How to Make an Apple Pie from Scratch

by Harry Cliff

  However, there was a way to do it: to look at how the particles curved when they passed through a magnetic field. The heavier the particles were, the less they would bend (think about a truck driving round a corner: the heavier the truck’s load, the more friction needed in the tires to keep it from sliding off the road). The trouble was that the amount that the particles bend also depends on their speed and electric charge. A fast-moving particle bends less than a slow-moving one, while the bigger the electric charge carried by the particle the stronger the magnetic force it feels, and the more it gets bent. As a result, J.J. couldn’t measure their masses directly. He could, however, compare their mass to their electric charge.

  When J.J. did his calculations he found something shocking. The mass of the cathode ray divided by its electric charge was roughly a thousand times smaller than a hydrogen ion’s. There were only two possible interpretations: either the electric charge of a cathode ray was much bigger than a hydrogen ion’s, or its mass was smaller, perhaps thousands of times smaller. Could Thomson have glimpsed something even more fundamental than Dalton’s indestructible atom?

  On Friday, April 30, 1897, J.J. caught the train down to London to present his findings at one of the Royal Institution’s famous Friday Evening Discourses. The steeply ranked rows of the oak-paneled theater were stuffed with the great and the good of the scientific establishment, dressed formally in their evening wear. Standing behind the desk where Humphry Davy had once captivated audiences with his dramatic chemical experiments, Thomson made his case for a new theory of the atom. Speaking with a slight Lancashire lilt and shuffling about the lecture theater in his own inimitable way, he guided his audience through his recent experiments, laying out the evidence that cathode rays were tiny negatively charged particles, far smaller than the smallest atom. That might have been a startling enough claim on its own, but he was building up to his final, radical conclusion. These particles, which Thomson named “corpuscles,”*2 were the basic building blocks of all atoms. The electric forces inside his glass tubes were literally tearing atoms apart, setting streams of negatively charged particles free from their atomic prisons. He had divided the indivisible atom!

  There was general disbelief among the scientists in the audience. One witness later said he thought that Thomson was “pulling their legs.” While many were happy to accept his claim that cathode rays were negatively charged particles, the idea that they were the constituents of atoms was a step too far. Thomson had clearly gotten carried away, going far beyond what was supported by his experiments. If he was going to convince the doubters of his extraordinary claim, he was going to need really extraordinary evidence.

  Back in the lab, Thomson enlisted his laboratory assistant Ebenezer Everett, described as “the best glassblower in England” to handcraft him a cathode ray tube of such strength and precision that it would let him settle the debate once and for all. The new tube needed to have additional electrodes bled through the glass so that an extra electric field could be applied to the cathode rays, and it had to be able to withstand incredibly high vacuums, since almost every last trace of air would have to be pumped out of the vessel for the experiment to work. That unenviable task fell to Everett, who spent several days laboriously pumping the tube down by hand.

  The pair worked hard over the summer of 1897. Everett, who wasn’t about to let his cack-handed boss anywhere near his beautifully blown glassware, did most of the hands-on stuff, while Thomson was kept at a safe distance and only allowed in close to take readings. By comparing how magnetic and electric fields of different strengths deflected the cathode rays, Thomson arrived at a far more precise measurement of the mass-to-charge ratio, in perfect accordance with his earlier results. Their hard work had paid off; Thomson’s corpuscles really did seem to have a mass thousands of times smaller than hydrogen.

  In October that year he released a new paper, reaffirming his bold claim that corpuscles were the building blocks of atoms. But this time he went even further, laying out a model of the atom in which the corpuscles arranged themselves in concentric rings in a sea of positive electric charge. Over a number of years, he developed his picture of the atom taking inspiration, rather appropriately given the title of this book, from a popular English dessert of the period. According to Thomson the atom was like a plum pudding, with negatively charged corpuscles acting as the plums, embedded in a positively charged sponge cake. At last Mendeleev’s periodic table could begin to be unraveled; the properties of the different chemical elements were the result of them containing different numbers of corpuscles.

  It took several years for Thomson’s ideas to be accepted. The specter of alchemy still haunted the physics community, with many unwilling to countenance the idea of a subatomic particle. One thing that never stuck was J.J.’s name for his particle—you’ve probably never heard of a corpuscle, and that’s because we now know them as electrons. To this day, every experiment that has ever been carried out suggests that electrons are truly fundamental objects.

  We have arrived at the first true ingredient of our apple pie.

  But the story of the atom is far from over. While J.J. had been playing around with electrons, one of his young students, only recently arrived from New Zealand, had begun a journey that would propel him and the entire field of physics into a brave new world. His name was Ernest Rutherford, and he was to change how we think about atoms forever.

  THE HEART OF THE ATOM

  Much as I’m proud of my home university, Carl Sagan was definitely giving Cambridge too much credit when he claimed that it was the place where the atom was first understood. In reality, it was in the forward-thinking industrial city of Manchester where the modern atom was forged. For more than a decade, a tight-knit band of physicists at the university’s Physics Laboratory unraveled the secrets of the atom, under the leadership of, in my view, the greatest experimental physicist of all time, Ernest Rutherford.

  Rutherford, a farmer’s son from Pungarehu on New Zealand’s North Island, had come to Britain in 1895 as one of J. J. Thomson’s research students at the Cavendish. He quickly made a name for himself as a brilliant experimenter, but it was only toward the end of his time there that he picked up the trail that would eventually lead him to uncover the true structure of the atom. In 1896, Henri Becquerel in Paris had discovered a new form of radiation that sprung spontaneously from minerals containing uranium, and Rutherford made the risky decision to drop his promising work on X-rays and throw himself headfirst into studying the mysterious phenomenon. It was a decision that would prove to be the making of him.

  In 1898 he had left Britain to head up the physics laboratory at McGill University in Montreal, Canada, at the tender age of twenty-seven, soon transforming the lab into one of the great centers for radioactive research. The other was in Paris, where Marie Curie and her husband, Pierre, carried out a series of grueling experiments that involved stirring steaming vats of pitchblende (a uranium-rich mineral) in the open air until they had painstakingly extracted a few tenths of a gram of an element that was millions of times more radioactive than uranium. They named it “radium.”

  With the Curies and Rutherford leading the charge, order was gradually imposed on the growing list of radioactive elements. One thing slowly became clear to both Ernest and Marie: radioactivity must come from somewhere inside the atom itself, and what’s more it seemed to change the original atom into a completely different one in the process. Working with the chemist Frederick Soddy at McGill, Rutherford amassed incontrovertible evidence that the radioactive element thorium decayed into a second element, which they dubbed “thorium-X” (now known to be radium), which in turn decayed into a radioactive gas. For the first time in history they had caught a supposedly immutable element transmuting into a totally different one. The alchemists seemed to be back in business.

  In 1907, Rutherford had been lured back to England by the prospect of being closer to t
he thick of the scientific action in Europe, much to the dismay of his colleagues at McGill. He arrived in Manchester like a whirlwind, reshaping the lab into something altogether new to science, a research school where almost all work was focused on what he now regarded as the most important problem in physics: the inner workings of the atom. Under his leadership, the number of staff and research students swelled, with Rutherford dishing out projects designed to attack the subject from every possible angle. Time and success had changed him from a slightly shy, if determined, young man, into a boisterous, self-confident, and inspiring leader, a larger than life figure whom one colleague compared to a living lump of radium. His booming voice would announce his approach long before he became visible as he made his daily rounds of the lab while tunelessly bellowing “Onward, Christian Soldiers,” dropping in on his staff and students to discuss whatever problem they were grappling with and dole out advice.

  That said, working with Rutherford wasn’t always easy. He had a volcanic temper, which would erupt without warning and overwhelm anyone unfortunate enough to be nearby at the time. Shortly after arriving in Manchester, he publicly dressed down the professor of chemistry, who had been slowly encroaching on lab space reserved for physics, bringing his fist down on a desk and exclaiming, “By thunder!” before pursuing the unfortunate prof to his office, shouting that he was “like the fag end of a bad dream.” His rages could be terrifying, as he loudly excoriated his victim from close range, although after the red mist cleared, he would almost always return to make a somewhat shamefaced apology.

  Despite his volatility, Rutherford was loved and revered by his team at Manchester. They were more than just a group of scientists; they were a family, bound together by a sense of being at the forefront of the most important scientific work being done anywhere in the world. Rutherford had an uncanny knack for choosing the right phenomenon for investigation, boasting that he never gave a student a dud project. Perhaps his greatest quality was his sheer doggedness, relentlessly hammering away at a problem until it gave up its secrets. One of his longest-serving colleagues, James Chadwick, was once asked if Rutherford had an acute mind. He replied that “acute” was the wrong word. “His mind was like the bow of a battleship. There was so much weight behind it, it had no need to be as sharp as a razor.”

  The problem that was now in Rutherford’s crosshairs was the structure of the atom itself. Despite all the progress in radioactivity over the past decade, many mysteries remained. No one knew why some atoms suddenly decided to change into others and spit out radiation. Even more mysterious was where the energy released in radioactivity came from. Rutherford had calculated that the radioactive decay of an atom released millions of times more energy than the most violent chemical reaction. There must be some vast well of energy deep within the atom itself, but what it could be was anyone’s guess.

  He hoped that he might be able to find answers by focusing on the particles that came flying out when a radioactive atom decayed. While still a student at the Cavendish, Rutherford had discovered that there were actually two different types of radiation being shot out by uranium: one that only flew a few centimeters through the air before stopping, and a second more penetrating ray that could travel much farther and even pass through strips of metal. Rutherford named these two types of radiation after the first letters of the Greek alphabet: alpha and beta.*3 Scientists had been quick to find that the highly penetrating beta rays could be bent using a magnetic field and soon realized that they were electrons.

  Rutherford’s first big success at Manchester was proving what he had long suspected, that alpha particles were helium atoms that had lost two electrons. The work had been done with a promising young German physicist, Hans Geiger, who pulled off the remarkable feat of building the first detector capable of counting alpha particles one by one.*4 While Geiger and Rutherford had been perfecting their detector, they had been irritated to find that the images left by a beam of alpha particles on a photographic plate became fuzzy after they had passed through the long tube of gas that made up the detector. This seemed to suggest that the alpha particles were being knocked off course by collisions with the gas molecules. Rutherford was puzzled; alpha particles got shot out of disintegrating atoms at incredibly high speeds, zipping along at a decent fraction of the speed of light. How could projectiles of such “exceptional violence,” as he put it, be deflected by something as insubstantial as a gas molecule?

  Once again, Rutherford had shown his uncanny ability to ask precisely the right question. He set Geiger the task of firing alpha particles through a range of different materials and measuring how much they got scattered. After trying out thin foils of various metals, Geiger found that the heavier the atoms in the metal foil, the more the alpha particles appeared to be deflected. Gold turned out to be the best deflector of all, often scattering the alpha particles through such large angles that it left Geiger and Rutherford scratching their heads.

  Why was this all so surprising? Well, according to J. J. Thomson’s plum pudding model, the atom was a weak and wobbly sphere of positive charge (the sponge cake), with tiny negatively charged electrons (the plums) stuck into it. It was hard to imagine how something so diffuse and insubstantial as an atom could cause any trouble to something as powerful and speedy as an alpha particle.

  It was while Rutherford was mulling over these peculiar results that he made an off-the-cuff suggestion: that one of the new students, Ernest Marsden, look to see if any alpha particles bounced backward off the gold foil. Rutherford was sure that Marsden wouldn’t see anything—there was absolutely no way a gold atom could knock an alpha particle backward—but it would be a good project to get him trained up in radioactive research.

  Counting alpha particles in those days was punishing work. I sometimes feel like a fraud when I describe myself as an experimental physicist—in truth I rarely come close to having to do anything hands-on. Data from the Large Hadron Collider is delivered via the internet straight to the comfort of my Cambridge office, an airport departure lounge, or even while sitting in bed with a nice cup of tea. At the start of the twentieth century, on the other hand, you would have to sit in a darkened room, peering through a microscope at a zinc-sulfide screen for hours on end, patiently and methodically counting faint flickers of light that were the telltale signs of alpha particles, until tired eyes forced you to call it a day. And all this with your head just a few centimeters from a powerful radioactive source.

  Before he began to take measurements, Marsden sat alone in the darkened lab for twenty minutes, slowly allowing his eyes to adjust to the gloom. On the workbench in front of him was a delicate glass cone containing the source of the alpha particles, a highly radioactive mix of radium, bismuth, and radon gas, with a thin mica window at one end to let the alpha particles escape. In their path hung the target, a thin golden foil that glimmered gently in the dim light of an electric lamp. On the same side of the foil as the radioactive source but shielded from it by a lead barrier to stop alpha particles from hitting it directly, was the zinc-sulfide screen and the microscope.

  Once his eyes had fully adjusted, Marsden leaned in and put one eye to the microscope. He knew that Rutherford hadn’t expected him to see anything at all, but he was astounded to see that the screen was alive with tiny flickers of light. They arrived sporadically, like the flashes of dozens of infinitesimal cameras at some microscopic movie premiere. Fearing Rutherford’s ire, the young undergraduate student checked and rechecked his results until he was absolutely convinced that he hadn’t messed anything up. After three days of eye-straining work, he finally gave Rutherford the jaw-dropping news as he was coming down the stairs from his study at the top of the laboratory: the alpha particles were being knocked backward!

  Rutherford was stunned. He later described it as “quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece
of tissue paper and it came back and hit you.” Neither Geiger nor Marsden had the foggiest idea what was going on. When they published their extraordinary results in July 1909, they didn’t even try to explain what they’d seen, except to make the tantalizing observation that it would require a magnetic field billions of times stronger than what was possible in the lab to bend alpha particles back on themselves. Whatever it was that was sending the alpha particles hurtling backward had to be the seat of almost unimaginably powerful forces.

  Even Rutherford was stumped. Having produced an astonishing fourteen scientific papers in 1908, his research slowed to a trickle as he brooded over the puzzle. He took to spending long periods deep in thought, locked away in his study at home, endlessly turning the problem over in his mind. At first, he wondered whether the alpha particles that came bouncing back had actually banged into multiple gold atoms, with dozens of tiny knocks building up until the alpha particle was turned back on its heels. But his calculations showed that the chances of that happening were vanishingly small, far too low to explain the number of flickers that Marsden had seen. The alpha particles had to be being reflected in a single collision with something with a lot of mass.

  It was over a weekend in December 1910, more than eighteen months after Marsden had given him the startling news, that Rutherford finally saw the answer clearly. Charles Galton Darwin,*5 a young student at the Manchester lab, had been invited round to the Rutherfords’ for Sunday dinner, and after they’d finished eating Rutherford shared his world-changing insight for the first time. His old mentor J.J. had it all wrong; the atom wasn’t a pudding-like blob, it was a tiny solar system, with negatively charged electrons held in orbit around an infinitesimal positively charged sun,*6 hidden deep at the heart of the atom. This atomic core, which Rutherford would later name the nucleus, contained 99.98 percent of the atom’s mass crushed into a tiny speck thirty thousand times smaller than the atom itself. It was this tiny but mighty nucleus that was responsible for scattering the alpha particles back on themselves. On the rare occasion that a positively charged alpha particle came close to the nucleus, it experienced an incredibly powerful electric repulsion, and if the collision was more or less head-on, the repulsion would send the alpha particle zooming backward.