By Don Lincoln, Fermilab
In the mid-1800s, researchers had an unsolved question: What conducts light? Remember that in the early 1800s, Thomas Young had demonstrated pretty convincingly that light was a wave. Furthermore, Einstein’s photoelectric effect was still decades in the future. At the time, the wave theory of light was pretty much unchallenged. But, the problem is that waves don’t move like particles do.
The Wave Theory
Throw a baseball and the ball moves from one spot to another. But if we put a rubber ducky on the surface of a lake far from the shoreline and watch the waves roll in, what we see is that the ducky mostly moves up and down but doesn’t actually get closer to the shore. The wave is the movement of energy through the water. But the water, or the ducky, doesn’t move in the direction the wave moves.
A similar thing is true with sound waves. With sound waves, air molecules move back and forth, but the air itself doesn’t experience any net motion. Thus, for all waves known to 19th century scientists, the waves needed a medium like water or air to pass through.
A second important feature is that relativity applies to most waves that is, the ordinary, 17th century, Galilean relativity. This implies that if we were on a fast-moving train and yelled from one end to the other, we’d measure a speed of sound. However, a person standing at a train station watching the train zoom by would measure a different speed of sound. Sounds travel through a medium and when we talk about the speed of a wave, we mean the speed when the medium is stationary.
When it came to light, the problem for 19th century scientists was that there didn’t seem to be anything through which light could travel. After all, space is completely empty—totally devoid of matter—and light from distant stars makes it to the Earth.
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Which Material Transmits Light?
Of course, scientists of the era didn’t know with 100% certainty that space was a vacuum, but they certainly had made hollow glass vessels and pumped all of the air out of them, leaving only a vacuum. And, when they did that, they could see through the glass and see whatever objects were on the other side of the glass. It was clear that light could pass through a vacuum.
And, if light could pass through a vacuum, the question was what was the material through which light was transmitted? There was a proposed answer that originated in the 1600s, when the question of the wave or particle nature of light was raging. Those who held to the wave theory of light proposed that the substance that transmitted light was called the luminiferous aether, or just aether for short.
Aether was first proposed in ancient Greek times as a substance found above the Earth, thought by some to be a substance that permeated the entire universe. It was everywhere and conducted light.
Light and the Aether
There were a few issues with the aether. One thing was that it wasn’t something that people could directly observe. The other problem was that, in general, waves travel quickly through very stiff and dense materials and slowly through ones that are low density or that can move easily. For instance, sound tends to travel quicker through liquids than through gases and quicker through solids than liquids. Light, as we have seen, is extremely fast. That suggests that the material that carries it should be solid and very, very, stiff. Yet, of course, we’d notice if the universe were solid and stiff, so this was a bit of a problem.
Proponents of the aether simply tabled the problem as a mystery still to be solved.
There was another aspect of light and the aether that troubled late 19th century scientists and that was the question of whether the aether was moving or whether the Earth was moving through the aether. And this particular question was made a little more concrete in the early 1860s, when Scottish physicist James Clerk Maxwell pulled together a ton of work by earlier researchers and invented what we now call Maxwell’s equations.
Maxwell’s equations describe how electric fields and magnetic fields interact and how we really shouldn’t talk about electricity and magnetism independently, but rather that there is but a single phenomenon called electromagnetism. This was significant, but it wasn’t central to the question of light.
However, the connection between electromagnetism and light became crystal clear when Maxwell used his equations and combined them. Taking a dash of electricity and a dollop of magnetism, combined with a pinch of calculus and a generous splash of algebra, he combined his equations and showed that they predicted that there existed electromagnetic waves. Furthermore, he was able to calculate the speed of electromagnetic waves and it turned out that his equations said that they moved at the speed of light.
It is important to note that this is a little different from saying that light actually was an electromagnetic wave, but it would have been a huge coincidence if it wasn’t. And, remember, this was before Einstein published his work that showed that light was also a particle, so people were pretty convinced that Maxwell’s work showed that light was simply an electromagnetic wave.
Common Questions about Transmission of Light and Electromagnetic Waves
With sound waves, air molecules move back and forth, but the air itself doesn’t experience any net motion. Thus, for all waves known to 19th century scientists, the waves needed a medium like water or air to pass through.
When it came to light, the problem for 19th century scientists was that there didn’t seem to be anything through which light could travel.
Maxwell’s equations describe how electric fields and magnetic fields interact and how we really shouldn’t talk about electricity and magnetism independently, but rather that there is but a single phenomenon called electromagnetism.