DOC. 42
SPECIAL AND GENERAL RELATIVITY 385
The
Experimental
Confirmation
of
the
General
Theory
of
Relativity
145
onds of
arc per
century.
The
uncertainty
of the
empirical
result
amounts to
a
few seconds
only.
(B) DEFLECTION
OF LIGHT
BY A
GRAVITATIONAL FIELD
In Section
12
it
has
been
already
mentioned
that
according
to
the
general theory
of
relativity,
a ray
of
light
will
experience
a curvature
of
its
path
when
passing through a gravitational
field,
this
curvature being
similar
to
that
experienced
by
the
path
of
a
body
which
is
projected through
a
gravitational
field.
As
a
result
of
this
theory,
we
should
expect
that
a ray
of
light
which
is
passing
close
to
a
heavenly body
would be deviated
towards the latter. For
a ray
of
light
which
passes
the
sun
at
a
distance
of
A
sun-radii
from its
centre,
the
angle
of deflection
(a)
should
amount to
[90]
[91]
1.7
seconds
of
arc
a = -.
It
may
be added
that,
according to
the
theory,
half
of this
deflection
is produced
by
the Newtonian
field
of
attraction of
the
sun,
and
the
other half
by
the
geo-
metrical modification
("curvature")
of
space
caused
by
the
sun.
This
result admits of
an
experimental
test by
means
of the
photographic regis-
tration of
stars
during a
total
eclipse
of
the
sun.
The
only reason
why
we
must
wait
for
a
total
eclipse
is
because
at every
D1
D1
D2
F
Fig.
5