near the surface of
the Earth. - An explanation of the advance of the perihelion of Mercury.
Research in general relativity continues. Two areas of research that
are being actively pursued in general relativity are the search for a