120
DOC.
2
RELATIVITY AND
ITS CONSEQUENCES
the
year,
one was
bound
to believe
that,
in
our
laboratories,
the ether
would
take
a slight
part
in
the motion of the earth the
way
it did in
the motion of the
liquid
in
Fizeau's
investigations.
From
this it would
seem
to follow
that the relative
velocity
of the ether
with
respect
to
our equipment
must
vary
with
time,
and
that
one
therefore
should
expect
that
an
apparent
spatial
anisotropy
be
observed
in
optical
phenomena,
i.e.,
that these
phenomena
should
depend
on
the orientation of the
equipment. Thus,
in
vacuum
or
in
the
atmosphere,
light
should
propagate
faster
in
the direction of the earth's motion than
in
the
opposite
direction.
Experimental
verification
of
this
consequence
of
the
theory
was
unthinkable,
because the order of
magnitude
of the
term
considered
is
that of the ratio
of the
velocity
of the earth
to
the
velocity
of
light, i.e.,
of the order of
10-4,
and
one
could
not
hope
to
attain
such
precision
in
the direct determination of
the
velocity
of
light.
Also-and
this is
a
most
important
point-all
terrestrial methods for
measuring
the
velocity
of
light
employ light rays
that
travel
along a
closed
(back
and
forth)
rather than
a
simple
path,
this
due
to
the
fact
that the
times of
departure
and
arrival
of
the
rays
must
be
determined
with
the
help
of
one
and
the
same device, as
for
example
a
notched
wheel.
Many
optical phenomena
are
known in which
variations
in
the
velocity
of
light
of
the
order of
10-4
become
capable
of
being
detected,
and
when
observing
these
phenomena
one
should
have
expected
to
find,
according
to
the
theory,
that the results
vary
with
the
orientation of the
apparatus
with
respect
to
the motion of the earth. Without
dwelling
on
these
experiments,
let
me only say
that
all
of them
gave negative
results.
Thus,
Fizeau's
experiment
led to
the
hypothesis
of
the relative
motion of
the
ether
with
respect
to
moving
bodies.
None of
the
other
experiments
confirmed this
hypothesis.
The
theory
of
Lorentz4
provided,
at
least
partly,
a
key
to
the solution of
this
puzzle.
A uniform
translation of
velocity v
of the
apparatus
with
respect
to
the ether
does have
an
influence
on
the
phenomena,
but
this
influence
on
the distribution of
visible
light
intensities
can
only
be
detected
starting
with
the
terms
of the order of
(v/c)2
in
the
Lorentz
equations
that
give
this
distribution,
c
being
the
velocity
of
light
in
a
vacuum.
This
seemed, therefore,
to
explain
the
negative
results of the
experiments
aimed at
showing
the relative motion of the earth
with
respect
to
the ether.
Still,
the
negative
result of
one
of these
experiments presented
a
real headache
for
the theoreticians:
I
am
speaking
of
the famous
investigations by
Michelson
and
Morley.5
These
physicists
based
themselves
on
the
following
observation:
Let M
and
N
be two
points
of
a
solid
body;
suppose
a ray begins
at
M
and travels
toward
N,
where
it
is reflected,
and then
returns
to M.
In
this
case,
if
the
body
undergoes
a
uniform translation
with
respect
to
the
ether,
4It
should be
added,
for the
sake
of
completeness,
that
Mr.
Lorentz
did not
consider
bodies
capable
of
rotating
the
plane
of
polarization
when
they are
not in
a
magnetic
field
(naturally
active
bodies).
5A.
A. Michelson and E. W.
Morley,
Amer. Jour.
of
Science 34
(1887):
333-345.
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