einstein

elative to the medium (the water or air) propagating them.
In other words, the speed at which an ocean wave reaches you will depend on how fast you are moving through the water toward or away from
the source of the wave. The speed of a sound wave relative to you will likewise depend on your motion relative to the air that’s propagating the
sound wave.
Those relative speeds add up. Imagine that you are standing in the ocean as the waves come toward you at 10 miles per hour. If you jump on a
Jet Ski and head directly into the waves at 40 miles per hour, you will see them moving toward you and zipping past you at a speed (relative to you)
of 50 miles per hour. Likewise, imagine that sound waves are coming at you from a distant boat horn, rippling through still air at 770 miles per hour
toward the shore. If you jump on your Jet Ski and head toward the horn at 40 miles per hour, the sound waves will be moving toward you and zipping
past you at a speed (relative to you) of 810 miles per hour.
All of this led to a question that Einstein had been pondering since age 16, when he imagined riding alongside a light beam: Does light behave
the same way?
Newton had conceived of light as primarily a stream of emitted particles. But by Einstein’s day, most scientists accepted the rival theory,
propounded by Newton’s contemporary Christiaan Huygens, that light should be considered a wave.
A wide variety of experiments had confirmed the wave theory by the late nineteenth century. For example, Thomas Young did a famous
experiment, now replicated by high school students, showing how light passing through two slits produces an interference pattern that resembles
that of water waves going through two slits. In each case, the crests and troughs of the waves emanating from each slit reinforce each other in some
places and cancel each other out in some places.
James Clerk Maxwell helped to enshrine this wave theory when he successfully conjectured a connection between light, electricity, and
magnetism. He came up with equations that described the behavior of electric and magnetic fields, and when they were combined they predicted
electromagnetic waves. Maxwell found that these electromagnetic waves had to travel at a certain speed: approximately 186,000 miles per
second.* That was the speed that scientists had already measured for light, and it was obviously not a mere coincidence. 4
It became clear that light was the visible manifestation of a whole spectrum of electromagnetic waves. This includes what we now call AM radio
signals (with a wavelength of 300 yards), FM radio signals (3 yards), and microwaves (3 inches). As the wavelengths get shorter (and the frequency
of the wave cycles thus increases), they produce the spectrum of visible light, ranging from red (25 millionths of an inch) to violet (14 millionths of an
inch). Even shorter wavelengths produce ultraviolet rays, X-rays, and gamma rays. When we speak of “light” and the “speed of light,” we mean all
electromagnetic waves, not just the ones that are visible to our eyes.
That raised some big questions: What was the medium that was propagating these waves? And their speed of 186,000 miles per second was a
speed relative to what?
The answer, it seemed, was that light waves are a disturbance of an unseen medium, which was called the ether, and that their speed is relative
to this ether. In other words, the ether was for light waves something akin to what air was for sound waves. “It appeared beyond question that light
must be interpreted as a vibratory process in an elastic, inert medium filling up universal space,” Einstein later noted. 5
This ether, unfortunately, needed to have many puzzling properties. Because light from distant stars is able to reach the earth, the ether had to
pervade the entire known universe. It had to be so gossamer and, shall we say, so ethereal that it had no effect on planets and feathers floating
through it. Yet it had to be stiff enough to allow a wave to vibrate through it at an enormous speed.
All of this led to the great ether hunt of the late nineteenth century. If light was indeed a wave rippling through the ether, then you should see the
waves going by you at a faster speed if you were moving through the ether toward the light source. Scientists devised all sorts of ingenious devices
and experiments to detect such differences.
They used a variety of suppositions of how the ether might behave. They looked for it as if it were motionless and the earth passed freely through
it. They looked for it as if the earth dragged parts of it along in a blob, the way it does its own atmosphere. They even considered the unlikely
possibility that the earth was the only thing at rest with respect to the ether, and that everything else in the cosmos was spinning around, including
the other planets, the sun, the stars, and presumably poor Copernicus in his grave.
One experiment, which Einstein later called “of fundamental importance in the special theory of relativity,” 6 was by the French physicist Hippolyte
Fizeau, who sought to measure the speed of light in a moving medium. He split a light beam with a half-silvered angled mirror that sent one part of
the beam through water in the direction of the water’s flow and the other part against the flow. The two parts of the beam were then reunited. If one
route took longer, then the crests and troughs of its waves would be out of sync with the waves of the other beam. The experimenters could tell if this
happened by looking at the interference pattern that resulted when the waves were rejoined.
A different and far more famous experiment was done in Cleveland in 1887 by Albert Michelson and Edward Morley. They built a contraption that
similarly split a light beam and sent one part back and forth to a mirror at the end of an arm facing in the direction of the earth’s movement and the
other part back and forth along an arm at a 90-degree angle to it. Once again, the two parts of the beam were then rejoined and the interference
pattern analyzed to see if the path that was going up against the supposed ether wind would take longer.
No matter who looked, or how they looked, or what suppositions they made about the behavior of the ether, no one was able to detect the elusive
substance. No matter which way anything was moving, the speed of light was observed to be exactly the same.
So scientists, somewhat awkwardly, turned their attention to coming up with explanations about why the ether existed but was undetectable in any
experiment. Most notably, in the early 1890s Hendrik Lorentz—the cosmopolitan and congenial Dutch father figure of theoretical physics—and,
independently, the Irish physicist George Fitzgerald came up with the hypothesis that solid objects contracted slightly when they moved through the
ether. The Lorentz-Fitzgerald contraction would shorten everything, including the measuring arms used by Michelson and Morley, and it would do so
by just the exact amount to make the effect of the ether on light undetectable.
Einstein felt that the situation “was very depressing.” Scientists found themselves unable to explain electromagnetism using the Newtonian
“mechanical view of nature,” he said, and this “led to a fundamental dualism which in the long run was insupportable.” 7
Einstein’s Road to Relativity
“A new idea comes suddenly and in a rather intuitive way,” Einstein once said. “But,” he hastened to add, “intuition is nothing but the outcome of
earlier intellectual experience.” 8
Einstein’s discovery of special relativity involved an intuition based on a decade of intellectual as well as personal experiences. 9 The most
important and obvious, I think, was his deep understanding and knowledge of theoretical physics. He was also helped by his ability to visualize
thought experiments, which had been encouraged by his education in Aarau. Also, there was his grounding in philosophy: from Hume and Mach he