DOC. 42 SPECIAL AND GENERAL RELATIVITY 327
84
Relativity
responds to
the influence
on
the
moving body
of the
gravita-
tional
field
prevailing relatively to
K'.
It
is
known that
a
gravitational
field influences
the
movement
of bodies
in
this
way, so
that
our
consideration
supplies
us
with
nothing
essen-
tially
new.
However,
we
obtain
a new
result of fundamental
impor-
tance
when
we carry
out
the
analogous
consideration
for
a ray
of
light.
With
respect to
the Galileian
reference-body
K,
such
a ray
of
light
is
transmitted
rectilinearly
with the
velocity c.
It
can
easily
be shown that the
path
of the
same ray
of
light is
no
longer
a
straight
line when
we
consider
it
with reference
to
the
accelerated chest
(reference-body K').
From this
we con-
clude,
that,
in
general, rays
of
light are
propagated
curvilinearly
in
gravitational
fields.
In
two respects
this result
is
of
great
importance.
In the
first
place,
it
can
be
compared
with
the
reality.
Al-
though a
detailed examination of
the
question
shows
that
the
curvature
of
light
rays
required by
the
general theory
of
rela-
tivity
is
only exceedingly
small for
the
gravitational
fields
at
our
disposal
in
practice, its
estimated
magnitude
for light
rays
passing
the
sun
at grazing
incidence
is
nevertheless
1.7
sec-
onds of
arc.
This
ought to
manifest
itself
in
the
following
way.
[43]
As
seen
from
the
earth,
certain
fixed
stars appear to
be in the
neighbourhood
of the
sun,
and
are
thus
capable
of observation
during
a
total
eclipse
of the
sun.
At such
times,
these
stars
ought
to
appear to
be
displaced
outwards
from
the
sun
by
an
amount
indicated
above,
as
compared
with their
apparent po-
sition
in
the
sky
when the
sun
is
situated
at
another
part
of the
heavens.
The
examination of the
correctness
or
otherwise
of
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