How to Make an Apple Pie from Scratch

by Harry Cliff

  Unfortunately, at the start of 2019 Borexino ran into an unexpected and potentially terminal problem. Not a scientific one, mind, but a legal one, whose history can be traced all the way back to 2002. That summer, human error led to some of the liquid hydrocarbon used in the detector escaping into the groundwater. Since then, environmental safety standards and procedures have been significantly tightened at the lab and everyone I spoke to was convinced that the risks of a similar incident were now absolutely tiny. But the damage to local community relations was done. A determined, decade-long campaign by environmental activists finally came to a head just two months before my visit, when it was announced that three senior managers at LNGS would face criminal prosecution. Another consequence of the court case is that all new experimental work has been suspended and Borexino will have to shut down in a little under two years.

  Hence the race against time. The big question in the researchers’ minds is whether two years will be enough to see neutrinos from the CNO cycle. The rarity of CNO neutrinos means that to have any chance of spotting them the team at Borexino will have to control radioactive backgrounds at an unprecedented level.

  Walking farther along the gantry several meters above the cavern floor we passed the control room and moved toward the back of the experimental hall. The mechanical chirruping grew louder until we finally came face-to-face with Borexino itself: a 17-meter-tall domed tank, covered in silver insulating foil that glistened softly in the artificial light. Toward the top, loops of blue piping were wrapped around its girthy 18-meter diameter. The whole thing looked like some nineteenth-century vision of an alien spacecraft. The shiny insulation and blue piping, Aldo told me, had only been added recently in the hope that it might make seeing CNO neutrinos possible at long last.

  When Borexino was first proposed way back in the 1990s, no one had thought that it had any chance of seeing neutrinos from the CNO cycle. The signal was just too weak and the radioactive backgrounds too high. However, in recent years the team realized that a peculiar feature of the underground environment might make it possible after all.

  The surrounding mountain rock keeps the cavern floor on which Borexino sits at a more or less constant temperature of 8 degrees. This, it turns out, is cooler than the average temperature 17 meters higher up at the top of the tank. Since hot things go up and cool things go down, the consequence is that the liquid inside Borexino remains almost completely still. Crucially this keeps any radioactive contamination that seeps out of the spherical nylon container inside the tank in place, stopping it from mixing with the liquid used to detect the neutrinos. The job of the new insulation and blue water pipes is to maintain as steady a temperature as possible, which in turn will keep the liquid inside from flowing about, which maybe, just maybe, will give Borexino a fighting chance of spotting the last missing reaction that the Sun uses to make helium.

  We climbed down a staircase to the stand at the foot of the towering detector. It is by far the strangest observatory I have ever seen, a silent giant, deep under a mountain, that waits patiently for whispers from ghostly emissaries from the center of our Sun. As we left Hall C to return to the car, I asked Aldo what he thought the chances of seeing the CNO cycle were before Borexino’s life came to an end. He looked at me sideways. “We’ll see…I would say, by the end of the year.”

  A fortnight before my trip to Italy I’d had a Skype call with Gianpaolo Bellini, known affectionately as the “father of Borexino,” from his holiday villa in rural Italy. Although now in his eighties and retired (at least officially) he still radiates enthusiasm and delight in the success of the experiment that he first imagined in the early 1990s. Finally snaring the CNO cycle would be both a sweet end to a long career and a fitting reward for the hundred-strong team who have worked tirelessly to perfect their observatory. Once Borexino goes offline in two years’ time, there are no new detectors in the works with similar capabilities. If Borexino doesn’t see the CNO cycle, perhaps no one will.

  That evening at my hotel, I thought about taking a drive up into the mountains to catch the sunset, but in the end I decided sod it, I was tired and it was time for a beer and a pizza. You’ve seen one sunset you’ve seen them all, and in any case, unlike sunshine, neutrinos don’t get blocked when the Sun goes down; they stream right through the Earth just as strong as ever. So, as I sat on the terrace of my hotel in the dwindling light sipping my beer I tried to imagine an invisible flood of neutrinos, trillions upon trillions strong, streaming through my body, a short moment on their long journey from the center of the Sun out into the depths of space.

  Skip Notes

  *1 Nuclear fusion reactors would produce no carbon dioxide and unlike nuclear fission reactors, which generate their energy by breaking uranium nuclei apart, no long-lived radioactive waste.

  *2 We’ll see later they do have a role to play in making elements beyond iron, including gold.

  *3 E = mc2 doesn’t actually appear in Einstein’s paper. Instead he uses a combination of symbols and words to spell out the same relationship.

  *4 A similar effect was discovered around the same time by two American physicists, Clinton Davisson and Lester Germer at Bell Labs in New York. Davisson shared the Nobel Prize with G. P. Thomson.

  *5 Or indeed agrees if it actually is necessary. For a great account see Beyond Weird by Philip Ball.

  *6 The Americans Ronald Gurney and Edward Condon came up with the same solution as Gamow at the same time.

  *7 The future “father of the atom bomb.”

  *8 The accepted modern value.

  *9 At the time, Bethe incorrectly believed that the CNO cycle was the dominant power source of the Sun thanks to an overestimate of its core temperature.

  CHAPTER 6

  Starstuff

  It’s several months now since I first caught the train down to my parents’ house on a mission to break a Mr. Kipling Bramley apple pie down into its chemical elements. I keep the products in sealed test tubes on my desk at home as a reminder that no matter how deep into the strange and abstract world of particle physics we get, we are still ultimately chasing the origins of ordinary stuff. Also, they’re fun to look at.

  The lump of carbon, the charred remnants of the apple pie, is my favorite: hard, jagged, and jet-black, with small reflective faces that catch the light. Of all the elements, carbon has got to be the most charismatic. Its many and varied personas, from charcoal to diamond, make it the David Bowie of the periodic table, but its real mystique comes from its role as the key building block of life. All living things, from apple trees to Mr. Kipling himself,*1 are built up from molecules with backbones of carbon.

  The atoms that make up that little lump of carbon are truly ancient. They were forged long, long ago, before the first living thing, before the Earth formed, even before the Sun first flickered into light, somewhere, out there, in a distant part of the cosmos. The question is, where?

  We have taken the first step toward a recipe for the elements in an apple pie. Thanks to more than a century of work by astronomers and physicists, we know that stars like our Sun are giant thermonuclear ovens that cook helium from hydrogen over billions of years. If the stars can make helium from hydrogen, then perhaps they can also make the heavier elements. Since an ordinary carbon nucleus is made of six protons and six neutrons, making one should be a simple matter of melding three helium nuclei together.

  In fact, Hans Bethe made exactly that suggestion when he published his famous paper on why the stars shine way back in 1939. However, he ran into the same problem that Arthur Eddington had encountered when he first suggested that the Sun might be powered by fusing hydrogen into helium—the stars didn’t seem to be hot enough. As we saw in the last chapter, getting two protons to fuse meant finding a way to overcome the gigantic electrical repulsion between the two positively charged particles. The solution in that case was pro
vided by Gamow, Houtermans, and Atkinson when they showed that quantum mechanics allowed a proton to quantum tunnel through the walls surrounding the nuclear fortress, allowing fusion to take place at temperatures found in the cores of the Sun and stars.

  Unfortunately, forcing three helium nuclei together to make carbon is far more challenging. A helium nucleus has an electric charge of +2, which means that the repulsive force between three helium nuclei is far stronger than for hydrogen. Hans Bethe realized that to get helium to fuse would require temperatures of hundreds of millions, perhaps billions, of degrees, far hotter than anyone imagined could possibly exist inside a star.

  But if stellar ovens weren’t hot enough to cook the heavy elements, then where do they come from? One daring solution was proposed by George Gamow and his PhD student Ralph Alpher in 1948. If the elements beyond helium weren’t made in the stars, there was only one other furnace that could conceivably be hot enough—the primordial fireball at the dawn of time.

  The idea that the universe had begun with a big bang had been gaining support since the 1920s, when astronomers had discovered that the universe seemed to be expanding. Winding the clock back, this implied that in the past the universe must have been smaller, and if you went back far enough you found that everything would once have been squashed into a single point.

  According to Gamow and Alpher’s theory, billions of years ago the entire universe was concentrated into a tiny, unimaginably hot embryonic blob, filled with a superheated gas of neutrons. For reasons unknown, that blob started to expand rapidly, and as it grew and cooled, the neutrons successively bumped into one another, building up the elements one by one in a frenzy of nuclear reactions, starting with hydrogen and running all the way through the periodic table.

  Unfortunately, it soon became clear that the theory had a fatal flaw. Nature had rather unobligingly failed to include a chemical element with mass 5. That meant once you made helium (mass 4) the path was blocked. Adding another neutron to helium results in a nucleus so fantastically unstable that it falls apart in around a billionth of a trillionth of a second, far, far too short a time for another neutron to have a chance of smacking into it and getting you to mass 6. One way over the chasm at mass 5 might be to bang two helium nuclei together to make a nucleus of mass 8, but again the resulting nucleus’s life is so fleeting—one ten-thousandth of a trillionth of a second—that there just isn’t enough time to let you leapfrog on to the next stable element at mass 9.

  Gamow and Alpher’s big bang had crashed and burned. And yet, we live in a universe with carbon, oxygen, iron, and uranium. They must have come from somewhere. But where?

  Fortunately, at the very time that Gamow and Alpher were working on their theory in the United States, a young theoretical physicist from England named Fred Hoyle was puzzling over the same problem.

  THE RECIPE FOR CARBON

  Fred Hoyle was one of the most influential and controversial astronomers of the twentieth century. Born in Yorkshire in the north of England to a poor family in the wool trade, he often played truant from school before catching the science bug when he borrowed a book from his local library by Arthur Eddington titled Stars and Atoms, two subjects that would come to dominate his life. Thanks in large part to the dogged efforts of a devoted teacher, Fred secured a scholarship to study at Cambridge, and by cock-up more than conspiracy, wound up as the PhD student of Paul Dirac, the greatest quantum physicist in the known universe. In the mid-1930s, Dirac, who was an otherwise rather unwilling supervisor, gave Hoyle one life-changing piece of advice: the glory days of physics were over, the quantum revolution was done, and the time wasn’t yet ripe for new breakthroughs. If the ambitious young man wanted to make his mark on science, he should look elsewhere. So Fred Hoyle turned his attention to the stars.

  During his long and eclectic career, Hoyle became famous for his contrarian, sometimes wacky scientific views, bitter disputes with fellow academics, and as a talented writer of science fiction, including the smash-hit BBC television series A for Andromeda. Today, he’s probably best known as a die-hard opponent of the big bang theory, which he dismissed as pseudoscience thanks to its inability to explain what actually caused it to happen in the first place.*2

  However, despite all the sound and fury that Hoyle generated, he was unquestionably a brilliant scientist. In fact, a key ingredient of his success was the same willingness to go against orthodox thinking that often landed him in hot water. As far as Hoyle was concerned, “It’s better to be interesting and wrong than boring and right.” One subject on which he turned out to be both interesting and right was the origins of the chemical elements.

  In late 1944, Hoyle got a chance to leave the gloom of wartime Britain to visit the United States for a meeting on radar technology. While stateside, he took the opportunity to drop in on the Mount Wilson Observatory in California, where he cadged a lift back to Pasadena from the greatest observational astronomer of his age, Walter Baade. Baade and Hoyle soon got chatting about the most energetic outbursts in the known universe: supernovae. These stupendously violent stellar explosions can briefly blast out more power than all of the hundreds of billions of stars in a galaxy combined. At the time, no one could explain the source of a supernova’s awesome power, but Hoyle’s mind was set whirring when he bumped into a former colleague as he waited for his flight home in Montreal.

  His name was Maurice Pryce, a British physicist who had mysteriously disappeared from the radar establishment in Portsmouth where Hoyle was based earlier that year. Although what exactly Pryce and his colleagues were doing in Montreal was a closely guarded secret, the presence of key nuclear physicists at the nearby Chalk River site left Hoyle in little doubt: they were working on an atomic bomb.

  During his conversations Hoyle picked up a hint of the problem they were struggling with. The objective was to design a bomb that used the radioactive isotope plutonium-239 as the nuclear explosive. To trigger a nuclear chain reaction the scientists and engineers at Chalk River were trying to find a way to crush a sphere of plutonium in on itself, in other words to create an implosion. If the plutonium could be imploded fast enough, then a runaway nuclear reaction would be set up, releasing a vast amount of destructive power in a far larger explosion.*3

  Back in Britain, Hoyle began to wonder whether a similar process might be responsible for supernovae. As a star ages, it gradually burns through its hydrogen fuel supplies until at last its entire core has been transformed into helium. Hoyle realized that robbed of the source of heat that keeps the star inflated, the core would start to collapse under the crushing force of its own gravity. As it implodes, this vast well of gravitational energy would be converted into heat, sending the temperature at the center of the star skyrocketing and ultimately resulting in an unimaginably violent explosion—a supernova. A year or so later, Hoyle calculated that a collapsing star could in principle generate temperatures of more than 4 billion degrees Celsius, hundreds of times higher than anyone had previously imagined possible. Maybe, just maybe, stars were the cosmic cookers that made the chemical elements after all.

  When he was released from his wartime work in 1945, Hoyle returned to Cambridge and to the world of astronomy. Although he had already put forward a theory of how the elements might be made in a collapsing star, the details were far from worked out, and without a specific recipe to get from helium to the heavier elements, it was doomed to fail just like Gamow and Alpher’s.

  Going back to Bethe’s seminal paper on nuclear fusion from 1939, Hoyle hit upon the idea of trying to revive what became known as the “triple-alpha process,” the reaction where three helium nuclei combined to form a nucleus of carbon-12. Bethe had originally dismissed the reaction for two reasons: first it required stupendously high temperatures beyond what was possible in any star, and second, the chances of three helium nuclei smacking into one another at the same instant were vanishingly small.

  Hoyl
e’s work on collapsing stars had already removed the first objection, and he thought he could spy a way of overcoming the second as well. Instead of three helium nuclei running into one another at the same time, what if two collided first to form a nucleus of beryllium-8 (four protons, four neutrons), before being hit by a third helium to make carbon-12. But hang on, didn’t we already say that there’s no stable element of mass 8? That’s true; beryllium-8 only lives for one ten-thousandth of a trillionth of a second before splitting apart into two helium nuclei. But Hoyle realized this might not be an insurmountable problem. In a star with the right temperatures and densities, so many helium nuclei would be crashing into one another that enough beryllium-8 would get made to make up for the fact that it was constantly falling apart. This balance of creation and destruction would mean there was always a stable concentration of beryllium-8 in the star’s interior, even though an individual nucleus lived such a fleeting existence.

  The question was, would there be enough beryllium-8 around at any one time so that a decent number would get hit by another helium to make carbon-12? In 1949, Hoyle gave the problem to one of his PhD students. If the answer came out the way he hoped, the door would be open to fuse all of the elements in the periodic table.