Einstein's third 1905 paper was entitled "On the Electrodynamics of
Moving Bodies." Although this paper challenged foundational notions
about space and time, each of its parts was simply a response to
an important problem facing the physics community of Einstein's
time.
One of these three challenges Einstein addresses is the
relationship between Maxwell's electromagnetic equations and the mechanical
worldview. Scientists in Einstein's day searched for a unifying
theory that would explain both electromagnetism and mechanics.
Einstein was attracted to this problem because he was troubled by
an electromagnetic principle that did not make sense according to
the mechanical world view: Faraday's 1831 magnet-coil experiment.
In this experiment, a magnet is moved near an electric circuit,
and then the circuit is moved near the magnet. According to Faraday,
an electric current should be formed whenever there is relative
movement, regardless of whether the magnet or the circuit is moving.
However, according to Maxwell's equations, an electric current
is only induced when the circuit is at rest and the magnet moving.
This asymmetrical explanation disturbed Einstein, who was committed
to aesthetic principles in his science. In order to resolve this
asymmetry, Einstein analyzed the arrangement of magnet and current
in terms of relative movement. He proposed that the existence of
an electric current depends on the relative velocity of the magnet
and circuit with respect to one another. His relativity theory
wa s thus the product of his aesthetic discomfort with an asymmetrical
explanation.
Einstein was not the first to formulate a relativity theory,
however: Galileo had
considered the concept in the early seventeenth century. According
to Galilean relativity, the laws of mechanics are useless to an
observer in a non-accelerating reference frame trying to determine
whether he or she is moving with respect to another reference frame.
When Newton revisited
this problem fifty years later, he attempted to solve it by postulating
an "absolute space" eternally at rest, relative to which any reference
frame was either at rest or in motion. However, the fundamental
pri nciple of relativity remained the same: the laws of mechanics
are the same in all inertial (non-accelerating) reference frames,
so it is impossible to determine whether an observer in one frame
is moving or stationary with respect to another frame of refe rence.
In Einstein's day, physicists questioned whether the relativity principle
could be applied to electrodynamic theory as well. Was it also
true that the laws of electrodynamics were the same in all reference
frames? Physicists were particularly interested in whether the earth's
velocity could be detected with respect to the ether, a substance
postulated by scientists as a medium through which light waves travel.
In the 1880s, the American physicists Albert Michelson and Edward
Morley constructed a dev ice called an interferometer to measure
the earth's velocity with respect to the ether, but were unable
to detect any movement. However, there is no evidence that Einstein
was familiar with these results when he dismissed altogether the
concept of the et her in his relativity paper. Einstein claimed
that it is impossible to detect whether or not one is moving with
respect to the ether, rendering meaningless the whole notion of an
ether. His dismissal of the ether also meant that every concept involving
space and time had to be considered in relative terms, a fundamental
challenge to all of nineteenth-century science.
Einstein's relativity theory was presented as a principled,
rather than a constructive, theory. A principled theory is one
that begins with principles and then uses these principles to explain
the phenomena; a constructive theory starts with the observat ions
and culminates in theories that explain and reconcile those observations.
Einstein's principled account began with the postulate that the
laws of science should appear the same to all freely moving observers.
In particular, all observers should mea sure the speed of light
as the same regardless of how fast they are moving. Thus, there
is no "universal time" that all clocks measure; rather, everyone
has his or her own personal time. If one person is moving with
respect to another, their clocks will not agree. To an observer
moving in one frame of reference with uniform velocity relative
to a second frame of reference, the clock in the second frame will
appear to move more slowly than his own clock. Moreover, since
velocity is the measurement of d istance per unit of time, a measuring-stick
in the second time frame would appear contracted to the observer
in the reference frame. Of course, we do not observe these effects
in everyday situations of movement; we do not see a ruler as contracted
if we are moving by on a bus. Rather, these phenomena are noticeable
only at speeds near the speed of light. Nonetheless, Einstein's
relativity paper showed that time and space are not a priori categories
of human understanding; rather, they are relative quan tities that
are defined operationally.
One implication of relativity is the famous "twin paradox,"
a hypothetical situation in which one twin embarks on a journey through
space while the other twin stays on earth. When the first twin
returns home after traveling at a velocity close to the spe ed of light,
he finds that he has aged by merely a couple of years, while his brother
on earth has been long since dead. This is because the twin on
earth has been traveling through space at a constant time (as the earth
orbits the sun), whereas the twin in the spaceship has had to decelerate
and then accelerate in order to turn back home, so she has not remained
in an inertial (non-accelerating) reference frame. This paradox
runs counter to our commonsense view of time, but it is a natural
consequence of relativity theory.
