218 ALBERT EINSTEIN
significantly depending on whether
it was observed from one side of
Earth’s orbit, as our planet moved
away from it at 20 miles/s (30 km/s),
or on the opposite side, when the
observer was moving toward it
at a similar speed.
Measuring Earth’s motion
through the ether became an
obsession for late 19th-century
physicists. Such a measurement
was the only way of confirming
the existence of this mysterious
substance, and yet the proof
remained elusive. However precise
the measuring equipment, light
always seemed to move at the
same speed. In 1887, US physicists
Albert Michelson and Edward
Morley devised an ingenious
experiment to measure the
so-called ether wind with high
precision, but once again found
no evidence for its existence. The
negative result of the Michelson-
Morley experiment shook the belief
in the ether’s existence, and similar
results from attempts to repeat it
over the following decades only
intensified the sense of crisis.
Einstein’s third 1905 paper, On
the Electrodynamics of Moving
Bodies, confronted the problem
head on. Special relativity, as
his theory became known, was
developed from an acceptance of
two simple postulates—that light
moves through a vacuum with a
fixed speed that is independent
of the motion of the source, and
that the laws of physics should
appear the same to observers in
all “inertial” frames of reference—
that is, those not subject to external
forces such as acceleration.
Einstein was undoubtedly helped
in accepting the first bold postulate
by his previous acceptance of
the quantum nature of light—
conceptually, light quanta are often
imagined as tiny self-containedpackets of electromagnetic energy,
able to travel through the vacuum
of space with particle-like
properties while still maintaining
their wavelike characteristics.
Accepting these two
postulates, Einstein considered
the consequences for the rest
of physics, and mechanics in
particular. In order for the laws of
physics to behave in the same way
in all inertial reference frames, they
would necessarily appear to be
different when looking from one
frame to another. Only relative
motion mattered, and when the
relative motion between two
separate frames of reference
approached the speed of light
(“relativistic” speeds) strange
things began to happen.The Lorentz factor
Although Einstein’s paper made
no formal references to other
scientific publications, it did
mention the work of a handful
of other contemporary scientists,
for Einstein was certainly not the
only person working toward an
unorthodox solution to the ether
crisis. Perhaps the most significant
of these was Dutch physicistMass and energy are both
but different manifestations
of the same thing.
Albert EinsteinAlbert Einstein Born in the southern German city
of Ulm in 1879, Einstein had a
somewhat bumpy secondary
education, eventually training at
Zurich Polytechnic to become a
mathematics teacher. After failing
to find teaching work, he took a
job at the Swiss Patent Office in
Bern, where he had plenty of
spare time to develop the papers
published in 1905. He attributed
his success in this work to the fact
that he had never lost his childlike
sense of wonder.
Following the demonstration
of general relativity, Einstein
was propelled to global stardom.He continued to explore the
implications of his earlier work,
contributing to innovations in
quantum theory. In 1933, fearing
the rise of the Nazi party,
Einstein elected not to return
to Germany from a foreign tour,
settling eventually at Princeton
University in the United States.Key works1905 On a Heuristic Viewpoint
Concerning the Production and
Transformation of Light
1915 The Field Equations
of Gravitation