DOC.
17
THE THEORY OF RELATIVITY
343
systems.
On the other
hand,
Lorentz's fundamental
hypothesis
of the
stationary
luminiferous ether
distinguishes
from
among
all
possible
acceleration-free
moving systems
those
in
a
state
of
motion,
namely systems
that
are
at rest
relative
to this
light
medium.
Thus,
even
though one
cannot
say according
to this
conception
that there
exists
an
absolute motion
in
the
philosophical
sense-because
this
is completely
out
of the
question, we can
only
conceive
of
relative
changes
of
position
of bodies-an absolute
motion
in
the
physical sense
is
affirmed
insofar
as we
have
privileged one
state
of
motion,
namely,
the
state
of
rest relative to
the ether.
Every body
that
is
at rest
relative
to
ether
can
be
designated,
in
a
certain
sense,
as absolutely
at rest.
Reference
systems
that
are
at
rest
relative
to
the ether
are distinguished
from all
other acceleration-free
systems.
In that
sense
Lorentz's
basic
concept
of
the
stationary
luminiferous ether does
not
satisfy
the
principle
of
relativity.
The
basic
concept
of the
stationary
luminiferous ether
leads
to
the
following
general
argument:
Let
a
reference
system
k be
at rest relative to
the
luminiferous ether. Let another reference
system
k'
be
in
uniform motion
relative to
the luminiferous ether. It
is
to
be
expected
that the
relative
motion of k'
with
respect
to
the ether
will have
an
influence
on
the
laws
of
nature
that
are
valid with
respect
to
k'.
Hence,
it
was
to
be
expected
that the
laws
of
nature with
respect
to
k'
would differ
from those
with
respect
to
k
on
account
of
the
motion of
k'
in
the luminiferous ether.
One
had
to
tell
oneself,
furthermore,
that the earth and
its
laboratories
could not
possibly
be
at rest relative to this
luminiferous medium
throughout
the entire
year,
i.e.,
that the
earth
must,
therefore,
play
the role
of
a
reference
system
k'.
One,
therefore,
had
to
assume
that
some
phenomenon
could be found
in which
the
influence
of
this
motion
on
the
experiments
in
our
laboratories
would
come
to
the fore. One
would
think
that,
because of
this relative
motion, our
physical space
in
the
form
we
find it
on
the earth
would show
different behavior
in
different directions. But there
was
not
a
single
case
in
which it
was possible
to
prove something
like
that.
One
was, thus,
in
an
awkward
position
with
regard
to
the ether. Fizeau's
experiment
says:
The ether does
not
move
with
matter,
i.e.,
there
does exist
a
motion of the
light
medium relative
to matter.
But
all
attempts
to
detect
this relative
motion
yielded
negative
results.
These
are
two
results that
seem
to
contradict
one
another,
and
physicists
found
it
enormously
distressing
that
they
could not
get
rid
of
this
unpleasant
conflict.
They were
bound
to ask
themselves whether
it
might
not
be
possible,
after
all,
to
reconcile the
principle
of
relativity-to which
no
exception
could be
found
in
spite
of
all
the
searching-with
Lorentz's
theory.
Before
delving
into
this
question, we
will
extract from
Lorentz's
theory
of
the
stationary
luminiferous ether the
following aspects
most
essential
to
us.
What
is
the
physical
meaning
of the
statement
that there
exists
a
stationary
luminiferous ether? The
most
important
content
of
this
hypothesis
can
be
expressed as
follows:
There
exists
a
reference
system (called
in
Lorentz's
theory
"a
system
at rest relative to
the
ether")
with
respect
to which
every light ray
propagates
in
a