DOC.
21
THEORY OF RELATIVITY
251
theory.
In
a
very
ingenious
theoretical
investigation,
H. A.
Lorentz showed
that,
to
a
first
degree
of
approximation,
the relative motion
is
without effect
on
the
way rays
proceed
in
arbitrary optical experiments.
There remained
only
one
optical experiment
[9]
employing
a
method
so
extremely
sensitive that
its
negative
outcome
could
not
be
accounted for
even by
H. A.
Lorentz's theoretical
analysis.
This
was
the
already
mentioned
experiment
of
Michelson,
the
setup
of which
was
in
essence
the
following.
Michelson's
Experiment
The
light ray
L
from
a
light
source
G
arrives
first
at
a
semitransparent
mirror
S,
where it
is
split
into
two
rays.
The first of these
travels toward the mirror
s1,
is
reflected
at
it,
comes
back
to
S,
and
is
then reflected
(partly)
toward
E;
the second
ray
travels
to
Fig.
2.
s2,
is
reflected
there,
travels back
to
S,
and,
after
passing through
S,
likewise heads
toward
E.
At E the
two
rays
interfere. The
whole
arrangement
that
we
have
just
described
was
mounted
on a
stone
plate
that
was
floating
on
mercury
so
that the
arrangement
as a
whole could be
brought
into
different
positions
relative
to
the
hypothetical
motion of the Earth with
respect
to
the
luminiferous
ether.
According
to
the
theory,
the
change
in
the
orientation
of
the
stone
plate
should have exerted
an
influence
on
the
position
of the interference
fringes
at
E that
was
large enough
to
be verified.
However,
the
experiment
turned
out
negative.
To reconcile the
negative
result of
this
experiment
with the
theory,
H. A.
Lorentz
and FitzGerald
set
forth the
hypothesis
that the
stone
plate together
with
all
the
objects
mounted
on
it
experience
a
minuscule
contraction in the
direction
of
the
Earth's
motion,
the
magnitude
of which
is
such that the
expected
effect
is
compen-
sated
by
an
opposite
effect
resulting
from this contraction.
[10]
The
Basic Idea
of the
Theory
of
Relativity
This
manner
of
providing
theoretical
justification
for
experiments
with
a
negative
outcome
by means
of
hypotheses
invented ad hoc
is
very
unsatisfactory.
It
becomes
[11]
hard
to
avoid the idea that
no
reality
attaches
to
this relative motion of the Earth with
respect
to
the
system
K,
i.e.,
that
it is in
principle impossible
to
demonstrate such
a
relative
motion. In other words:
we
arrive
at
the conviction that
the
relativity