How to Make an Apple Pie from Scratch

by Harry Cliff

  In March 1963, Gell-Mann was having lunch with some colleagues at Columbia University in New York when he got chatting with the physicist Robert Serber, who had also been thinking about subhadronic building blocks. Gell-Mann was dismissive when Serber asked him what he thought about the idea over lunch, but later that evening their conversation got Gell-Mann thinking: What if these fractionally charged nuggets were forever locked up inside hadrons, never able to escape into the external world? If that were true, the cherished principle of nuclear democracy could be preserved and the bootstrap model would still be viable.

  Gell-Mann, who had a talent for conjuring memorable monikers, dubbed these undetectable little particles “qworks”—a kind of nonsense word in the style of Lewis Carroll. Months later, while perusing the notoriously incomprehensible novel Finnegans Wake by James Joyce, his eyes alighted on the phrase “Three quarks for Muster Mark!” amidst Joyce’s gibberish. Gell-Mann immediately realized he had found a perfect opportunity to give his little building blocks a literary heritage and, more importantly, taking their names from such an obtuse work would only further impress upon his colleagues just how well-read and clever he really was. And so “qworks” became “quarks.”

  According to Gell-Mann, the symmetries in the hadrons could be explained if there were three such quarks, which he called “up,” “down,” and “strange.” The up quark had a charge of +²/₃, while the down and strange quarks both had charges of −¹/₃. By combining these three particles (and their antiversions) you could explain the properties of all the known hadrons. Mesons like the pion or the kaon were a pairing of a quark and an antiquark, while the baryons were a threesome of quarks. Most importantly for our purposes, the proton could be thought of as being made of two up quarks and a down quark, while a neutron was built from two downs and an up.

  Meanwhile, several thousand kilometers away at CERN near Geneva, a young Russian postdoc and former PhD student of Gell-Mann’s named George Zweig was thinking along very similar lines. Completely independently, Zweig had realized that the symmetries of the Eightfold Way could be explained if there existed three individual building blocks with electric charges +²/₃, −¹/₃, and −¹/₃, which he called “aces.”

  However, while the two ideas were identical when it came to accounting for the symmetries in the hadrons, Zweig and Gell-Mann had very different takes on what it all actually meant. Gell-Mann was happy to think of the quarks as mathematical conveniences rather than real physical entities. The really fundamental ingredients of hadrons, as far as he was concerned, were the mathematical symmetries that they appeared to obey. Quarks were just a convenient way of keeping track of these fundamental symmetries but would probably never be observable in the real world.

  For Zweig, on the other hand, quarks (or aces) could be just as real as protons, neutrons, and electrons. Unfortunately for the young physicist, such ideas were wildly unfashionable at a time when the weird but elegant bootstrap model was in vogue. To argue that the hadrons were made of smaller things seemed simpleminded, even childish. Gell-Mann himself teasingly referred to Zweig’s aces as “the concrete block model.” As a result, while Gell-Mann had no trouble getting his quark theory published in a respectable journal, Zweig faced such a barrage of criticism from his referees that his paper never saw the light of day, except as a lowly CERN preprint, an article put out by the lab itself rather than being published in a prestigious journal.

  However, while some theorists were sniffy about the idea of quarks, there were plenty of experimentalists for whom the prospect of discovering a new layer of reality was too good to pass up. Physicists began poring over tens of thousands of old bubble chamber photographs in search of fractionally charged particles that they might have missed. New particle beam experiments were hastily prepared at CERN and Brookhaven in the hope of spotting a quark being knocked free of one of its hadrons. Even some die-hard cosmic ray physicists got in on the act, searching for quarks amid the showers of particles that rain down from the heavens.

  But quarks were nowhere to be found. By 1966 twenty experiments had searched for them and come up empty-handed. Speaking at the Royal Society in London that year, Gell-Mann himself declared that “we must face the likelihood that quarks are not real.”

  Help would come from an unexpected source. At Stanford University in Northern California, the finishing touches were being made to the world’s largest and most expensive particle accelerator. Stretching 3.2 kilometers in a straight line through the rolling parkland of the Stanford campus and passing directly under Interstate 280, the Stanford Linear Accelerator was effectively an enormous particle cannon, capable of accelerating electrons to a whopping 20 GeV. Its enormous scale and $100 million price tag had earned it the nickname “the Monster,” and getting it built had taken more than a decade of planning, design, and construction, not to mention navigating the project through the U.S. Congress.

  At a time when most physicists were focused on the exciting new discoveries emerging from the high-energy proton accelerators at CERN and Brookhaven, the Monster was a bit of an odd beast. Unlike its circular cousins, which worked by steering beams of protons around a ring and accelerating them each time they completed an orbit, the Monster fired electrons down a dead-straight 3.2-kilometer tube,*3 accelerating them all the way until they slammed headfirst into a target at the far end. The aftermaths of these collisions were then recorded by towering spectrometers, which measured the energies and directions of the scattered electrons.

  In effect, the Monster was a colossal microscope capable of zooming right in on the proton to study its size and shape in unparalleled detail. The higher the energy of an electron beam, the shorter the distances it can probe, resolving ever finer details. The reason that higher energy particles allow you to explore shorter distances is down to the quantum mechanical phenomenon of wave-particle duality—specifically that a particle like an electron can be caught behaving like a wave, if you set up an experiment in the right way. The wavelength of an electron, or indeed any particle, depends inversely on the momentum of the particle; in other words, the faster the particle is moving, the shorter its wavelength.

  When it fired up in 1966, the Stanford Monster could accelerate electrons to 99.99999997 percent of the speed of light, giving them a wavelength of about 6 x 10-17 meters (sixty-millionths of a trillionth of a meter). Experiments had shown that protons and neutrons were about 1 x 10-15 meters across, and so in principle the Monster’s beam could resolve objects far, far smaller than these most basic building blocks of atoms.

  In the midsixties, theorists imagined the proton as a fuzzy, insubstantial sphere with no internal structure. As a result, when they fired their super-high-energy electron beam at the proton, the team working on the Monster expected most of the electrons to zip right through almost unimpeded. Remind you of anything?

  Back at the start of the twentieth century, physicists had pictured the atom as a similarly insubstantial pudding-like object, which is why Ernest Rutherford had been so thunderstruck when alpha particles came bouncing straight back off gold atoms. That famous result had completely changed our understanding of atoms, eventually leading the boisterous New Zealander to conclude that the atom has a tiny nucleus at its heart.

  Something eerily similar was about to happen at Stanford: their giant accelerator was really just Rutherford’s gold foil experiment writ large, albeit on a scale totally unimaginable in 1908. Sixty years after the discovery of the nucleus, physicists were still using Rutherford’s tried and tested technique of firing particles at a target and seeing how they bounced off.

  Stanford even had its own version of Rutherford in the fearsome figure of Richard Taylor, a towering presence whose angry, booming voice could often be heard echoing along the corridors. After the first set of electron scattering experiments ended in 1966, Taylor took charge of a joint Stanford-MIT team who began to probe ever deeper into th
e proton. In 1967, they got the first hints that something strange was going on. Electrons seemed to be losing far more energy as they passed through the proton than expected.

  At first the effect was dismissed as noise, but by early 1968 the team had convinced themselves that what they were seeing was real. Just like Rutherford’s alpha particles, the electrons were being scattered through far larger angles than you would expect if the proton really was just a diffuse sphere of electric charge. There seemed to be only one explanation—the electrons were bouncing off unimaginably tiny objects inside the proton.

  Against anyone’s expectations, this giant accelerator had peered deep within the most basic building blocks of matter and glimpsed a brand-new layer of reality. Despite the popularity of fancy ideas like the bootstrap model, the old, tried, and tested atomic view of matter appeared to have won out yet again. Protons, neutrons, and all the hadrons in the particle zoo really did seem to be made of even smaller particles.

  However, the Stanford-MIT team had a fight on their hands persuading people that they had really seen quarks. Such was the hold of the bootstrap model that at first their electron scattering results aroused little interest. It would take years more experimental and theoretical work, not to mention the enthusiastic advocacy of physics’s most charismatic communicator, Richard Feynman, to convince the world that the Monster really had seen the building blocks of the proton.

  It was in 1973, after CERN’s gargantuan bubble chamber named Gargamelle spotted neutrinos ricocheting off pointlike objects inside the proton, that the evidence for quarks became overwhelming. Comparing Gargamelle’s and the Monster’s results, physicists were able to discern three such particles within the proton; what’s more, these particles appeared to have fractional charges, just as Gell-Mann and Zweig had predicted. Despite Gell-Mann’s skepticism about the reality of his own inventions, quarks had finally become real physical objects that physicists could begin to believe in.

  Well, sort of. One great puzzle remained—no one had actually seen a quark. All the evidence for their existence came from bouncing particles off hadrons. No accelerator, no matter how powerful, had managed to break a single, solitary quark from its hadronic jail cell. Quarks seemed to be inexorably locked up inside.

  The reason, it turns out, has to do with the force that binds quarks together inside hadrons. This force—known simply as the strong force—is the most potent attraction ever discovered. The strong nuclear force that holds protons and neutrons together inside atomic nuclei is a kind of echo of this far mightier force. To break the bonds of the strong force and liberate quarks from inside protons and neutrons requires temperatures far hotter than the hottest star, temperatures of trillions of degrees.

  Such temperatures have not been seen in the universe since a millionth of a second after the big bang. It was during this first microsecond of cosmic time that the protons and neutrons from which we are made came into existence. To get at the ultimate origins of matter we’ll need to find a way to probe the physics of this trillion-degree universe. Incredibly, such temperatures are now routinely recreated here on Earth, just a few miles from the bustling heart of New York City.

  TRILLION-DEGREE SOUP

  For a country that prides itself as a freewheeling, beacon-of-liberty, keep-government-out-of-my-business, who-is-the-federal-government-to-tell-me-I-can’t-own-a-surface-to-air-missile? sort of place, the United States can be surprisingly officious. Ahead of my visit to Brookhaven National Laboratory I was required to fill out a multipage online application form, followed by a fairly lengthy back and forth with their (unfailingly helpful) administrative staff about the purpose of my visit. Crucially, I was told that on entering the United States I should take great care to get the correct stamp at immigration—get it wrong and I wouldn’t be allowed access to the site. So ensued a meandering conversation with two slightly bemused-looking border officials about what exactly I was doing in their country, during which I tried to explain that I just wanted to visit some government labs and chat to some scientists in a completely innocuous, non-espionagey kind of a way, while taking great pains to avoid using the word “nuclear.” By comparison, it used to be possible to get onto the CERN site by waving your Tesco Clubcard at an uninterested-looking security guard.*4

  So it was with some trepidation that I presented myself at the security hut on the wooded road running into the Brookhaven site, brandishing my passport complete with a worryingly faint immigration stamp. The woman on the desk eyed it suspiciously. “I think they were running out of ink,” I said, smiling weakly. After some tutting and a bit of tapping on a computer, to my relief her face brightened, and she handed back my passport. “Welcome to Brookhaven.”

  Brookhaven National Laboratory has a long and illustrious history when it comes to particle physics. Founded in 1947 on the site of an old U.S. Army training camp, its first major facility was an experimental nuclear reactor, followed by the billion-volt-barrier-busting Cosmotron accelerator in 1953, which played a leading role in exploring the particle zoo. Then in 1960 came the Alternating Gradient Synchrotron (AGS), which ruled the roost as the world’s highest energy accelerator for the best part of a decade.

  Among the AGS’s many achievements was a major discovery that sent particle physicists into a frenzy of excitement in 1974. The November Revolution, as it’s known in the field, began when a team led by Samuel Ting at Brookhaven discovered a striking new peak in their data at an energy of around 3.1 GeV, or just over three times the mass of the proton. Meanwhile, 4,000 kilometers away in California, Burton Richter’s group working on the Stanford Monster were staring astounded at exactly the same spike. Both groups announced their discoveries on November 11. The peak proved to be evidence of a hadron made from a brand-new, never-before-seen type of quark—the “charm quark”—a heavier cousin of the positively charged up found inside protons and neutrons.*5

  The AGS’s discovery removed all remaining doubts about the existence of quarks and did much to lay the foundations of our current theory of particle physics. Today, this venerable accelerator is still in operation as the feeder for an even larger and more powerful atom smasher—the Relativistic Heavy Ion Collider, or RHIC for short. It was this machine that I had come to see.

  To understand what the scientists at RHIC are up to, we need to delve a little further into the physics of the quarks that make up protons and neutrons. At the same time that the reality of quarks was becoming accepted in the early 1970s, physicists were trying to understand the mysterious strong force that keeps them locked up inside hadrons.

  By 1973 a candidate theory had emerged based on the very same SU(3) symmetry group that Gell-Mann and Ne’eman had used to categorize the hadrons in the Eightfold Way. However, this time the symmetry described the strong force itself.

  Just as protons and electrons attract each other through the electromagnetic force thanks to their opposite electric charges, quarks attract each other because they carry the equivalent charge of the strong force. But whereas there is only one kind of electric charge, which can be either positive or negative, SU(3) symmetry dictates that the strong force should have three different types of charge, each with its own positive and negative version. Again demonstrating his uncanny talent for picking terms that stick, Gell-Mann called these three strong charges “colors.” Not to be confused with actual color like the color of my sweater (orange, in case you were wondering), the color of a quark is just a word for the charge that determines how it feels the strong force. Originally Gell-Mann patriotically suggested that these three colors should be called red, white, and blue, but today physicists usually plump for the more neutral red, green, and blue.

  If quarks come in red, green, and blue varieties, antiquarks come in anti-red, anti-green, and anti-blue, and, just as with electric charge, like colors repel while opposites attract. So two red quarks will repel each other while a green quark and an anti-gree
n antiquark will want to get together. One of the things that makes the strong force more complicated than the electromagnetic force is that the three different colors also attract one another, so a red up quark, a green up quark, and blue down quark will draw one another together to form a proton. Hadrons (particles made from quarks) are always colorless overall; either a color paired with its anti-color in a meson, or all three colors mixed together in a baryon. Thanks to all this color business, the theory has a very cool-sounding name: “quantum chromodynamics” (QCD), the quantum theory of color.

  As well as dictating that quarks come in three colors, QCD tells us that the strong force is transmitted by particles called “gluons,” literally because they glue quarks together. At first glance, gluons look a lot like photons, the force carriers of electromagnetism. Like photons they have zero mass and a spin of 1. However, the particular requirements of the SU(3) symmetry group mean that while there is only one type of photon, there are eight different types of gluon. And, crucially, while the photon carries no electric charge, gluons are colored. It’s this final fact that explains why, even to this day, no one has ever seen a quark flying solo.

  Here’s why. Photons only interact directly with electrically charged particles like protons and electrons. Since photons are electrically neutral, this means that if you fire two photons at each other they’ll (almost) always just zip past each other without so much as a gentle handshake. They pass like ships in the night.