250 DOC.
21
THEORY OF RELATIVITY
respect
to all
coordinate
systems
K'
that
are
in motion relative
to
K. Thus,
the
theory
does
not
agree
with the
relativity principle.
We
can
also
carry
out
this
argument
without
making
use
of the
concept
of the luminiferous ether.
According
to
Lorentz's
theory,
there exists
a
coordinate
system
K
relative
to
which
every
light ray propagates
in
a vacuum
with the definite
constant
velocity
c.
If
we
refer such
a
light ray
to
a
coordinate
system
K'
that
is
in motion relative
to K-moving,
say,
in
the direction
of
the
propagation
of
the
light-then
we
feel
intuitively compelled
to
assume
that
this
same
light ray
has
a
different
propagation velocity
relative
to
K'.
Thus,
one
would
have
to
conclude
that-in
contradiction
to
the
relativity
principle-the
coordinate
system
K
is
privileged
with
respect
to all
coordinate
systems
K' that
are
in motion
relative
to it.
The fundamental assertion of Lorentz's
theory,
that
every light ray
in
a
vacuum
always propagates
(at
least with
respect
to
a
certain coordinate
system
K)
with the
definite
constant
velocity c, we
will
call the
principle
of
the constancy
of
the velocity
of
light.
The
previously
indicated
difficulty
with Lorentz's
theory
consists
in
the fact
that the
principle
of the
constancy
of the
velocity
of
light
seems
to
be
incompatible
with
the
relativity principle.
The
successes
of Lorentz's
theory
were so significant
that
physicists
would
have
dropped
the
relativity principle
without hesitation had
it not
been for
an important
experimental result,
of which
we
must
now
speak, namely,
the
experiment
of
Michelson. [8]
The
Motion
of the
Earth and
the
Concept
of the
Stationary Luminiferous
Ether
If
one
maintains, in
accordance with Lorentz's
theory,
that there exists
a privileged
coordinate
system
K
in
which the
velocity
of
light
in
a vacuum
equals
c,
then
one
cannot
assume
that
the Earth
is
at
rest
relative
to
this coordinate
system.
Because
one
cannot
then
assume
that the
(stationary)
ether
participates
in
the
motion of the Earth
around the
Sun.
For
at
least
a
part
of the
year
we
would therefore have
to
possess
a
velocity
of the order of
magnitude
30
km
per
second relative
to
the
system
K.
This
gives
rise
to
the
problem
of
demonstrating
this relative motion of
our
laboratories and
apparatus
with
respect
to
K and the ether.
A
great
number of
experiments
were
carried
out to
demonstrate this relative motion. One started with the idea that the
orientation of sensitive
optical apparatus
with
respect
to
the direction of that relative
motion
must
have
an
influence
on
the
optical processes.
But
it
proved wholly
impossible
to
demonstrate
experimentally
such
a privileged
direction.
However,
most
of these
negative findings
did
not
prove anything against
the