How to Make an Apple Pie from Scratch

by Harry Cliff

  However, as the universe cooled, the fact that the neutron is slightly heavier than the proton began to upset the balance. The extra bit of energy needed to turn a proton into a neutron starts to make the reaction less likely than its reverse, and the number of neutrons compared to protons falls. When the first second is up, the temperature of the universe drops to the point where particles no longer have enough energy to turn a proton into a neutron and the number of neutrons freezes out, with about one neutron for every six protons.

  All we have to do now is wait a couple of minutes for the universe to cool down enough for nuclear fusion to get going. However, during that time, yet another factor comes into play—the fact that the neutron is unstable, only living for about fifteen minutes on average before it decays into a proton, an electron, and an antineutrino. As a result, during this two-minute wait, a decent fraction of the neutrons decay into protons, leaving just one neutron for every seven protons when fusion kicks off.

  In the next minute or so almost all those neutrons get fused into helium-4, which, as we said, is made of two neutrons and two protons. So if we start out with fourteen protons for every two neutrons, then for every helium nucleus we expect twelve protons left over. Since helium weighs four times as much as a proton, that’s a ratio of 4:12 helium to hydrogen. In other words, the big bang theory predicts that 25 percent of the mass of atoms in the universe should end up as helium, with the remaining 75 percent left over as unfused hydrogen. This is precisely what we see in the universe today!

  Apologies for the mental arithmetic required there, but I hope the final message is clear enough: the big bang theory does a great job of predicting the amount of hydrogen and helium that astronomers see when they look outward into space. Hoyle himself came to the same conclusion in a paper he published with a younger colleague, Roger Tayler, in 1964. However, while Tayler took this as clear evidence for the big bang, the dogmatic Hoyle still wouldn’t let go of the steady state, clinging stubbornly to his unseen dark stars.

  By the mid-1960s the great war over the history of the universe was all but done.

  The killer blow came in 1965 when two American radio astronomers, Arno Penzias and Robert Wilson, discovered a faint microwave glow emanating from the entire sky. The pair had been planning to use a large antenna at Bell Labs in New Jersey to study radio emissions from objects in the Milky Way. However, while they were calibrating their equipment, they were plagued by a low-level microwave noise that they just couldn’t seem to get rid of. Realizing that the noise would make precise astronomical observations impossible, they spent the best part of a year trying to figure out where it was coming from.

  They ruled out a catalog of potential sources, both in space and on Earth, including stray radio broadcasts from New York City, just a few miles away across Lower Bay. In fact, no matter where they pointed the antenna, the noise remained stubbornly constant. It was only after much confusion and endless checks of the giant radio horn, which famously included evicting some roosting pigeons and cleaning up the “white dielectric material” that they had left behind, that the significance of their discovery was realized. The faint microwave signal wasn’t pigeon shit, it was the afterglow of creation.

  Forgotten to almost everyone, back in 1948 Alpher and Herman had predicted that the fearsome light that had dominated the early fireball of the big bang should still be around today. Around 380,000 years after time zero the universe should have cooled down enough for negatively charged electrons to bind to positively charged nuclei and form the first neutral atoms. Before this pivotal moment in cosmic history, photons couldn’t travel far through space without pinballing off charged particles in the primordial fireball. However, as the first neutral atoms formed, the universe changed from a fiery plasma to a transparent gas of hydrogen and helium. Suddenly, the photons were free to travel unimpeded through space.

  That light has been traveling through the cosmos ever since. As it traveled, the expansion of space gradually stretched what started out as short wavelength visible light with a temperature of around 3,000 degrees to a weak microwave signal, just 2.7 degrees above absolute zero. It was this faint glow from the universe’s fiery birth that Penzias and Wilson had stumbled upon. It appeared to come from the entire sky because the big bang happened everywhere, or to put it another way, everywhere was once inside that ancient fireball.

  The discovery of what is now known as the “cosmic microwave background” was the final piece of evidence that convinced cosmologists that our universe really did begin with a bang. I can’t think of a scientific discovery more profound than that. In the space of just a few decades we’d gone from believing that our Milky Way galaxy was the entire universe to finding ourselves gazing out at an enormously vaster, ever-expanding cosmos whose origins can be traced back to a single unimaginably violent event that took place 13.8 billion years ago. Penzias and Wilson were rightly rewarded with a Nobel Prize for the painstaking work that had made the discovery possible, a great example of how scrupulous experimental care can lead to seriously big discoveries. As the science fiction writer Isaac Asimov once wrote, “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’ (I found it!) but ‘That’s funny…’ ”

  Gamow, Alpher, and Herman, on the other hand, were left feeling more than a little bitter. Their original prediction of the cosmic microwave background had been all but ignored. It was the Princeton physicists Robert Dicke and Jim Peebles who’d realized the significance of Penzias and Wilson’s microwave buzz, but when they published their paper, they were completely unaware that Gamow, Alpher, and Herman had made the same prediction almost two decades earlier. In fact, despite all their many contributions to our understanding of the origins of the elements, the stars, and the universe itself, neither George Gamow nor Fred Hoyle received a Nobel Prize. Perhaps Gamow’s refusal to take anything seriously and his tendency for embarrassing drunken antics had played a part in him being passed over. In Hoyle’s case, his downright rudeness and increasingly bonkers scientific views in later life had alienated many of his colleagues, views which included the idea that flu outbreaks were caused by microbes raining down from outer space and that the fossil of the birdlike dinosaur archaeopteryx was a fake.

  Accolades or no, together Gamow and Hoyle laid the foundations of our understanding of where the chemical elements come from and, ironically, they were both right and wrong at the same time. The elements weren’t all made in stars as Hoyle had earnestly hoped, nor were they all made in the fiery maelstrom of Gamow’s big bang. They were made in both. The big bang gave birth to our universe, and in the process seeded space with the hydrogen and helium*4 that went on to form the first stars. These in their turn fused everything else, from the carbon in our apple pie to the uranium that warms our planet’s core. We, and everything around us, are products of these awesome events. We are children of both the big bang and the stars.

  We’ve reached a turning point on our cosmic cookery story. At last we know where the chemical elements that bubbled out of that first silly apple pie experiment came from. The carbon was made in stars like our Sun as they reached the end of their lives, the oxygen was blasted out across space by terrifying supernovae. The stars, in their turn, ultimately formed from the hydrogen and helium left over from the big bang. But there is one apple pie ingredient whose origin we still haven’t explained, the simplest of all, the raw materials from which all the others got made: hydrogen.

  In a sense, we already know where hydrogen comes from: the first hydrogen atoms formed 380,000 years after the big bang when protons and electrons got together for the first time. When I say we don’t yet know where hydrogen came from what I really mean is we don’t know where protons and electrons came from. To answer that question, we must finally leave the chemical elements behind, delving into the wonderful world of particles, while drilling ever deeper into the very first seco
nd of cosmic history.

  Skip Notes

  *1 Today the figure is even larger, 2.5 million light-years. A light-year is the distance traveled by light in one year, roughly 9.5 trillion kilometers, which is more than sixty thousand times the distance from the Earth to the Sun.

  *2 In fact, he was so pissed off that he literally tore up a year’s worth of notes and flushed them down the toilet.

  *3 If you don’t know what dark matter is, don’t worry, neither do physicists. We’ll come to that later…

  *4 And some tiny amounts of the third element, lithium.

  CHAPTER 8

  How to Cook a Proton

  The first time I saw data from the Large Hadron Collider was a gray Friday morning in April 2010. I was at my desk in the bowels of the new Cavendish Laboratory, a heap of concrete drabness built in the 1970s after the famous lab had outgrown its creaking city-center site and relocated to a windswept field on the edge of Cambridge.

  I shared my windowless office with two other graduate students. One was a depressive Italian who spent most of his time bemoaning the backward state of British plumbing—“Why you don’t have mixed taps?” was a frequent refrain. “When I wash my face I either freeze or I scald myself. It’s not fit for human life…”—and a sardonic final-year student in the midst of writing up her dissertation, whose gallows humor made me and the Italian fear what ordeals still lay ahead.

  I had just gotten back to Cambridge after spending the winter at CERN preparing for the first high-energy collisions at the LHC. Having lived the last few weeks in a state of mild terror at the prospect of being summoned to the control room to fix a problem that I had no idea how to solve, delays with the collider meant that I had left Geneva just as the first protons smashed into one another inside the LHCb detector. Almost as soon as the news arrived, so did an email from my supervisor asking whether I’d looked at the data yet.

  The algorithm used to sift through the collision data in search of the specific particles we were interested in was already written and prepared. It was more or less a simple job of pressing go and waiting. The data had been steadily accumulating since the first collisions on March 30, with each collision adding a little more to the small but rapidly growing store of new information about the subatomic world.

  Today, running over the vast LHCb dataset takes weeks, but in those early days so few collisions had been recorded that I had the results a little more than an hour after setting the algorithm running. Opening up the data file I hurriedly navigated my way to examine the key graph that I knew would tell us whether or not we were in business.

  A shaky double click on the mass spectrum, and there it was, a clear spike rising high above the low-level background noise, the unmistakable signature of the particle we were looking for. I remember feeling a rush of excitement. Up until that day I had only studied computer simulations, but here on my screen, clear as day, was proof that these particles actually did exist in the real world. Not only that, but what I was seeing was a product of the most ambitious scientific project ever attempted, a particle collider the size of a city that had taken decades to design and construct, an impossibly complex detector assembled by an international collaboration of more than seven hundred scientists, a worldwide grid of computer farms that stored, processed, and distributed the data across the globe, and, at the end of it all, the little algorithm that I had written. Somehow, miraculously, it had all worked.

  I bashed out an excited email to my supervisor, Val Gibson, head of the Cambridge group, attaching a copy of the telltale graph. The spike was a clear sign that D mesons—exotic particles about twice as heavy as a proton—were being produced in the collisions in the heart of our detector. Seeing them wasn’t in any way groundbreaking—they were first discovered in the 1970s—but we could now begin a raft of detailed studies in the hope of seeing some evidence of them misbehaving, at least as far as our accepted theory of particle physics is concerned.

  D mesons only live for around a half a trillionth of a second, so they don’t hang around in the wider world. They’re created at the LHC when the enormous kinetic energy of two colliding protons is converted into new matter. Accompanying them is a veritable cornucopia of other particles that come flying out from the impact point like the glowing embers of a firework. Among the hundreds of different types are familiar ones like protons, neutrons, and electrons, but also ones with strange and exotic names: pions, kaons, lambdas, deltas, eta primes, rhos, sigmas, psis, phis, upsilons, xis, omegas. A typical collision looks like someone has stuck a stick of dynamite in a can of Greek alphabet soup.

  What are all these particles and where do they come from? The answer to this question is deeply tied up with our search for the ultimate recipe for apple pie. It turns out that the protons and neutrons of which we are made are just two members of a much larger family of related particles that gradually began to appear in experiments from the 1930s onward. Their arrival on the scene was unwelcome, at first at least, and caused no end of confusion for many years. But slowly and surely a pattern began to emerge that seemed to hint at a more fundamental structure. The discovery of what lay beneath would open the way to a far deeper understanding of the nature of matter and would unlock the ultimate origin of the protons that make up our universe.

  WHO ORDERED THAT?

  Just around the corner from my office at the Cavendish is a corridor lined with wooden cabinets crammed full of what you might be forgiven for thinking is the kind of junk you’d find in your granddad’s shed. In fact, if particle physics had a hall of fame, this would be it. Among the historical curios are the cathode ray tube that J. J. Thomson used to discover the electron, Chadwick’s battered brass tube that revealed the neutron, and at the far end a large bulb from the particle accelerator that first smashed the atomic nucleus to bits. Easily missed amid the experimental bric-a-brac is an unassuming brass and glass contraption that revolutionized particle physics: the first cloud chamber.

  As the name suggests, the cloud chamber was originally designed to create artificial clouds in the lab after its inventor, the Scottish physicist Charles Wilson, fell in love with the dramatic atmospheric effects he saw while working on the summit of Ben Nevis in the Scottish Highlands. To test the idea that clouds form when water vapor attaches to airborne grains of dust, he built a water-vapor-filled chamber with as little contamination as possible, expecting that without any dust particles to act as seeds no clouds would be able to form. However, on examining the chamber he was surprised to see delicate wisps of water droplets streaming in every direction, like the contrails left by a fleet of tiny passenger jets. (Although since it was 1895, the comparison wouldn’t have occurred to him.)

  Completely by accident, Wilson had invented the first instrument capable of making individual subatomic particles visible to the human eye. Each fleeting track was caused by a single charged particle zipping through the chamber and knocking electrons off gas molecules as it went, leaving a trail of positive and negatively charged ions in its wake. These ions attract water molecules out of the vapor, growing until they form trails of droplets big enough to see.

  The cloud chamber was a literal revelation. For the first time, physicists had a window into the hidden world of atoms and particles, allowing them to watch and even photograph them going about their otherwise invisible business. Ernest Rutherford called it “the most original and wonderful instrument in scientific history” and it became the prime tool of subatomic physics for the first half of the twentieth century, leading directly to three Nobel Prize–winning discoveries.

  One unarguable master of cloud chamber photography was the American physicist Carl Anderson. Anderson spent much of the 1930s using cloud chambers to take photographs of cosmic rays—particles that rain down on the Earth from outer space. In 1932 he had rocked the physics world when he snapped a photograph of the first antiparticle, a positively charged mi
rror image of the electron known as the “positron.”

  The positron wasn’t entirely unexpected—the British theoretical physicist Paul Dirac had predicted its existence three years earlier—but in 1936 Anderson and his colleague Seth Neddermeyer would discover another particle that really upset the apple cart. A year earlier they had decided that to get better images of cosmic rays they needed to get closer to the source of the action, so they loaded their cloud chamber onto a flatbed trailer bought at a used car lot near their lab at Caltech in Pasadena and set off for the Colorado Rockies. They erected their equipment on the summit of Pikes Peak, a 4,300-meter-high pink granite mountain near Colorado Springs, camping out each night in a bunkhouse halfway down the mountain. After months of long days and nights working at high altitude, they returned to Pasadena to develop their photographs and analyze the results. Examining the beautiful cloud chamber traces, each showing dozens of particle tracks curving elegantly in their powerful magnetic field, they discovered a particle unlike anything they’d seen before.

  They convinced themselves that these new particles were neither featherweight electrons nor the relatively bulky protons. In fact, their rough measurements suggested that they had a mass somewhere between the two—about two hundred times heavier than an electron or about one-tenth the mass of a proton. Given its in-between mass, Anderson and Neddermeyer coined the term “mesotron”—mesos in Greek means “middle”—but today we know it as the muon.

  The muon didn’t appear to be a constituent of an atom—it only seemed to be found in cosmic rays—so what was it for? Well, at first at least, it looked to be a good match for a particle predicted by the Japanese theoretician Hideki Yukawa, who had been pondering the force that keeps protons and neutrons held together inside the atomic nucleus. Since protons are all positively charged, they should exert a stupendously large repulsive force on one another when squashed into a space as cramped as the nucleus of an atom. The only way nuclei could be holding together was if a much stronger attractive force between their constituents overwhelmed the electrical repulsion. The puzzling thing about this “strong nuclear force,” as it became known, was that it didn’t seem to have any effect until two protons or neutrons were almost touching each other. At distances longer than about a thousandth of a trillionth of a meter the force seemed to disappear entirely.