322 DOC. 42 SPECIAL AND GENERAL RELATIVITY
Inertial
and
Gravitational Mass 79
observer in the
railway
carriage experiences
a
jerk
forwards
as
a
result
of the
application
of the
brake,
and
that
he
recognises
[36]
in this the
non-uniformity
of motion
(retardation)
of
the
car-
riage.
But he
is
compelled by nobody
to
refer this
jerk to
a
"real"
acceleration
(retardation)
of the
carriage.
He
might
also
interpret
his
experience
thus:
"My
body
of reference (the
carriage)
remains
permanently at rest.
With reference
to
it,
however,
there exists
(during
the
period
of
application
of
the
brakes)
a
gravitational
field which
is
directed forwards and
which
is
variable with
respect to
time. Under the influence of
this
field,
the
embankment
together
with
the
earth
moves
nonuniformly
in such
a manner
that their
original velocity
in
[37]
the backwards direction
is
continuously
reduced."