252 DOC.
21
THEORY OF RELATIVITY
principle
is
universally
and
strictly
valid. On the other
hand-as
already men-
tioned-the
foundation of Lorentz's
theory,
and therewith also the
principle
of the
constancy
of the
velocity
of
light,
seem
to be
incompatible
with the
relativity
principle.
But
anyone
who has made
a
thoroughgoing attempt
to
replace
Lorentz's
theory
with another
one
that would do
justice
to
the
experimental
facts would have
to admit
that,
given
the
current state
of
our
knowledge,
this
undertaking seems quite
hopeless.
Inadmissibility
of the
Ether
Hypothesis
Given this
state
of
affairs,
one
has
to
ask oneself
again
whether Lorentz's
theory
and
the
principle
of the
constancy
of the
velocity
of
light
are
really incompatible
with the
relativity principle.
Close examination shows that the
two
principles
are
compatible
with each
other,
that Lorentz's
theory
does not
contradict the
relativity principle.
However,
our
conception
of
space
and time
must
be
subjected
to
a
fundamental
revision.
Further, it is
easy
to
see
that
we
must
also abandon the idea of
introducing
a
luminiferous
ether into the
theory.
Because
if
every light
ray
in
vacuum
is
supposed
to
propagate
with the
velocity
c
with
respect
to
K,
then
we
must
conceive of this
luminiferous ether
as
being everywhere
at rest
with
respect
to
K. However,
if
the
laws
of
propagation
of
light
with
respect
to
the
system
K'
(in
motion relative
to
K) are
the
same as
those with
respect
to
K,
then
we
would have
to
assume
with
as
much
right
the existence of
a
luminiferous ether that is
at rest
with
respect
to
K'. Since
it is
absurd
to
assume
that the luminiferous ether
is
simultaneously
at rest
with
respect
to
both
systems,
and since
it would be
hardly
less absurd
for
the
theory
to
privilege
one
of the
two
(or
of
infinitely many) physically
equivalent
systems over
the
others,
one
must
dispense
with the introduction of that
concept,
which
was
anyway just
a
useless
accessory
of the
theory
ever
since the mechanical
interpretation
of
light
had been
abandoned.
It has
already
been shown
that,
as
far
as
its
interpretation
in
theoretical
physics
is concerned,
the coordinate
system
is
nothing
but
a
rigid measuring
framework
on
which
to
mark off the values of the
spatial
coordinates with the
help
of
rigid
rods.
We
now
have
to
pose
the
question
of the
physical
meaning
of the
temporal
determinations that
usually
occur
in
physics
in
association with
spatial
determinations.
We shall
now
investigate
this
question.
The
Physical Meaning
of Temporal
Determinations
We
usually
measure
time
by
means
of
a
clock. We
designate
as a
clock
a
system
that