DOC.
2
RELATIVITY AND
ITS CONSEQUENCES
121
the
theory predicts
that the
time
t
needed
for
the
light
to travel
the
closed
path
MNM
will
vary
depending
on
whether the
body is
moving
in
the direction MN
or
perpendicu-
lar
to
it.
True,
the difference
is
very
small, being
of the order of
,
that
is to
say,
of
v2
c
the order of
magnitude
10-8
if
v
is
taken
as
the
velocity
of the earth. But
Michelson
and
Morley
were
able
to
devise
an
interference
experiment
in which this
slight
difference
would
have to become detectable.
The essential features of their
arrangement were as
follows:
Light rays coming
from
the
source
S
(Fig.
1)
are
split
into
two beams
by
means
B/'
I
B
ts
Fig. 1.
of
a
transparent
mirror
at A.
One of the
beams
is
then reflected
at B
and
returns to
A,
where
it
splits
and
yields
a
ray traveling
to I.
The other
crosses
the mirror and
travels
to B',
where
it
is
reflected
toward
A;
there
it
splits, yielding,
too, a ray
that
goes
to
I,
where the
two
rays
interfere. The
position
of the
fringes
depends
on
the difference
between the
routes ABA
and
AB'A
taken
by
the
two
rays
during
their
travels. This
difference between
the routes should have
depended on
the orientation of
the
equipment;
one
should have
observed
a
displacement
of
the
fringes
the
moment
AB',
instead of
AB,
coincided with
the direction of the earth's motion.
However,
nothing
of the
kind
was
observed,
and
as a
result the foundation of Lorentz's
theory
seemed
extremely shaky.
To
save
the
theory,
Lorentz
and
FitzGerald resorted
to
a strange hypothesis:
they
assumed
that
each
body
in
motion
with
respect
to
the ether
contracts in
the direction of motion
[8]
by
a
fraction
equal to
\2
,
or-which
amounts to
the
same
if
only
terms
of
second
1
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