326 DOC. 42 SPECIAL AND GENERAL RELATIVITY
TWENTY-TWO
A
Few
Inferences
from
the
General
Principle
of
Relativity
The
considerations
of Section
20
show that the
general
principle
of
relativity puts
us
in
a
position to
derive
properties
of the
gravitational
field in
a
purely
theo-
retical
manner.
Let
us
suppose,
for instance,
that
we
know the
space-time
"course"
for
any
natural
process
whatsoever,
as
regards
the
manner
in
which
it
takes
place
in
the Galileian
domain relative
to
a
Galileian
body
of reference
K.
By means
of
purely
theoretical
operations
(i.e.
simply by
calculation)
we
are
then able
to
find
how
this
known natural
process appears,
as seen
from
a
reference-body
K'
which
is
accelerated rela-
tively
to
K.
But since
a
gravitational
field
exists with
respect
to
this
new
body
of reference
K',
our
consideration
also
teaches
us
how the
gravitational
field influences the
process
studied.
For
example,
we
learn that
a
body
which
is
in
a
state
of
uniform rectilinear motion with
respect to
K (in
accordance
with
the
law
of
Galilei) is
executing
an
accelerated and
in
general
curvilinear
motion
with
respect to
the accelerated
reference-body
K' (chest).
This
acceleration
or
curvature
cor-
83