A PARADIGM SHIFT 221
In particular, he predicted that
relativistic effects such as time
dilation should occur in strong
gravitational fields. The closer a
clock is to a source of gravitation,
the more slowly it will tick. This
effect remained purely theoretical
for many years, but has now been
confirmed using atomic clocks.
Space-time manifold
Meanwhile, also in 1907, Einstein’s
former tutor Hermann Minkowski
had hit upon another important
part of the puzzle. Considering the
effective trade-offs between
the dimensions of space and time
involved in special relativity, he
developed the idea of combining
the three dimensions of space
with one of time in a space-time
manifold. In Minkowski’s
interpretation, relativistic effects
could be described in geometrical
terms by considering distortions in
the way that observers in relative
motion observe the manifold in a
different frame of reference.
In 1915, Einstein published his
complete theory of general relativity.
In its finished form, it was nothing
less than a new description of the
nature of space, time, matter, and
gravity. Adopting Minkowski’s
ideas, Einstein saw the “stuff of the
universe” as a space-time manifold
that could be distorted thanks to
relativistic motion, but could also
be warped by the presence of large
masses such as stars and planets
in a way that was experienced as
gravity. The equations that described
the link between mass, distortion,
and gravity were fiendishly complex,
but Einstein used an approximation
to solve a long-standing mystery—
the way in which Mercury’s closest
approach to the Sun (aphelion)
precesses, or rotates, around the
Sun much more quickly than
predicted by Newtonian physics.
General relativity solved the puzzle.Gravitational lensing
Einstein published at a time when
much of the world was swept up in
World War I, and English-speaking
scientists had other things on their
minds. General relativity was a
complex theory and might have
languished in obscurity for many
years had it not been for the
interest of Arthur Eddington, a
conscientious objector to the war,
and, as it happened, Secretary of
the Royal Astronomical Society.
Eddington became aware of
Einstein’s work thanks to letters
from Dutch physicist Willem
de Sitter, and soon became its
chief advocate in Britain. In 1919,a few months after the end of the
war, Eddington led an expedition
to the island of Príncipe, off the
west coast of Africa, in order to
test the theory of general relativity
and its prediction of gravitational
lensing in the most spectacular
circumstances. Einstein had
predicted as early as 1911 that a
total solar eclipse would allow the
effects of gravitational lensing to be
seen, in the form of apparently out-
of-place stars around the eclipsed
disk (a result of their light being
deflected as it passed through the
warped space-time around the
Sun). Eddington’s expedition
delivered both impressive images
of the solar eclipse and convincing
proof of Einstein’s theory. When
published the following year, they
proved to be a worldwide sensation,
propelling Einstein to global fame
and ensuring that our ideas about
the nature of the universe would
never be the same again. ■Arthur Eddington’s photographs of
a solar eclipse in 1919 provided the first
evidence for general relativity. Stars
around the Sun appeared out of place,
just as Einstein had predicted.