324 DOC. 42 SPECIAL AND GENERAL RELATIVITY
Mechanics
and
Relativity
81
of
motion)?
What is
the
reason
for
this
preference?
In
order
to
show
clearly
what I
mean
by
this
question,
I shall
make
use
of
a comparison.
I
am
standing
in
front
of
a gas
range. Standing alongside
of
each other
on
the
range
are
two pans
so
much alike
that
one
may
be mistaken
for
the other. Both
are
half full
of
water.
I
notice
that
steam
is
being
emitted
continuously
from
the
one
pan,
but
not
from
the other.
I
am
surprised at
this,
even
if
I
have
never seen
either
a
gas
range or a pan
before. But if
I
now
notice
a
luminous
something
of bluish colour under
the
first
pan
but
not
under the
other,
I
cease
to
be
astonished,
even
if
I
have
never
before
seen
a gas
flame.
For
I
can
only
say
that
this bluish
something
will
cause
the
emission
of
the
steam,
or
at
least
possibly
it
may
do
so.
If,
however, I
notice the bluish
something
in
neither
case,
and if
I
observe
that
the
one
con-
tinuously
emits
steam
whilst
the
other does
not,
then
I
shall
remain astonished and dissatisfied until
I
have discovered
some
circumstance
to
which
I
can
attribute the different be-
haviour
of the
two pans.
Analogously,
I
seek in vain
for
a
real
something
in
classical
mechanics
(or
in the
special theory
of
relativity)
to
which I
can
attribute the different behaviour of bodies considered with
respect to
the
reference-systems
K
and
K'.1
Newton
saw
this
[4]
objection
and
attempted
to
invalidate
it,
but
without
success.
But E. Mach
recognised
it
most
clearly
of
all,
and
because
of
1
The
objection
is of
importance
more
especially
when the
state
of motion of the reference-
body
is of such
a
nature
that
it
does
not
require any
external
agency for its maintenance,
e.g.
in
the
case
when the
reference-body is rotating uniformly.