Evening journal., 1921-05-28
BY LEIGH PAGE, Ph. d.
assistant Professor OF Physics In
Yale University

chicago IN 1881 ana repeated twice
since with great care. first by Michel
son and Morley and the last time
by Morley and Miller. In order TO
appreciate the Significance OF this
experiment, IL is necessary for the
reader 20 recall some OF the charac-
teristics OF the motion OF light
A century ago the English physic-
list Young and the French physicist
Fresnel showed conclusively that
light is a wave motion similar TO the
water waves which everyone has
seen running across the surface OF a
pond. Even earlier the velocity
with which light travels had been
measured and had been found to
have the enormous value OF $4,000
miles a second This speed is so
prodigious that a shot fired from the
most powerful gun in existence would
appear hardly to move al all as COM
pared with a light wave. In fact.
If a ray Of light could be guided
around the earth at the equator by
a suitable arrangement OF mirrors,
il would circumnavigate the globe
seven times in one second.
Water waves travel on the surface
Of water, sound waves are carried by
the air. but the physicist was very
puzzled TO md out the nature OF the
medium through which light waves
travel. Light passes easily from the
sun to the earth taking about nine
minutes 10 make the ninety three
million mile trip. But for most Of
the distance between the sun and
the earth there Is no matter pres
ent therefore light cannot travel
through matter. Hence the physic-
list was forced to the conclusion that
light travels through a weightless,
unperceived m d ium extending
through all space. and this medium
he named the ether.
Now the earth moves around the
sun in a circle OF ninety-three mil
lion miles radius once a year. It's
speed, therefore. is about eighteen
miles a second. This is not a very
great speed compared TO that OF
light, but still Il is great enough "a
make possible the very interesting
experiment which was thought out
by Nicholson. Michelson's idea
was as follows: Suppose a ray oi
light starts out from the earth in
the same direction as that in which
the earth is moving. Then in one
second II will have moved ahead
$4,000 miles through the ether. bul
in the same time the earth itself vii
have Travelled IS miles forward
Hence at the end Of the second tn,
light wave will be 1S5,9S2 miles IT
front of the earth II. however
the light wave had started out In thu
direction opposite 20 that in which
the earth was moving. IL would b'
1s6.01s miles behind the earth's a
the end Of a second.
Michelson's experiment was ingen
iously devised 10 compare these tw'
velocities. But what was his aston
ishment 20 fnd that at the end Of
second the light wave was just 1S6,
900 miles from It's starting point OF
the earths surface, no matter wheth
er il was travelling with the cart
or against the earth. The carefu
repetition OF the experiment men
toned above confirmed the result
obtained on the first trial. and show
ed that Michelson's conclusions coul
not be in error by more than one pc
cent
This experiment excited a grea
/ deal Of interest among scientists. an'
2 led TO the enunciation Of the prin
ciple OF special relativity by Einstei
B in 1905. Einstein makes the state
I ment that all motion is relative. an
in particular that the measured velo
% city OF light will come out TO be ls6,
D 600 miles a second. no matter no,
l fast the observer who makes IN
81 measurement may be moving, NJ
matter whether the light is travelin
Z with him or against him. Obvious
2 ~ this cannot be true unless spac
] and time are very different froT
p what they are ordinarily suppose
10 be. SO Einstein goes on TO In
- vestigate the characteristics whic
5 space and time must have in orde
Z to fit his principle. First he find
r that an object such as a rod, whic
is moving relative TO the observe
y in the direction Of its length, mus
have a shorter measured length tha
, the same or a similar rod when
/, IS at rest This shortening is nS
= noticeable at such low speeds as an
: ordinarily under observation-sucl
for instance, as the speed of riff
bullet-but for speeds approachin
4 the velocity OF light it becomes ver
marked. Tho Nicholson Morley q
pertinent proves that the shortenir
. actually occurs, as predicted by n
theory. Secondly, a moving cloc
=, runs slow as compared with identcl
clocks at rest. Again the effect
y great enough to be noticed on
II when the speed Is very great ThirS
> ~, the measured mass OF a movir
body becomes greater the faster
goes. This prediction Of the theoi
y has been very accurately conhrms
II in the case OF beta rays, which al
II minute particles shot OFF by radiu
% with velocities almost as great
p the velocity of light. Fourthly. u
y theory offers simple explanation
H. why electric currents attract or r
le pei one another In fact, the actis
le OF every generator and every mot
in this age OF electricity Is indispu
m able evidence Of the correctness
l. the RELATIVELY principle. Finally, Il
lg theory predicts the impossibiity
le motion with a velocity greater th'
"A that Of light. This Is strictly in a
cord with the fact that beta ra
have velocities right up TO the vel
ID city OF light. but never greater.
IS II must be emphasized that all q
effects mentioned above are recipr
cal. II observer A is moving Ia
relative to observer Il with a spe
D. half that of light. then a rod held
u. AS hand with it's length parallel