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In the early years of the last century, R.A. Millikan measured the charge of the electron. He was one of the greatest experimentalists to ever live, not only isolating and measuring something so incredibly small but doing other important work with things like cosmic rays. As such, when I saw he had a book, named The Electron, I figured he ought to know a thing or two about the subject. He did; it’s a complete, informative, and up-to-date book, so long as that date occurs within World War I.
The book has been eye-opening, not because of the new physics, but because of all the outmoded and discarded theories that he mentions and dismisses on the way. What if electrons didn’t have a fixed charge, but a statistical distribution that averaged out to what we think of as a fixed charge? This was a viable theory until Millikan disproved it looking at his oil droplets. What really got me though was when he spent his last chapter describing wave-particle duality. Only there was no such thing when he wrote the book. At that point all modern physics had was a real head-scratcher of a problem. Sample quote:
To be living in a period which faces such a complete reconstruction of our notions as to the way in which either waves are absorbed and emitted by matter is an inspiring prospect.
Wait, ether waves? Let’s begin early. How does light get from the sun to you, the discerning customer? The Greeks would have told you it emits minute particles, but they hadn’t tumbled to the notion of measuring the world experimentally, so their explanations are suspect. Ol’ Ike Newton liked that theory pretty well and he put it on a sound mathematical basis. About 1680, a Dutch feller name of Huygens watching ripples in water proposed that light was actually a wave traveling through an interstellar medium, the luminiferous ether. During the 1800s, the ether theory reigned supreme because it was best able to explain what light does. However you describe it, your description has to fit what we see light actually doing:
- Light interferes with itself. That is, it generates patterns of high and low intensity much like any wave would.
- The speed of light in water is slower than that of air (which is slower than light in a vacuum).
- Radio waves exist, and as you lengthen the wavelength of a radio wave, it becomes a static electric field.
- The speed of light is independent of the motion of the source.
Add to that the presence of an ether is useful for describing electricity and magnetism. An electrostatic field is a strain in the ether. These days we don’t believe in ether, but we still talk about electrostatic fields. At least I paid a college professor good money to talk about ’em for a whole semester. Doesn’t sound like such a good idea in retrospect.
As the year turned 1900, life was pretty good for a proponent of the luminiferous ether. All four of those tests are difficult to impossible to explain with a “corpuscular” theory where light is a particle, and relatively easy to explain where light is a wave. The problems come when you look at some exciting new experiments.
- X-rays passing through matter will eject not every electron, but only one out of a multitude.
- The photoelectric effect — where shining light on a metal produces an electric current — depends only on the wavelength of the light, not on the total amount of energy. A strong red light won’t produce electrons when a weak blue light might.
If light is a wave then the total energy of that wave is spread out uniformly over that wavefront. Why then does only one electron get the boot by your x-ray, not every single one the wave passes? J. J. Thompson, the guy shooting x-rays in this instance (There were a lot of people flinging x-rays around back then. I gather they took lumps of radium to parties to irradiate stuff and see what happened.) Thompson proposed that the ether wasn’t a continuous medium, but instead consisted of innumerable strings binding the universe together, and that electromagnetic forces travel down these lines.
Now, how these strings don’t get tangled like the backside of a TV cabinet nobody ever seems to have explained. But what that does do is that it lets your light travel as a wave, but keep all the energy localized in one spot. That lets your x-rays only hit the occasional electron all right, but then how does one ether ray interfere with another?
Right about this time the clock strikes 1905. Probably anyone who’s read this far knows what’s coming. A furry little patent clerk, name of Albert Einstein, shows up on the scene. He drops four scientific papers like a rapper droppin’ the mic and struts off stage. Along with special relativity, and an explanation of Brownian motion (another thing Millikan experimentally confirmed), he explains the photoelectric effect by means of Thompson’s ether-strings theory. He borrows Plank’s constant and describes with it the energy of escaping electrons.
Now, at the moment, nobody was able to test that. But in short order, Einstein’s equations were tested and shown to be valid all along. So all’s peachy in string land? Well, nobody’s worked out how one string interferes with another, but there’s another problem. If all the ether is strings, then it can’t possibly vary continuously in an electrostatic field. This is something Millikan could see and measure with his oil droplets. Therefore, no strings. But if there are no strings, how come Einstein’s equation describing the photoelectric effect works? That question baffled Millikan.
There’s one more way to explain why x-rays only eject the occasional electron, rather than all of them. What if the x-ray doesn’t provide the energy? What if the energy is stored up in the atom, and the x-ray only triggers it in passing? Ignoring how precisely the triggering works, this would explain why only the occasional electron gets ejected. The problem then has to do with why the ejected electron’s energy depends on the incident x-ray’s frequency. It makes sense if that’s where the energy comes from, but if the atom provides it, then why does the frequency matter? Does each atom have a number of electrons for any given frequency waiting to be ejected?
That’s the point where Millikan finishes his book; he doesn’t have the answer. The real answer, by which I mean the accepted by a century’s worth of physicists since, is that light exists both as a wave and a particle, that everything when you get that small does. In short, that quantum mechanics and all its freakiness exists. And if you’re not satisfied with that explanation, well, now you know why the alternatives didn’t cut it, and what questions you’d have to answer to provide your own. Good luck.Published in