328 DOC.
42
SPECIAL AND GENERAL RELATIVITY
A
Few
Inferences
from
the
General
Principle
of
Relativity
85
this deduction
is
a
problem
of the
greatest importance,
the
[44]
early
solution
of
which
is
to
be
expected
of
astronomers.1
In the second
place
our
result
shows that,
according to
the
general
theory
of
relativity,
the
law
of the
constancy
of
the
velocity
of
light
in
vacuo,
which constitutes
one
of the
two
fundamental
assumptions
in the
special theory
of
relativity
and
to
which
we
have
already frequently
referred,
cannot
claim
any
unlimited
validity.
A
curvature
of
rays
of
light
can
only
take
place
when the
velocity
of
propagation
of
light
var-
ies
with
position.
Now
we
might
think that
as a
consequence
of
this,
the
special
theory
of
relativity
and with it the whole
theory
of
relativity
would
be
laid
in the dust. But
in
reality
this
is
not
the
case.
We
can only
conclude
that
the
special theory
of
relativity cannot
claim
an
unlimited domain of
validity;
its
results hold
only
so
long
as
we are
able
to disregard
the in-
fluences of
gravitational
fields
on
the
phenomena
(e.g.
of
light).
Since it
has
often
been
contended
by opponents
of the
theory
of
relativity
that the
special theory
of
relativity
is
over-
thrown
by
the
general theory
of
relativity,
it
is
perhaps
advis-
able
to
make the
facts
of
the
case
clearer
by
means
of
an
appropriate comparison.
Before the
development
of electro-
dynamics
the
laws
of electrostatics
were
looked
upon
as
the
laws
of
electricity.
At the
present
time
we
know
that
electric
fields
can
be derived
correctly
from electrostatic consider-
ations
only
for
the
case,
which
is
never
strictly
realised,
in
which the electrical
masses are
quite at rest relatively to
each
1 By means
of the
star photographs
of
two expeditions equipped
by a
Joint
Committee of the
Royal
and
Royal
Astronomical
Societies,
the existence of the deflection of
light
demanded
by
theory was
first
confirmed
during
the
solar
eclipse
of 29th
May,
1919. (Cf. Appendix 3.)