As long as there have been people, we have tried to understand the world in which we live. It's been a rocky road, and there has been much in the way of pitfalls, blind alleys, and backtracking. Many different cultures have approached this understanding in many ways, with ideas over time weaving and separating like the threads of a tapesty. The threads are at times tangled, at times cleanly woven. Some threads are cut, new threads are introduced, and colors sometimes disappear only to appear later. Parts of the tapestry have led to modern physics, with many ideas that seem strange at first, including the Theory of Relativity.
In the view of Aristotle, the world was very much an absolute place. You could easily tell whether you were moving or stopped. Rocks fell in "natural motion" toward the Earth, and smoke rose into the air, but eventually, even they stopped. The sun, moon, stars, and planets appeared to move and never stopped, but they were embedded in crystalline spheres that turned.
Aristotle's views weren't the only ones to be developed in ancient Greece. Heraclitus held that all things were in motion, or a contant state of flux though possibly not visibly so, a surprisingly modern view. However, Aristotle's ideas prevailed and held sway in European culture for millennia.
Around the time of the Renaissance, many people noticed that some of Aristotle's ideas weren't good enough and set out to improve them. Some people, such as Tycho Brahe, became interested in making more and more accurate measurements of the motions of the planets and the stars. His technology was simple, but it seems that he looked harder and more carefully than anyone had before. This led Johannes Kepler to come up with his three laws of planetary motion, including the idea that the planets moved in ellipses (so much for crystalline spheres). Later, Galileo made some significant advances in astronomy, developed five laws of motion, and came up with the first known modern statement of the principle of relativity, This played an important role in the long-standing question of whether the earth was stationary or moved.
Isaac Newton developed three laws of motion that relied on Galileo's principle of relativity. Relativity also enabled him to produce a theory of gravitation with only an attractive force, improving on Kepler's work. Yet if Galileo came up with relativity, and Newton embraced it, why all the fuss about relativity over the past century?
One important puzzle that has fascinated people is the nature of light. Newton thought that light was "corpuscular," that it came in chunks. As it turned out, he was right, though his reasons for thinking it were wrong. The idea of light as a particle, however, didn't seem to explain how light worked, so most gradually came to think of light as a wave. In the middle of the 20th century, quantum electrodynamics re-established light as a particle, but at the time relativity was being developed, the wave theory was very much in vogue.
The wave theory of light has a problem: all known waves, such as sound and water waves, move through a medium. It was well known that the stiffer the medium, the faster the waves. Sound traveled much faster though an iron bar than through air. Light traveled so fast that it was generally assumed to be instantaneous. In the 19th century, people started to measure the speed and found that, while it was not infininte, it was still really fast, fast enough to be able to circle the Earth seven times a second. This medium seemed to be the luminiferous ether, an old idea of Aristotle. It had to be incredibly stiff for light to go so fast in it, but at the same time incredibly soft and insubstantial. After all, people could walk through it easily. In fact, it was so insubstantial that nobody had ever detected it.
The luminiferous ether seemed to reintroduce a bit of the Aristotelian absolute to the world. Whatever the ether was, it was obviously important enough to be considered, in some sense, an absolute. So, the old relativity of Galileo was called into question. In the middle of the 19th century, James Clerk Maxwell came up with the Maxwell's equations. They unified the phenomena of electrical charge and magnetism. They predicted that, when there was a change in an electric field, a disturbance would travel out from it at the speed of light. This was identified as light and later also as radio. The equations didn't seem to depend on the speed of the source of the light. This was unlike, say, throwing a ball out of a moving car, but it was just like what you'd expect for light in the luminiferous ether. Aha! people thought. Now we have a way of measuring the speed of the Earth through the ether! We just have to set up a light source and a meter stick and see the difference when we point it in the direction the Earth is going compared to some other direction. The speed of the light doesn't depend on how fast the light source is moving, just like the speed of a boat doesn't change the speed of the waves in water. But surely it must depend on how fast the observer is moving through the medium. Sailors can tell their speed by dropping things into the water and watching how the water carries them along, so let's drop some light and watch the ether carry it along.
The most famous attempt was the Michelson-Morley experiment, and it showed no significant difference. It was controversial for many years. The experiment was hard to perform with the existing technology, and people came up with a lot of other reasons to explain away the null result. (Nowadays, cheap lasers, good metallurgy, and precise machining make it easy to do the experiment using high-school quality equipment, and it comes out the same as Michelson-Morley.) Eventually, a consensus emerged that the null results were real, and the idea of the ether mostly faded from popularity.
At first many people thought that maybe Maxwell's equations were wrong. Since they were newer, it seemed more plausible for them to be wrong than what had been believed for hundreds of years. Some tried changing Maxwell's equations to have terms for the speed of the observer, but this seemed to predict other effects that were not confirmed by experiment. After a while, a consensus emerged that Maxwell's equations
were also probably correct, or at least correct enough that the solution to the puzzle must lie elsewhere.
Hendrik Lorenz tinkered with the numbers and came up with the idea that the Michelson-Morley experiment could be explained if you assumed that objects shortened in the direction of travel by a certain amount. The equations turned out to be mostly correct, and we still refer to most of the math in the Special Theory of Relativity as the Lorenz transformations. They are improvements over the Galilean transformations, the common ideas of speeds adding up that are still good enough for most everyday purposes.
Henri Poincaré suggested that the old assumptions were wrong, that no matter how counterintuitive it sounded, there should be no way at all to tell whether you were moving or at rest or how fast you were moving except relative to something else, and so resurrected the principle of relativity.
This meant that either the speed of something was affected both by the speed of the source and the speed of the observer (like a ball thrown out of a car), or it was affected by neither. If the speed were ever affected by one but not the other, then all we'd have to do is make sure that the source and the observer were stationary relative to each other and detect a variation in speed from what we would expect. Poincaré presumed this was impossible.
To see this more clearly, consider that only Maxwell's equations or only the null result of the Michelson-Morley experiment, taken separately, don't pose much of a problem. The two, taken together, lead to the problem. Consider the four possibilities:
- The speed depends on the speed of the source and the speed of the observer.
This works fine for rocks, baseballs, rockets, etc. but is inconsistent with Maxwell's equations when applied to light
- The speed depends on the speed of the source but not the speed of the observer.
This is inconsistent with Maxwell's equations for light and doesn't work for rocks, either.
- The speed depends on the speed of the observer but not the speed of the source.
This is inconsistent with the null results of the Michelson-Morley experiment and still doesn't work for rocks.
- The speed depends neither on the speed of the source nor on the speed of the observer.
This is consistent with both Maxwell's equations and the null results of the Michelson-Morley experiment for light. It doesn't work for rocks.
Therefore, to have ideas consistent with what had been observed and with the equations that nobody had been able to break, it became clear that the speed of light had to be completely independent of both the speed of the source and the speed of the observer. While ordinary matter works like case 1, light works like case 4. This basic idea is the starting point for the Theory of Relativity. In Galilean relativity, all speeds were relative both to the speed of the source and the speed of the observer. To have a special kind of thing, light, with a special speed relative to neither required a lot of rethinking and led to some conclusions that seem very strange indeed. It was ironic that the notion that the speed of light was relative had to be abandoned to save relativity, but that's the way it was.
As can be seen, the ideas of relativity were developed by many people. The basic principle was from Galileo, embraced by Newton, restated and refined by Poincaré . The mathematics was already pretty much figured out by Lorenz and Minkowsky. The experiments were provided by Michelson and Morley and others later. Innumerable others made theoretical contributions as well. In many cases, several people came up with the same ideas independently (such as Lorentz and FitzGerald). All these threads, however, still looked like a big tangle.
In 1905, Albert Einstein added a few of his own threads and weaved the whole into the Special Theory of Relativity, at once a rigorous scientific theory making predictions of its own and beautiful story that made all these weird observations and theories fit together. That is the subject of the next installment.