This perception must have given Einstein the idea of explaining an experimental observation of Heinrich Hertz, who was generating radio waves to verify Maxwell's theory. Hertz did this by striking sparks between two metal balls. In the course of this work he noticed that sparks would jump across the gap more readily if the balls were freshly polished. He suspected that the polishing enabled the electric charge to leave the surface. Being curious, he spent some time studying the effect of light on metal surfaces. He noticed that the blue-violet light of the spark was essential in drawing charges out of the metal surface. These charges fueled the cycle by aiding the formation of sparks. Hertz reasoned that polishing removes oxides, which interfere with the interaction of light with a metal surface.
The blue-violet light was stimulating electrons to pour out of the metal, which at the time seemed a bizarre effect. Experimenters systematically studied the phenomenon and came up with these curious facts:
Red light is incapable of releasing electrons, even if the light is extraordinarily intense.
Violet light, even if relatively faint, releases electrons easily.
The shorter the wavelength (the more violet the light), the higher the energy of the released electrons.
Einstein realized that Planck's idea that light appears in bundles could be the key to understanding the photoelectric mystery. Imagine an electron, minding its own business in the metal of one of Hertz's highly polished balls. What kind of light can give that electron enough energy to jump out of the surface? Einstein, using Planck's equation, noted that if the wavelength of light is short enough, the electron receives enough energy to breach the surface of the metal and escape. Either the electron swallows the entire bundle of energy or it doesn't, reasoned Einstein. Now, if the wavelength of the bundle swallowed is too long (not energetic enough), the electron cannot escape; it doesn't have enough energy. Drenching the metal with impotent (long-wavelength) bundles of light energy doesn't do any good. Einstein said that what's important is the energy of the bundle, not how many bundles you have.
Einstein's idea works perfectly. In the photoelectric effect the light quanta, or photons, are absorbed rather than, as with the Planck theory, emitted. Both processes seem to demand quanta with energy E = hf. The quantum concept was gaining. The photon idea wasn't convincingly proven until 1923, when American physicist Arthur Compton succeeded in demonstrating that a photon could collide with an electron much as two billiard balls collide, changing direction, energy, and momentum and acting in every way like a particle—but a very special particle somehow connected with a vibration frequency or wavelength.
Here was a ghost arisen. The nature of light was an old battleground. Recall that Newton and Galileo held that light consisted of "corpuscles." The Dutch astronomer Christiaan Huygens argued for a wave theory. This historic battle of Newton's corpuscles and Huygen's waves had been settled in favor of waves by Thomas Young's double-slit experiment (which we will review soon) early in the nineteenth century. In quantum theory, the corpuscle was resurrected, in the form of the photon, and the wave-corpuscle dilemma was revived with a surprise ending.
But there was even more trouble ahead for classical physics, thanks to Ernest Rutherford and his discovery of the nucleus.
SMOKING GUN NO. 3: WHO LIKES PLUM PUDDING?
Ernest Rutherford is one of those characters almost too good to be true, as if he were delivered to the scientific community by Central Casting. A big, gruff New Zealander with a walrus moustache, Rutherford was the first foreign research student admitted to the famed Cavendish Laboratory, run at the time by J. J. Thomson. Rutherford arrived just in time to witness the discovery of the electron. Good with his hands (unlike his boss, J.J.), he was an experimenter's experimenter a worthy rival to Faraday as the best ever He was known for his profound belief that swearing at an experiment made it work better a notion backed up by experimental results, if not theory. In evaluating Rutherford one must especially add in his students and postdocs, who, under his baleful eye, carried out great experiments. There were many: Charles D. Ellis (discoverer of beta decay), James Chadwick (discoverer of the neutron), and Hans Geiger (of counter fame), among others. Don't think it's easy to supervise some fifty graduate students. For one thing, one must read their papers. Listen to one of my best students begin his thesis: "This field of physics is so virginal that no human eyeball has ever set foot in it." But back to Ernest.
Rutherford had ill-concealed contempt for theorists, though, as you'll see, he wasn't such a bad one himself. And it's a good thing there wasn't the press coverage of science at the turn of the century that there is today. Rutherford was so quotable he'd have skewered himself out of a truckful of grants. Here are a few Rutherfordisms that have leaked down to us over the decades.
"Don't let me catch anyone talking about the universe in my department."
"Oh that stuff [relativity]. We never bother with that in our work."
"All science is either physics or stamp collecting."
"I've just been reading some of my early papers and, you know, when I finished, I said to myself, 'Rutherford, my boy, you used to be a damned clever fellow.'"
This damned clever fellow put in his time with Thomson, then crossed the Atlantic to work at McGill University in Montreal, then trekked back to England for a post at Manchester University. By 1908 he had won a Nobel Prize for his work with radioactivity. That would seem a fitting climax to a career for most men, but not for Rutherford. Now his work began in Ernest.
One cannot talk about Rutherford without talking about the Cavendish Lab, created in 1874 as the research laboratory of Cambridge University. The first director was Maxwell (a theorist as lab director?). The second was Lord Rayleigh, followed by Thomson in 1884. Rutherford arrived from the boonies of New Zealand as a special research student in 1895 at a fantastic time for rapid developments. One of the major ingredients for professional success in science is luck. Without this, forget it. Rutherford had it. His work on the newly discovered radioactivity—Becquerel rays they were called—honed him for his most important discovery, the atomic nucleus, in 1911. He made that discovery at the University of Manchester then returned in triumph to the Cavendish, where he succeeded Thomson as director.
You'll recall that Thomson had seriously complicated the issue of matter by discovering the electron. The chemical atom, thought to be the indivisible particle put forth by Democritus, now had little guys running around inside. These electrons had a negative charge, which presented a problem. Matter is neutral, neither positive nor negative. So what offsets the electrons?
The dramatic story begins quite prosaically. The boss comes into the lab. There sit a postdoc, Hans Geiger, and an undergraduate hanger-on, Ernest Marsden. They are engaged in alpha-particle scattering experiments. A radioactive source—for example, radon 222—naturally and spontaneously emits alpha particles. The alpha particles, it turns out, are nothing but helium atoms without their electrons—that is, helium nuclei, as Rutherford discovered in 1908. The radon source is placed in a lead case with a narrow hole that aims the alpha particles at a piece of extremely thin gold foil. As the alphas pass through the foil, their paths are deflected by the gold atoms. The angles of these deflections are the subject of the study. Rutherford had set up what became the historical prototype of a scattering experiment. You shoot particles at a target and see where they bounce. In this case the alpha particles were little probes whose purpose was to find out how atoms are structured. The gold-foil target is surrounded on all sides—360 degrees—by zinc sulfide screens. When a zinc sulfide molecule is struck by an alpha particle, it emits a flash of light, which allows the researchers to measure the angle of deflection. The alpha particle zips into the gold foil, hits an atom, and is deflected into one of the zinc sulfide screens. Flash! Most of the alpha particles are deflected only slightly and strike the zinc sulfide screen directly behind the gold foil. It was a tough experiment to do. They had no particle counter—Geiger hadn't invented it yet—s
o Geiger and Marsden were forced to sit in a dark room for several hours to adapt their eyesight to see the flashes. Then they had to spot and catalogue the number and positions of the little sparks.
Rutherford—who didn't have to sit in dark rooms because he was the boss—said: "See if any of the alpha particles are reflected from the foil." In other words, see if any of the alphas hit the gold foil and bounce back toward the source. Marsden recalled, "To my surprise I was able to observe the effect....I told Rutherford when I met him later; on the steps leading to his room."
The data, later published by Geiger and Marsden, recorded that one in 8,000 alpha particles was reflected from the metal foil. Rutherford's now-famous reaction to this news: "It was quite the most incredible event that ever happened to me in my life. It was as if you fired a fifteen-inch artillery shell at a piece of tissue paper and it came back and hit you."
This was May 1909. Early in 1911 Rutherford, acting now as a theoretical physicist, cracked the problem. He greeted his students with a broad smile. "I know what the atom looks like and I understand the strong backward scattering." In May of that year, his article declaring the existence of the nuclear atom was published. This was the end of an era. The chemical atom was now seen, correctly, as complex, not simple, and as cuttable, not at all a-tomlike. It was the beginning of a new era, the era of nuclear physics, and it marked the demise of classical physics, at least inside the atom.
Rutherford took at least eighteen months to think through a problem that is now solved by physics majors in their junior year. Why was he so puzzled by the ricocheting alpha particles? It had to do with how scientists at the time viewed the atom. Here is the massive, positively charged alpha particle charging into a gold atom and bouncing backward. The 1909 consensus was that the alpha should have blasted right through, like an artillery shell through tissue paper, to use Rutherford's metaphor.
The tissuelike model of the atom went back to Newton, who said forces have to cancel out if one is to have mechanical stability. Thus the electrical forces of attraction and repulsion had to be balanced in a stable atom that you could trust. The theorists of that turn-of-the-century epoch went in for a frenzy of model making, trying to arrange the electrons to make a stable atom. Atoms were known to have lots of negatively charged electrons. Therefore they had to have an equal amount of positive charge distributed in some unknown way. Since the electrons are very light and the atom is heavy, either an atom must have thousands of electrons (to make the weight) or the weight must reside in the positive charge. Out of the many models proposed, by 1905 the leading model of the day had been postulated by none other than J. J. Thomson, Mr. Electron. It was called the plum-pudding model because it had the positive charge spread out in a sphere covering the entire atom, with the electrons embedded throughout like plums in a pudding. Such an arrangement appeared to be mechanically stable and even allowed the electrons to vibrate around equilibrium locations. But the nature of the positive charge was a complete mystery.
Rutherford, on the other hand, calculated that the only configuration capable of knocking an alpha particle backward was one in which the entire mass and positive charge were concentrated in a very small volume in the center of a relatively huge (atom-size) sphere. The nucleus! The electrons would be spaced throughout the sphere. In time and with better data, Rutherford's theory was refined. The central positive charge (nucleus) occupies a volume no more than one trillionth of the volume of the atom. According to the Rutherford model, matter is predominantly empty space. When we pound on a table, it feels solid, but it is the interplay of electrical forces (and quantum rules) among atoms and molecules that creates the illusion of solidity. The atom is mostly void. Aristotle would be appalled.
Rutherford's surprise at the alpha particles bouncing backward may be appreciated if we abandon his artillery shell and think instead of a bowling ball thundering down the alley toward an array of tenpins. Picture the bowler's shock if the ball were stopped by the pins and then rebounded, careening back to the bowler, who would then run for her life. Could this happen? Well, suppose somewhere in the middle of the triangular array of pins there was a special "fat pin" made out of solid iridium, the densest metal known. This pin is heavy! It weighs fifty times more than the ball. A sequence of time-lapse photos would show the ball impinging on the fat pin, deforming it but coming to rest. Then, as the pin re-forms to its original shape and, indeed, recoils just a little bit, it imparts a resounding force to the ball, which reverses its velocity. This is what happens in any elastic collision, say of a billiard ball and cushion. Rutherford's more picturesque military shell metaphor was derived from his preconception, and that of most physicists of his day, that the atom was a sphere of pudding tenuously spread over a large volume. For a gold atom, this was an "enormous" sphere of radius 10−9 meters.
To get a sense of the Rutherford atom, if we picture the nucleus as the size of a green pea (about a quarter inch in diameter) the atom is a sphere of radius 300 feet, something that can surround six football fields, packed into a rough square. Rutherford's luck held here too. His radioactive source just happened to produce alphas with an energy of about 5 million electron volts (we write it 5 MeV), which was ideal for discovering the nucleus. The energy was low enough that the alpha particle never got too close to the nucleus but was turned back by its strong positive electric charge. The surrounding cloud of electrons had much too little mass to have any appreciable effect on the alpha. If the alpha had had much more energy, it would have penetrated the nucleus, sampling the strong nuclear force (we'll learn about this later) and greatly complicating the pattern of scattered alpha particles. (The vast majority of alphas pass through the atom so far from the nucleus that their deflections are small.) As it was, the pattern of scattered alpha particles, as subsequently measured by Geiger and Marsden and eventually by a host of continental competitors, was mathematically equivalent to what would be expected if the nucleus were a point. We know now that nuclei are not points, but if the alpha parades don't get too close, the arithmetic is the same.
Boscovich would have been pleased; the Manchester experiments backed up his vision. The outcome of a collision is determined by the force fields around the "point" things. Rutherford's experiment had implications beyond the discovery of the nucleus. It established that very large deflections imply small "pointlike" concentrations, a crucial idea experimenters eventually employed when going after quarks, the real points. In the slowly emerging view of the structure of the atom, Rutherford's model was a clear milestone. It was very much a miniature solar system: a dense, positively charged central nucleus with a number of electrons in various orbits such that the total negative charge exactly canceled the positive nuclear charge. Maxwell and Newton were duly invoked. The orbiting electron, like the planets, obeyed Newton's commandment, F = ma. F was now the electrical force (Coulomb's law) between charged particles. Since this is also an inverse-square force like gravity, one would assume at first glance that stable, planetary orbits would follow. And there you have it, the nice neat solar system model of the chemical atom. Everything was fine.
Well, it was fine until the arrival in Manchester of a young Danish physicist of theoretical persuasion. "Name's Bohr, Niels Henrik David Bohr Professor Rutherford. I'm a young theoretical physicist and I'm here to help you." We can only imagine the reaction of the gruff, earthy New Zealander.
THE STRUGGLE
The evolving revolution known as quantum theory didn't spring fully grown from the foreheads of theorists. It was slowly induced from data that emerged from the chemical atom. One can look at the struggle to understand this atom as practice for the real contest, understanding the sub-atom, the subnuclear jungle.
This slow unfolding of the real world is probably a blessing. What would Galileo or even Newton have done if the full data emerging from Fermilab had somehow been revealed to them? A colleague of mine at Columbia, a very young, very bright, articulate, enthusiastic professor, was given a unique teaching as
signment. Take the forty or so freshmen who had declared physics as their major and give them two years of intensive instruction: one professor, forty aspiring physicists, two years. The experiment turned out to be a disaster. Most of the students switched to other fields. The reason came later from a graduating mathematics major: "Mel was terrific, best teacher I ever had. In those two years not only did we get through the usual—Newtonian mechanics, optics, electricity and so on—but he opened a window on the world of modern physics and gave us a glimpse of the problems he was facing in his own research. I felt there was no way I could ever handle such a difficult set of problems, so I switched to mathematics."
This raises a deeper question, whether the human brain will ever be prepared for the mysteries of quantum physics, which in the 1990s continues to disturb some of the very best physicists. Theoretician Heinz Pagels (who died tragically a few years ago in a mountain-climbing accident) suggested, in his fine book The Cosmic Code, that the human brain may not have evolved enough to understand quantum reality. Perhaps he's right, although a few of his colleagues seem convinced that they have evolved much more than the rest of us.
The overriding point is that quantum theory, as a highly refined and dominant theory of the 1990s, works. It works in atoms. It works in molecules. It works in complex solids, metals, insulators, semiconductors, superconductors, and anywhere it has been applied. The success of the quantum theory accounts for a significant fraction of the industrial world's total gross national product (GNP). But more important for us, it is the only tool we have as we proceed down into the nucleus, into its constituents and down, down into the vast minuteness of primordial matter—where we will confront the a-tom and the God Particle. And it is there that quantum theory's conceptual difficulties, dismissed by most working physicists as mere "philosophy," may play a significant role.
The God Particle: If the Universe Is the Answer, What Is the Question? Page 20
