New-York tribune., 1920-06-13

Let us go back to the observer with
a ring of mirrors surrounding him,
from all Of which the reflections of
his flash Of light reach him at the
same instant. If he thinks that he
is at rest in space he will say that
these mirrors are distributed around
a perfect circle, with his own posi-
ton as center.
Now, suppose he chooses differ
ent frame of reference, in uniform
motion compared with his original
one. That is, suppose that he thinks
that he and the mirrors together
are moving uniformly in some par
ticular direction and at high ve-
locity.
He will now say, "If these mir
rors were really on a circle tho light
would take longer to reach me from
those which were in the direction of
my path than from those at right
angles. Since the light returns
simultaneously from all, the mirrors
are not arranged on a circle but on
an ellipse, which is longer at right
angles to the direction of my motion
than it is the other way"
If, as in the case previously dis
cussed, he supposes himself to be
moving with half the speed of light,
he will conclude that the longer
diameter of this ellipse is about Is
per cent greater than the shorter
diameter. If he estimates his own
velocity higher, he will regard it as
differing still more from a circle.
The Lorentz Theory
But although the mirrors in this
case are not all at equal distances
from him, he cannot find this out by
measuring the distance with a meas
uring rod. In fact, if he does so,
their distance will all appear to be
exactly the same, if the principle of
relativity is true. For otherwise, by
combining an optical experiment and
a direct measurement he would have
a method by which he could distin-
guish between rest and uniform
motion; and this is, by the very
hypothesis, impossible.
Hence nature must be so consti-
tuted that his measuring rod would
automatically change in length when
turned from position parallel to
his motion to one at right angles
to it.
This sounds strange enough, but
something of the sort is entirely
necessary in order to explain the
Michelson-Morley experiment. The
assumption that material bodies,
when moving through space, con-
tract slightly in the direction Of mo,
ton was made by Lorentz in order
to explain this experiment before
the more general theory had been
developed. At such speeds as are
actually reached by the planets in
their orbits the contraction is less
than one part in one hundred mill
lion, and beyond detection by any,
thing except the most refined inves-
LEGATIONS.
We have now seen that, according
to the principle of relativity, the
answer to the question whether two
material rods laid on the table at
right angles to one another are of
the same length or of different
lengths depends on whether we
choose to think that we and the
room in which the apparatus is sit
uated and the rest of the world are
at rest in space or are moving in
different directions with high uni
form speeds.
Measurement of Time
The fact that when the two rods
are laid side by side they are OBJ
viously exactly equal does not prove
that they are the same length when
we turn them so that they make an
angle with one arother.
So much for the measuring of
distances and the measuring of the
lengths of things.
Now, how about measuring time?
Let US go back to our observer
with his mirror and call him A, and
suppose that at the mirror there is
a second observer whom we will call
Is, and that both observers have