124
DOC.
2
RELATIVITY AND
ITS CONSEQUENCES
encountered
so
far
are mainly
due to this
fact;
the
deeper
reasons
will
become
apparent
later
on.
Be
that
it
as
it
may,
it
is
so
much
the
more
improper to
accept a theory
that
is
not
consistent
with
the
principle
of
relativity,
the
more so
because
this
principle
has not
been contradicted
by a single
experimental
fact.
§5.
On
Two
Arbitrary Hypotheses
Contained
Implicitly
in the
Customary
Notions of Time
and
Space
We
have
seen
that,
assuming
the
existence
of the
ether,
we were
led
by
experiment
to
view this
medium
as stationary.
Further,
we
have
seen
that the
theory
so
founded
permits us
to
predict
the
main
experimental
facts
but
leaves
something
to be
desired
in
one
respect:
It
does not admit
of the
principle
of
relativity,
contrary
to
everything
that
experimental
investigations
have
taught us.
The
question
arises: Is
it
really
impossible
to
reconcile the essential
foundations
of Lorentz's
theory
with
the
principle
of
relativity?
If
we
wish to
attempt
such
a
reconciliation,
the
first
step
we
must
take
is
to
give
up
the ether.
For,
on
the
one hand, we
have
been
obliged
to
admit that the ether
is
stationary,
whereas,
on
the other
hand,
the
principle
of
relativity
demands that the
laws
of natural
phenomena
referred
to
a uniformly moving
coordinate
system
S'
be identical
with
the
laws
of
these
same
phenomena
referred
to
a system
S at rest with
respect
to
the ether. But there
is
no reason
to
assume
the
immobility
of
ether,
which
is
demanded
by
theory
and
experiment,
any more
with
respect
to
the
system
S' than
with
respect
to
the
system
S;
these
two
systems
cannot
be
distinguished
from each
other,
and it
is
therefore
improper
to
make
one
of them
play a special
role
by saying
that
it
is
at rest
with
respect
to
the ether. From
this it follows
that the
only way
to
arrive
at
a
satisfactory
theory
is
to
give
up
the notion of
a
medium
filling
all
of
space.
This
is
the
first
step
to be
taken.
To
go a
step further,
we
must
reconcile the
principle
of
relativity
with
an
essential
consequence
of Lorentz's
theory,
because
giving
up
this
consequence
would amount to
giving
up
the
most
fundamental
formal
properties
of the
theory.
And here
is
the
consequence
in
question:
A
ray
of
light
in
vacuum always propagates
with
the
same velocity c,
which
velocity
is
independent
of the motion of
the
body
that
emits the
ray.
We
will
see
in section 6
that
we
are
raising
this
consequence
to
the the
status
of
a
principle.
For the
sake
of
brevity,
we
will
from
now on
call it the
principle
of
the
constancy
of
the
velocity
of
light.
In Lorentz's
theory
this
principle
holds
only
for
a
system
in
a special
state
of
motion:
In
effect,
the
system
must
be
at rest
relative
to
the ether. If
we
want
to
preserve
the
principle
of
relativity,
we
must
assume
that the
principle
of the
constancy
of the
velocity
of
light
holds
for
any arbitrary
system
not in
accelerated
motion.
At
first
glance
this
seems impossible.
For let
us
consider
a light ray
that
propagates
with
velocity
c
with
respect
to
the
system
S,
and
suppose
that
we
seek
to
determine the
velocity
of