By the late 1890s, physicists thought they had it all together All of electricity, all of magnetism, all of light, all of mechanics, all moving things, as well as cosmology and gravity—all were understood by a few simple equations. As for atoms, most chemists felt that the subject was pretty much closed. There was the periodic table of the elements. Hydrogen, helium, carbon, oxygen et al. were indivisible elements, each with its own invisible, indivisible atom.
There were some mysterious cracks in the picture. For example, the sun was puzzling. Using then-current beliefs in chemistry and atomic theory, the British scientist Lord Rayleigh calculated that the sun should have burned up all its fuel in 30,000 years. Scientists knew that the sun was a lot older than that. This aether business was also troubling. Its mechanical properties would have to be bizarre indeed. It had to be totally transparent, capable of slipping between atoms of matter without disturbing them, yet it had to be as rigid as steel to support the huge velocity of light. Still, it was hoped that these and other mysteries would be solved in due time. Had I been teaching back in 1890, I might have been tempted to send my physics students home, advising them to find a more interesting major. All the big questions had been answered. Those issues that were not understood—the sun's energy, radioactivity, and a number of other puzzles—well, everybody believed that sooner or later they would yield to the power of the Newton-Maxwell theoretical juggernaut. Physics had been neatly wrapped up in a box and tied with a bow.
Then suddenly, at the end of the century, the whole package began to unravel. The culprit, as usual, was experimental science.
THE FIRST TRUE PARTICLE
During the nineteenth century, physicists fell in love with the electrical discharges produced in gas-filled glass tubes when the pressure was lowered. A glass blower would fashion an exquisite three-foot-long glass tube. Metal electrodes were sealed into the tube. The experimenter would pump out the air as best he could, then bleed in a desired gas (hydrogen, ait, carbon dioxide) at low pressure. Wires from each electrode were attached to an external battery. Large electrical voltages were applied. Then, in a darkened room, experimenters were awed as splendid glows appeared, changing shape and size as the pressure decreased. Anyone who has seen a neon sign is familiar with this kind of glow. At low enough pressure, the glow turned into a ray, which traveled from the cathode, the negative terminal, toward the anode. Logically, it was dubbed a cathode ray. These phenomena, which we now know to be rather complex, fascinated a generation of physicists and interested laypersons all over Europe.
Scientists knew a few controversial, even contradictory details about these cathode rays. They carried a negative charge. They traveled in straight lines. They could spin a fine paddle wheel sealed into the glass. Electric fields didn't deflect them. Electric fields did deflect them. A magnetic field would cause a narrow beam of cathode rays to bend into a circular arc. The rays were stopped by thick metal but could penetrate thin metal foils.
Interesting facts, but the critical mystery remained: what were these rays? In the late nineteenth century, there were two guesses. Some researchers thought cathode rays were massless electromagnetic vibrations in the aether. Not a bad guess. After all, they glowed like a light beam, another kind of electromagnetic vibration. And obviously, electricity, which is a form of electromagnetism, had something to do with the ray.
Another camp thought the rays were a form of matter. A good guess was that they were composed of gas molecules in the tubes that had picked up a charge from the electricity. Another guess was that cathode rays were composed of a new form of matter, small particles never before isolated. For a variety of reasons, the idea that there is a basic carrier of electric charge was "in the air." We'll let the cat out of the bag right now. Cathode rays weren't electromagnetic vibrations and they weren't gas molecules.
If Faraday had been alive in the late 1800s, what would he have said? Faraday's laws strongly implied that there were "atoms of electricity." As you'll recall, he did some similar experiments, except that he passed electricity through liquids rather than gases, ending up with ions, charged atoms. As early as 1874 George Johnstone Stoney, an Irish physicist, had coined the term "electron" for the unit of electricity that is lost when an atom becomes an ion. Had Faraday witnessed a cathode ray, perhaps he would have known in his heart that he was watching electrons at work.
Some scientists in this period may have strongly suspected that cathode rays were particles; maybe some thought they had finally found the electron. How do you find out? How do you prove it? In the intense period before 1895, many prominent researchers in England, Scotland, Germany, and the United States were studying gaseous discharges. The one who struck pay dirt was an Englishman named J. J. Thomson. There were others who came close. We'll take a look at two of them and what they did, just to show how heartbreaking scientific life is.
The guy who came nearest to beating Thomson was Emil Weichert, a Prussian physicist, who demonstrated his technique to a lecture audience in January 1887. His glass tube was about fifteen inches long and three inches in diameter. The illuminated cathode rays were easily visible in a partially darkened room.
If you're trying to corral a particle, you must describe its charge (e) and mass (m). At the time, the particle in question was too small to weigh. To get around this problem many researchers independently seized upon a clever technique: subject the ray to known electric and magnetic forces and measure its response. Remember F = ma. If indeed the rays were composed of electrically charged particles, the force experienced by the particles would vary with the quantity of charge (e) they carried. The response would be muted by their inertial mass (m). Unfortunately, therefore, the effect that could be measured was the quotient of these two quantities, the ratio elm. In other words, researchers couldn't find individual values for e or m, just a number equal to one value divided by the other. Let's look at a simple example. You are given the number 21 and told that it is the quotient of two numbers. The 21 is a clue only. The two numbers you're looking for might be 21 and 1, 63 and 3, 7 and 1/3, 210 and 10, ad infinitum. But if you have an inkling of what one number is, you can deduce the second.
To go after elm, Weichert put his tube into the gap of a magnet, which bent the beam into an arc. The magnet pushes on the electric charge of the particles; the slower the particles, the easier it is for the magnet to bend them into a circular arc. Once he figured out the speed, the deflection of particles by the magnet gave him a fair value for elm.
Weichert understood that if he made an informed guess as to the value of the electric charge, he could deduce the approximate mass of the particle. He concluded: "We are not dealing with atoms known from chemistry because the mass of these moving [cathode ray] particles turns out to be 2,000 to 4,000 times smaller than the lightest known chemical atom, the hydrogen atom." Weichert almost hit the bull's-eye. He knew he was looking at some new kind of particle. He was damn close to the mass. (The electron's mass turned out to be 1,837 times smaller than that of hydrogen.) So why is Thomson famous and Weichert a footnote? Because he simply assumed (guessed) the value of the electric charge; he had no evidence for it. Weichert was also distracted by a job change and a competing interest in geophysics. He was a scientist who reached the right conclusion but didn't have all of the data. No cigar Emil!
The second runner-up was Walter Kaufmann, in Berlin. He got to the finish line in April 1897, and his shortcoming was the opposite of Weichert's. The book on him was good data, bad thinking. He also derived elm using magnetic and electric fields, but he took the experiment a significant step further. He was especially interested in how the value of elm might change with changes in pressure and with the gas used in the tube—air hydrogen, carbon dioxide. Unlike Weichert, Kaufmann thought that cathode ray particles were simply charged atoms of the gas in the tube, so they should have a different mass for each gas used. Surprise—he discovered that elm does not change. He always got the same number no matter what gas, what pressure. Kaufmann wa
s stumped and missed the boat. Too bad, because his experiments were quite elegant. He got a better value for elm than the champ, J. J. It's one of the cruel ironies of science that he missed what his data were screaming at him: your particles are a new form of matter, dummkopf! And they are the universal constituents of all atoms; that's why elm doesn't change.
Joseph John Thomson (1856–1940) started out in mathematical physics and was surprised when, in 1884, he was appointed professor of experimental physics at the famous Cavendish Laboratory at Cambridge University. It would be nice to know whether he really wanted to be an experimentalist. He was famous for his clumsiness with experimental apparatus but was fortunate in having excellent assistants who could do his bidding and keep him away from all that breakable glass.
In 1896 Thomson sets out to understand the nature of the cathode ray. At one end of his fifteen-inch glass tube the cathode emits its mysterious rays. These head for an anode with a hole that permits some of the rays (read electrons) to pass through. The narrow beam thus formed goes on to the end of the tube, where it strikes a fluorescent screen, producing a small green spot. Thomson's next surprise is to insert into the glass tube a pair of plates about six inches long. The cathode beam passes through the gap between these plates, which Thomson connects to a battery, creating an electric field perpendicular to the cathode ray. This is the deflection region.
If the beam moves in response to the field, that means it is carrying an electric charge. If, on the other hand, the cathode rays are photons—light particles—they will ignore the deflection plates and continue on their way in a straight line. Thomson, using a powerful battery, sees the spot on the fluorescent screen move down when the top plate is negative, up when the top plate is positive. He thus proves that the rays are charged. Incidentally, if the deflection plates carry an alternating voltage (varying rapidly plus-minus-plus-minus), the green spot will move up and down rapidly, creating a green line. This is the first step in making a TV tube and seeing Dan Rather on the CBS nightly news.
But it is 1896, and Thomson has other things on his mind. Because the force (the strength of the electric field) is known, it is easy, using simple Newtonian mechanics, to calculate how far the spot should move if one can figure out the velocity of the cathode rays. Now Thomson uses a trick. He places a magnetic field around the tube in such a direction that the magnetic deflection exactly cancels the electric deflection. Since this magnetic force depends on the unknown velocity, he has merely to read the strength of the electric field and the magnetic field in order to derive a value for the velocity. With the velocity determined, he can now go back to testing the deflection of the ray in electric fields. What emerges is a precise value for elm, the ratio of the electric charge on a cathode ray particle divided by its mass.
Fastidiously, Thomson applies fields, measures deflections, cancels deflections, measures fields, and gets numbers for elm. Like Kaufmann, he double-checks by changing the cathode material—aluminum, platinum, copper on—and repeating the experiment. All give the same number. He changes the gas in the tube: air, hydrogen, carbon dioxide. Same result. Thomson does not repeat Kaufmann's mistake. He concludes that the cathode rays are not charged gas molecules but fundamental particles that must be part of all matter.
Not satisfied that he has enough proof, he hits on using the idea of conservation of energy. He captures the cathode rays in a metal block. Their energy is known; it is simply the electrical energy given to the particles by the voltage from the battery. He measures the heat generated by the cathode rays, and notes that in relating the energy acquired by the hypothetical electrons to the heat generated in the metal block, the ratio elm appears. In a long series of experiments, Thomson gets a value for elm (2.0 × 1011 coulombs per kilogram), that is not very different from his first result. In 1897 he announces the result: "We have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state." This "subdivision of matter" is an ingredient in all matter and is part of the "substance from which the chemical elements are built up."
What to call this new particle? Stoney's term "electron" was handy, so electron it became. Thomson lectured and wrote about the corpuscular properties of cathode rays from April to August 1897. This is known as marketing your results.
There was still one puzzle to be solved: the separate values of e and m. Thomson was in the same fix as Weichert a few years earlier. So he did something clever. Noting that the elm of this new particle was about a thousand times bigger than that of hydrogen, the lightest of all the chemical atoms, he realized that either the electron's e was much bigger or its m was much smaller. What's it to be: big e or little m? Intuitively, he went with little m—a brave choice, for he was guessing that this new particle had a tiny mass, far smaller than that of hydrogen. Remember, most physicists and chemists still thought that the chemical atom was the indivisible a-tom. Thomson now said that the glow in his tube was evidence of a universal ingredient, a smaller constituent of all chemical atoms.
In 1898 Thomson went on to measure the electric charge of his cathode rays, thus indirectly measuring the mass as well. He used a new technique, the cloud chamber, invented by his Scottish student C. T. R. Wilson in order to study the properties of rain, not a rare commodity in Scotland. Rain happens when water vapor condenses on dust to form drops. When the air is clear, electrically charged ions can stand in for dust, and that's what happens in a cloud chamber. Thomson measured the total charge in the chamber using an electrometric technique and determined the individual charge on each droplet by counting them and dividing the total.
I had to build a Wilson cloud chamber as part of my Ph.D. thesis, and I've hated the technique ever since, hated Wilson, hated anyone who had anything to do with this contrary and mulelike device. That Thomson got the correct value of e and hence a measurement of the mass of the electron is miraculous. And that's not all. During the whole process of pinning down this particle, his dedication had to be unwavering. How does he know the electric field? Does he read the label on the battery? No labels. How does he know the precise value of his magnetic field in order to measure velocity? How does he measure the current? Reading a pointer on a dial has its problems. The pointer is a bit thick. It may shiver and shake. How is the scale calibrated? Is it meaningful? In 1897 absolute standards were not catalogue items. Measuring voltages, currents, temperatures, pressures, distances, time intervals were all formidable problems. Each required a detailed knowledge of the workings of the battery, the magnets, the meters.
Then there was the political problem: how to convince the powers that be to give you the resources to do the experiment in the first place. Being the boss, as Thomson was, really helped. And I left out the most crucial problem of all: how to decide which experiment to do. Thomson had the talent, the political know-how, the stamina, to carry through where others had failed. In 1898 he announced that electrons are components of the atom and that cathode rays are electrons that have been separated from the atom. Scientists thought the chemical atom was structureless, uncuttable. Thomson had torn it apart.
The atom was split, and we had found our first elementary particle, our first a-tom. Do you hear that giggle?
5. THE NAKED ATOM
There's something happening here.
What it is ain't exactly clear.
—Buffalo Springfield
ON NEW YEAR'S EVE 1999, while most of the world prepares for the last blowout of the century, physicists from Palo Alto to Novosibirsk, from Cape Town to Reykjavik, will be resting, having exhausted themselves almost two years earlier celebrating the one hundredth anniversary (in 1998) of the discovery of the electron—the first truly elementary particle. Physicists love to celebrate. They'll celebrate any particle's birthday, no matter how obscure. But the electron, wow! They'll be dancing in the streets.
After its discovery, the electron was frequently toasted in its birthplace, the Cavendish Laboratory at Cambr
idge University, with: "To the electron, may it remain forever useless!" Fat chance. Today, less than a century later; our entire technological superstructure is based upon that little fellow.
Almost as soon as the electron was born, it began causing problems. It still perplexes us today. The electron is "pictured" as a sphere of electric charge that spins rapidly around an axis, creating a magnetic field. J. J. Thomson struggled mightily to measure the electron's charge and mass, but now both quantities are known to a high degree of precision.
Now for the spooky features. In the curious world of the atom, the radius of the electron is generally taken to be zero. This gives rise to some obvious problems:
If the radius is zero, what spins?
How can it have mass?
Where is the charge?
How do we know the radius is zero in the first place?
Can I get my money back?
Here we meet the Boscovich problem face to face. Boscovich solved the problem of "atoms" colliding by making them into points, things with no dimension. His points were literal mathematicians' points, but he allowed these point particles to have conventional properties: mass and something we call charge, the source of a field of force. Boscovich's points were theoretical, speculative. But the electron is real. Probably a point particle, but with all other properties intact. Mass, yes. Charge, yes. Spin, yes. Radius, no.
Think of Lewis Carroll's Cheshire Cat. Slowly the Cheshire Cat disappears until all that's left is its smile. No cat, just smile. Imagine the radius of a spinning glob of charge slowly shrinking until it disappears, leaving intact its spin, charge, mass, and smile.
The God Particle: If the Universe Is the Answer, What Is the Question? Page 18
