Michael Fowler, UVa Physics 3/3/08
At this point in the course, we finally enter the twentieth century—Albert
Einstein wrote his first paper on relativity in 1905. To put his work in context, let us first
review just what is meant by “relativity” in physics. The first example, mentioned in a
previous lecture, is what is called “Galilean relativity” and is
nothing but Galileo’s perception that by observing the motion of objects,
alive or dead, in a closed room there is no way to tell if the room is at rest
or is in fact in a boat moving at a steady speed in a fixed direction. (You can tell if the room is
accelerating or turning around.) Everything
looks the same in a room in steady motion as it does in a room at rest. After
What exactly do we mean by a frame “at rest” anyway? This seems obvious from our perspective
as creatures who live on the surface of the earth—we mean, of course, at
rest relative to fixed objects on the earth’s surface. Actually, the earth’s rotation
means this isn’t quite a fixed frame, and also the earth is moving in
orbit at 18 miles per second. From
an astronaut’s point of view, then, a frame fixed relative to the sun
might seem more reasonable. But why
stop there? We believe the laws of
physics are good throughout the universe. Let us consider somewhere in space far
from the sun, even far from our galaxy. We would see galaxies in all directions,
all moving in different ways. Suppose we now set up a frame of
reference and check that
As we mentioned in the last lecture, in the middle of the nineteenth century there was a substantial advance in the understanding of electric and magnetic fields. (In fact, this advance is in large part responsible for the improvement in living standards since that time.) The new understanding was summarized in a set of equations called Maxwell’s equations describing how electric and magnetic fields interact and give rise to each other, just as, two centuries earlier, the new understanding of dynamics was summarized in the set of equations called Newton’s laws. The important thing about Maxwell’s equations for our present purposes is that they predicted waves made up of electric and magnetic fields that moved at 3×108 meters per second, and it was immediately realized that this was no coincidence—light waves must be nothing but waving electric and magnetic fields. (This is now fully established to be the case.)
It is worth emphasizing that Maxwell’s work predicted the speed of light from the results of experiments that were not thought at the time they were done to have anything to do with light—experiments on, for example, the strength of electric field produced by waving a magnet. Maxwell was able to deduce a speed for waves like this using methods analogous to those by which earlier scientists had figured out the speed of sound from a knowledge of the density and the springiness of air.
We now come to Einstein’s major insight: the Theory of Special Relativity. It is deceptively simple. Einstein first dusted off Galileo’s discussion of experiments below decks on a uniformly moving ship, and restated it as :
The Laws of Physics are the same in all Inertial Frames.
Einstein then simply brought this up to date, by pointing out that
the Laws of Physics must now include Maxwell’s equations describing
electric and magnetic fields as well as
Demanding that Maxwell’s equations be satisfied in all inertial frames has one major consequence as far as we are concerned. As we stated above, Maxwell’s equations give the speed of light to be 3×108 meters per second. Therefore, demanding that the laws of physics are the same in all inertial frames implies that the speed of any light wave, measured in any inertial frame, must be 3×108 meters per second.
This then is the entire content of the Theory of Special Relativity: the Laws of Physics are the same in any inertial frame, and, in particular, any measurement of the speed of light in any inertial frame will always give 3×108 meters per second.
Just as Galileo had asserted that observing gnats, fish and dripping bottles, throwing things and generally jumping around would not help you to find out if you were in a room at rest or moving at a steady velocity, Einstein added that no kind of observation at all, even measuring the speed of light across your room to any accuracy you like, would help find out if your room was “really at rest”. This implies, of course, that the concept of being “at rest” is meaningless. If Einstein is right, there is no natural rest-frame in the universe. Naturally, there can be no “aether”, no thin transparent jelly filling space and vibrating with light waves, because if there were, it would provide the natural rest frame, and affect the speed of light as measured in other moving inertial frames as discussed above.
So we see the Michelson-Morley experiment was doomed from the start. There never was an aether wind. The light was not slowed down by going “upstream”—light always travels at the same speed, which we shall now call c,
c = 3×108 meters per second
to save writing it out every time. This now answers the question of what the speed of light, c, is relative to. We already found that it is not like sound, relative to some underlying medium. It is also not like bullets, relative to the source of the light (the discredited emitter theory). Light travels at c relative to the observer, since if the observer sets up an inertial frame (clocks, rulers, etc.) to measure the speed of light he will find it to be c. (We always assume our observers are very competent experimentalists!)
The Truth we are referring to here is the seemingly innocuous and plausible sounding statement that all inertial frames are as good as each other—the laws of physics are the same in all of them—and so the speed of light is the same in all of them. As we shall soon see, this Special Theory of Relativity has some surprising consequences, which reveal themselves most dramatically when things are moving at relative speeds comparable to the speed of light. Einstein liked to explain his theory using what he called “thought experiments” involving trains and other kinds of transportation moving at these speeds (technically unachievable so far!), and we shall follow his general approach.
To begin with, let us consider a simple measurement of the speed of light carried out at the same time in two inertial frames moving at half the speed of light relative to each other. The setup is as follows: on a flat piece of ground, we have a flashlight which emits a blip of light, like a strobe. We have two photocells, devices which click and send a message down a wire when light falls on them. The photocells are placed 10 meters apart in the path of the blip of light, they are somehow wired into a clock so that the time taken by the blip of light to travel from the first photocell to the second, in other words, the time between clicks, can be measured. From this time and the known distance between them, we can easily find the speed of the blip of light.
Meanwhile, there is another observer, passing overhead in a spaceship traveling at half the speed of light. She is also equipped with a couple of photocells, placed 10 meters apart on the bottom of her spaceship as shown, and she is able to measure the speed of the same blip of light, relative to her frame of reference (the spaceship). The observer on the spaceship will measure the blip of light to be traveling at c relative to the spaceship, the observer on the ground will measure the same blip to be traveling at c relative to the ground. That is the unavoidable consequence of the Theory of Relativity.
(Note: actually the picture above is not quite the way it would really look. As we shall find, objects moving at relativistic speeds are contracted, and this combined with the different times light takes to reach the eye from different parts of the ship would change the ship’s appearance. But this does not affect the validity of the statements above.)