einstein

The Background
CHAPTER SIX
SPECIAL RELATIVITY 1905
The Bern Clock Tower
Relativity is a simple concept. It asserts that the fundamental laws of physics are the same whatever your state of motion.
For the special case of observers moving at a constant velocity, this concept is pretty easy to accept. Imagine a man in an armchair at home
and a woman in an airplane gliding very smoothly above. Each can pour a cup of coffee, bounce a ball, shine a flashlight, or heat a muffin in a
microwave and have the same laws of physics apply.
In fact, there is no way to determine which of them is “in motion” and which is “at rest.” The man in the armchair could consider himself at rest and
the plane in motion. And the woman in the plane could consider herself at rest and the earth as gliding past. There is no experiment that can prove
who is right.
Indeed, there is no absolute right. All that can be said is that each is moving relative to the other. And of course, both are moving very rapidly
relative to other planets, stars, and galaxies.*
The special theory of relativity that Einstein developed in 1905 applies only to this special case (hence the name): a situation in which the
observers are moving at a constant velocity relative to one another—uniformly in a straight line at a steady speed—referred to as an “inertial
reference system.” 1
It’s harder to make the more general case that a person who is accelerating or turning or rotating or slamming on the brakes or moving in an
arbitrary manner is not in some form of absolute motion, because coffee sloshes and balls roll away in a different manner than for people on a
smoothly gliding train, plane, or planet. It would take Einstein a decade more, as we shall see, to come up with what he called a general theory of
relativity, which incorporated accelerated motion into a theory of gravity and attempted to apply the concept of relativity to it. 2
The story of relativity best begins in 1632, when Galileo articulated the principle that the laws of motion and mechanics (the laws of
electromagnetism had not yet been discovered) were the same in all constant-velocity reference frames. In his Dialogue Concerning the Two Chief
World Systems, Galileo wanted to defend Copernicus’s idea that the earth does not rest motionless at the center of the universe with everything
else revolving around it. Skeptics contended that if the earth was moving, as Copernicus said, we’d feel it. Galileo refuted this with a brilliantly clear
thought experiment about being inside the cabin of a smoothly sailing ship:
Shut yourself up with some friend in the main cabin below decks on some large ship, and have with you there some flies, butterflies, and other
small flying animals. Have a large bowl of water with some fish in it; hang up a bottle that empties drop by drop into a wide vessel beneath it.
With the ship standing still, observe carefully how the little animals fly with equal speed to all sides of the cabin. The fish swim indifferently in all
directions; the drops fall into the vessel beneath; and, in throwing something to your friend, you need throw it no more strongly in one direction
than another, the distances being equal; jumping with your feet together, you pass equal spaces in every direction. When you have observed all
these things carefully, have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You
will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still. 3
There is no better description of relativity, or at least of how that principle applies to systems that are moving at a constant velocity relative to each
other.
Inside Galileo’s ship, it is easy to have a conversation, because the air that carries the sound waves is moving smoothly along with the people in
the chamber. Likewise, if one of Galileo’s passengers dropped a pebble into a bowl of water, the ripples would emanate the same way they would
if the bowl were resting on shore; that’s because the water propagating the ripples is moving smoothly along with the bowl and everything else in
the chamber.
Sound waves and water waves are easily explained by classical mechanics. They are simply a traveling disturbance in some medium. That is
why sound cannot travel through a vacuum. But it can travel through such things as air or water or metal. For example, sound waves move through
room temperature air, as a vibrating disturbance that compresses and rarefies the air, at about 770 miles per hour.
Deep inside Galileo’s ship, sound and water waves behave as they do on land, because the air in the chamber and the water in the bowls are
moving at the same velocity as the passengers. But now imagine that you go up on deck and look at the waves out in the ocean, or that you
measure the speed of the sound waves from the horn of another boat. The speed at which these waves come toward you depends on your motion