Albert Abraham Michelson was born on December 19, 1852 in Strelno, Prussia
(today Strzelno, Poland). He came to the United States with his parents when
he was two years old. From New York, the family made its way to Virginia
City, Nevada and San Francisco.
At 17, Michelson entered the U.S. Naval Academy at Annapolis, Maryland,
and graduated in 1873. Early on Michelson was fascinated with the sciences
and the problem of measuring the speed of light in particular. After two
years of studies in Europe he resigned from the navy in 1881 and set out
to determine the speed of light with an unprecedented accuracy using his
interferometer. His value remained the best for a generation and when it
was improved, Michelson was the one who did it.
In 1883 he accepted a position as professor of physics at the Case
School of Applied Science in Cleveland and concentrated on improving his
interferometer. By 1887 with the help of his colleague Edward Williams Morley
he conducted what was to be known as the Michelson-Morley experiment. Their
experiment showed that there was no significant motion of the Earth relative
to the ether, the hypothetical medium in which light waves were supposed
to travel. This result later became the foundation of Einstein's Theory of
After serving as professor at Clark University at Worcester, Massachusetts
from 1889, in 1892 Michelson was appointed professor and the first head of
the department of physics at the newly organized University of Chicago. In
1907 Michelson became the first American to receive a Nobel prize in physics.
Michelson died on May 9, 1931, in Pasadena, California.
Michelson's interferometer and the ether debate
The nature of light was a matter of intense study during the second half
of last century and until the beginning of this century. There was mounting
evidence that light is an electromagnetic wave that behaves according to
Maxwell's formulas. The strongest and long-known argument was that white
light can be split up into a spectrum of colors through a prism or a diffraction
grating. The reason that diffraction is not more commonly observed was readily
explained by the short wavelength of light. However, so far every wave had
to have a medium in which it traveled. Sound waves travel through air, ocean
waves on water and so forth. Light was known to travel through the apparent
empty space of evacuated laboratory vessels and through the vast distances
of interstellar space. This puzzle left physicists to wonder what the medium
was that light was traveling in.
At this point scientists had postulated a hypothetical medium that
they called the "ether" (also spelled "aether") that was thought to be all
pervading, or penetrating any enclosure with ease. However, if that were
true, the motion of the Earth around the Sun would result in a noticeable
motion of the Earth relative to the ether. Much like a boat cruising over
the ocean, this motion should be measurable. Earth's velocity in its orbit
is substantial at about 30 kilometers per second (18.6 miles per second),
but it is still only 1 ten-thousanth of the speed of light. Therefore, any
measurements of this effect had to be extremely accurate.
The situation is much like having two swimmers go the same distance
but in different directions or paths. The first swimmer swims the width of
a river that is 50 meters wide. The second swimmer swims up-stream, 50 meters
along the bank of the river. If both swim at the same speed relative to the
water, the swimmer who has to swim up-stream will take longer to return than
the swimmer who has to swim the width of the river.
Michelson used the same principle of the swimmers in his first interferometer.
He split up one ray of light into two beams and sent them on two equally
long, separate paths that are at a right angle to each other. Then he reunited
the two light beams into one. He expected any change in the time the two
beams would take along their paths to change the relative position of the
crests and troughs of the two light waves. This would result in a changing
interference pattern that he would be able to observe.
The surprising result of this experiment, and it has since been
repeated over and over again with very high accuracy, is that there is no
measurable motion of the Earth relative to the ether. This result left physicists
stunned for many years and some of them postulated that the ether, while
real, was in principle unobservable. Albert Einstein finally took a brave
step forward with the publication of his theory of special relativity in
1906. His apparently innocent and reasonable argument was that, if the ether
was unobservable, or in other words that there was no experimental proof
of it whatsoever, the simplest explanation was that it did not exist.
The surprising consequence of this innocent statement was however,
that time itself did pass at different rates along the two paths of Michelson's
interferometer. It was this stunning consequence that took scientists years
to accept and only in light of the overwhelming experimental proof did they
accept as fact what seemingly runs against all intuition.
The first measurement of a star's diameter
The stars on the night sky appear point-like because the distance to
them is so large that our eyes do not resolve their disks. The only star
on the sky that we can see as an extended disk is our own star, the Sun.
Remember that light from the Sun travels only eight minutes to reach Earth,
whereas the light travelling time even from the closest star, Proxima Centauri,
is more than 4 years.
This large distance is the reason why we need extremely high resolving
telescopes to see and measure the disks of even the closest stars. Here,
the resolving power of a telescope is determined by the diameter, or the
largest distance between two mirror elements, of a telescope.
In 1919, Albert Michelson enhanced the resolution of the then largest
telescope in the world, the 100-inch Hooker telescope on Mt. Wilson, to measure
for the first time the diameter of a star. Together with his colleague Pease,
Michelson mounted a 20-foot long beam that carried small mirrors on top of
the 100-inch telescope. By adding these mirrors, they had increased the effective
diameter of the telescope and therefore the resolution of the telescope.
The increase in resolution was sufficent to measure for the first time the
diameter of the bright, red giant star Betelgeuse. It was also the debut
of interferometry in astronomy when it was applied to overcome the limitations
of existing instruments.