How to Make an Apple Pie from Scratch

by Harry Cliff

  If two of these neutron stars are produced close enough together, they can end up colliding, sending violent ripples out through space and time. When LIGO first detected one of these signals, astronomical observatories around the world swung their telescopes to point at the patch of sky where the waves appeared to have come from. Extraordinarily, they saw light, and when they analyzed that light spectroscopically, they found evidence of vast quantities of heavy metals being created, from gold through to uranium. In fact, by one estimate the collision produced enough gold to make thirty solid-gold planet Earths. But before you get on the phone to Elon Musk with a get-rich-quick scheme you should know that the collision happened in a galaxy around 130 million light-years away, a long ride in even the zippiest of rockets.

  For decades it was thought that the heavy elements in our universe were made in supernova explosions, but Jennifer and her colleagues now suspect that a large fraction came from these cataclysmic neutron star collisions instead. It’s extraordinary to think that most of the gold in an ordinary piece of jewelry is really a little bit of a neutron star. (Admittedly there isn’t much gold in an apple pie, but perhaps we can make it a fancy one with a bit of edible gold leaf on top.)

  At Apache Point, Karen labored tirelessly through the night, working her way through target after target, all the while making sure the Sloan Telescope and APOGEE were functioning as expected. Each time they had collected enough light for a given patch of sky, Viktor, the cold observer, had to trudge out into the pitch-black armed only with a small flashlight, to unload the spent 150-kilo cartridge and install the next one. By one a.m. I was flagging and crept off to grab a few hours’ sleep in the nearby dorm building before getting up again at five a.m. to see Karen close up for sunrise.

  I found her, tea in hand, looking a little bleary-eyed but pleased with how the night had gone. Observing conditions had been more or less perfect and the Sloan had functioned flawlessly. Within just a few hours the night’s observations would be available to the hundreds of scientists working on the Sloan Digital Sky Survey all around the world.

  For Jennifer and her colleagues, the hunt is now on for the oldest stars in the universe. As the universe has evolved, stars have increasingly enriched interstellar space with what astronomers refer to as metals, by which they mean any element heavier than helium. As a result, younger stars tend to be rich in metals, while the oldest stars are metal poor. Using APOGEE to determine what elements are present in the atmosphere of a star therefore allows astronomers to infer their age. The dream would be to find a star that formed from the pristine, unpolluted gas that filled the universe before the first stars blinked into light.

  Such a star, if one is ever found, would be an ancient relic from the birth of the universe, made of about 75 percent hydrogen and 25 percent helium. However, this in itself raises a question. Over the past 13 billion years or so, generations of stars have transformed just 2 percent of the matter in the universe into heavier elements. If that’s the case, and assuming that all matter began as the simplest element, hydrogen, then where did all the helium come from? It was a question that Fred Hoyle and his collaborators were unable to answer back in the 1950s.

  Outside, in the cold, crisp air, a faint light was growing behind the wooded ridge to the east. Karen and I walked quietly down to the Sloan Telescope, which stood gazing at the final target of the night. As she closed up the telescope, I asked how she coped with so many sleepless nights, so far away from home. She turned and gestured at the view. Across the Tularosa Basin, the lights of Alamogordo twinkled softly in the morning air, and the tips of the San Andres Mountains were catching the first light of the rising Sun. “This,” she said, “makes it worth it.”

  Skip Notes

  *1 Or so I thought. Turns out that Mr. Kipling never actually existed. He’s a fake, a fraud, like the Wizard of Oz or Ronald McDonald, invented by brand consultants in the 1960s to flog baked goods. That said, branding consultants are still carbon-based life-forms (my brother is one) so the point stands.

  *2 Hoyle is credited with coming up with the phrase “big bang” in a 1949 BBC radio interview. Some claimed the term was meant as an insult, although Hoyle insisted that he was just trying to conjure a striking image.

  *3 A plutonium implosion bomb was successfully tested on July 16, 1945, in the New Mexico desert. Another plutonium device was detonated over the Japanese city of Nagasaki a few weeks later on August 9, killing between 39,000 and 80,000 people.

  *4 An MeV is a megaelectron volt, the energy that an electron gets when you zap it with a million volts.

  *5 The reason why different chemical elements absorb and emit characteristic frequencies of light is all down to the quantum structure of atoms. As discussed in chapter 3, electrons orbit the atomic nucleus in discrete, quantized energy levels, which are unique to each chemical element. When an electron makes a quantum jump to a different energy level it must either absorb or emit a photon, whose energy must be equal to the difference in energy between the two levels. To jump to a higher energy level, the electron must absorb a photon, and when falling to a lower energy level it emits a photon. Now the energy of a photon depends directly on its frequency (higher frequency = more energy) and a given atom will therefore absorb and emit photons of the specific frequencies that match the arrangement of its unique tower of energy levels.

  *6 That said, life on Earth will have become pretty uncomfortable long before this—as the Sun ages it gets smaller and hotter, and in a mere billion years from now the Earth will become so hot that the oceans will boil, assuming we don’t manage to do it ourselves first.

  *7 The term “planetary nebula” was coined in the late eighteenth century when astronomers didn’t have much of a clue what they were looking at, thinking that they resembled fading planets.

  *8 Shortly after my trip to Apache Point there was a frenzy of speculation that Betelgeuse might be about to blow after it dimmed unexpectedly over the winter of 2019–20, but that was eventually put down to dust blocking its light.

  *9 LIGO stands for Laser Interferometer Gravitational-Wave Observatory.

  CHAPTER 7

  The Ultimate Cosmic Cooker

  The atoms that make up our bodies were forged billions of years ago deep inside stars.

  That has got to be the most poetic idea ever to come out of science. It connects our ordinary, humdrum lives to the cosmic. We, and everything we see around us, apple pies included, are part of the story of the lives and the deaths of stars. Unsurprisingly, the discovery of our celestial origins quickly caught the imaginations of artists, writers, and musicians. In 1969, Joni Mitchell wove the idea into her countercultural anthem “Woodstock,” which expresses a young generation’s yearning to achieve a more perfect state of harmony with itself and with nature: “We are stardust (billion year old carbon) / We are golden (caught in the devil’s bargain) / And we’ve got to get ourselves / Back to the garden.” That and to get whacked off its tits in a field. Of course, this does all raise another question, namely, where did the stuff that the stars were made of ultimately come from? To a certain extent, the answer is other stars, which died and blew their matter into space, which then got mixed up with other dust and gas to form yet more stars. But at a certain point this chain of logic must break down.

  The fact that there is still a large amount of hydrogen in the universe implies one of two things. If the universe is infinitely old and stars are continuously turning hydrogen into heavy elements, then new hydrogen must somehow be being created to replenish what the stars use up. The alternative is that the universe is not infinitely old and star formation began at some point in the past, billions of years ago perhaps, but certainly not infinity ago.

  So the question of where the matter that makes the stars comes from is thus inextricably mixed up with an even more profound question, arguably the most profound questio
n scientists have ever asked: Did the universe begin?

  While Joni Mitchell was singing about stardust, a long-fought, sometimes bitter argument over the origins (or not) of the universe was coming to an end. On one side were those who argued that the universe has always been here, and that despite all the dynamism that we see in the sky, at the largest scales the universe is ultimately unchanging and eternal, with neither beginning nor end. Chief among the proponents of this steady-state universe was habitual contrarian and architect of stellar nucleosynthesis Fred Hoyle.

  Arrayed against Hoyle and his collaborators were the supporters of what Hoyle himself had labeled the “big bang,” who argued that the universe had been born billions of years ago, bursting into existence from a single point of unimaginable density and creating space, time, light, and matter in the process.

  Hoyle hated the idea of the big bang. As far as he was concerned, it was unscientific, involving a moment of creation whose ultimate cause could never be probed scientifically. Worse still, as an avowed atheist, it had the unpleasant whiff of religion about it. Allow the universe to have a beginning and you open the door to all kinds of mystical nonsense about how that beginning came to pass.

  However, as we’ll soon see, both the big bang and the steady state involve moments of creation of one kind or another. The big bang gets creation done all in one go at the beginning of the universe. Meanwhile, the steady state requires an infinite number of microscopic moments of creation, with individual particles of matter continuously popping into existence throughout all of space and time.

  I’m sure you know how this debate ends—after all, there’s no sitcom called The Steady State Theory—but the discoveries that led to the big bang and the steady state, as well as the observations that finally saw the big bang win out, are absolutely crucial to our quest for the origins of matter. From here on in, as we leave the chemical elements behind and venture deeper into the structure of matter, we will find that there is only one oven that matters, the one at the beginning of the universe.

  THE UNIVERSE EXPLODES

  A few years ago, I attended a particle physics conference just outside Melbourne, Australia. It was a bit of a sweet gig (as nobody says in academia): the venue was in the seaside resort of Torquay, a mecca for surfers and the gateway to the Great Ocean Road, an improbable 151-mile stretch of asphalt that snakes its way westward past sheer limestone cliffs, long stretches of white sand, and lush rain forests. Before the week of intense PowerPoint presentations kicked off, I hired a car and spent a few days exploring the famous route, stopping off in pretty seaside towns along the way.

  One night, I was driving back to my hostel after a slightly disappointing evening bobbing about on a lake on what had been advertised as a “platypus watching tour,” during which the platypuses had remained conspicuously absent. Emerging from the forest, the unlit road turned along the coast, back toward my hostel at Apollo Bay. It was a particularly clear night and as I was miles from the nearest town I decided to pull over and take a look at the night sky.

  Switching off the headlights, I stepped out of the car and looked up. What I saw made my head swim. Above me, stretched out across the sky, was the Milky Way, thousands of stars shining more brilliantly than I had ever seen before. I was suddenly overcome by what felt like vertigo, and for a brief moment I lost my balance and reached out to steady myself against the roof of the car.

  Having spent most of my life living in or close to big cities, I had only seen the faint trace of the Milky Way a handful of times, but here, on a moonless night, far from any sources of light pollution, it dominated the sky. Directly above me was the glowing bulge of the galactic core, wreathed in the immense shadow of the Great Rift, a colossal band of molecular dust that hangs like smoke against the galaxy behind. Just to one side were two luminous patches—the Large and Small Magellanic Clouds—dwarf galaxies in orbit around the much larger Milky Way. The night sky at my home in London is a two-dimensional thing, a dark sheet pierced by a few pricks of light, but this scene was so brilliant, so detailed, that for the very first time I felt as though I was looking at a huge three-dimensional object.

  I think that moment was the first time that I felt awe in the true sense of the word: a mixture of wonder, delight, and fear. The way the galaxy loomed above left me feeling insignificant and yet exhilarated at the same time. The experience reminded me of the Total Perspective Vortex from Douglas Adams’s Hitchhiker’s Guide to the Galaxy, a torture device that drove its unfortunate victims mad by showing them the unfathomable vastness of the universe accompanied by a microscopic dot on a microscopic dot labeled “You are here.”

  Until the 1920s, most astronomers thought that the Milky Way was the entire universe: a gigantic island of stars, alone in the darkness. However, there was a debate, occasionally fierce, over whether spiral nebulae, faint whirlpool-like smudges scattered across the night sky, were clouds of dust and gas in the Milky Way, or perhaps their own island universes far beyond the borders of our galaxy. The problem was that there was no way to measure how far away they were, that is until a crucial breakthrough made by the pioneering American astronomer Henrietta Swan Leavitt.

  In 1904 Leavitt discovered a number of faint stars in the Small Magellanic Cloud whose brightness seemed to change over time. During the next few years she found hundreds more of these variable stars and by 1912 had noticed a clear relationship between their brightness and how fast they brightened and dimmed; the brighter the star was on average, the slower it pulsed.

  Leavitt’s law, as it became known, was the crucial clue that would allow astronomers to measure the distances to objects outside our local galactic neighborhood for the first time. By taking the pulse of one of these variable stars, an astronomer could figure out how brightly it shines, and if you compare that to how bright it appears (more distant stars look dimmer than nearby ones), you can tell how far away it is.

  Then in 1923, the American astronomer Edwin Hubble discovered a variable star in the largest spiral nebula in the night sky, Andromeda. Using Leavitt’s law, he estimated that Andromeda was almost a million light-years from Earth,*1 a shockingly huge number considering that the size of the entire universe had recently been estimated at only a thousand light-years. At a stroke the cosmos had grown in size by a factor of a thousand.

  In just a few short years, the way people imagined the universe was transformed. It became clear that spiral nebulae weren’t clouds of dust and gas in the Milky Way but galaxies containing billions of stars that lay far beyond the edge of our own. The universe was suddenly a much larger place, but an even more significant discovery was yet to come.

  A decade earlier, Vesto Slipher, who despite sounding like a character from Star Wars was actually a real-life astronomer at the Lowell Observatory in Arizona, had made the startling discovery that the Andromeda nebula appeared to be hurtling toward the Earth at a speed of around 300 kilometers per second. As he studied other nebulae, he found that they all seemed to be moving, but most of them were actually moving away from the Earth, some at incredibly high speeds of more than 1,000 kilometers per second. At first Slipher tried to make sense of their motions by suggesting that the Milky Way itself might be drifting through space relative to the nebulae, but without knowing how far away they were it was impossible to draw firm conclusions.

  Armed with Leavitt’s law, Hubble was now ready to attack Slipher’s puzzle. Working at the Mount Wilson Observatory in California, he made careful studies of variable stars in twenty-four galaxies outside the Milky Way and calculated their distances. Comparing his results with Slipher’s estimate of their speeds he found an intriguing pattern. With the exception of the very closest galaxies to the Milky Way like Andromeda, every galaxy in the night sky appeared to be moving away from the Earth, and the farther away they were the faster they were retreating. It was as if the entire universe was expanding away from us, although when he publi
shed his results in 1929 Hubble himself was careful not to make such a bold claim.

  At first some questioned whether Hubble’s results were reliable, but by 1931 he had produced new measurements including galaxies more than 100 million light-years away. The new data left little room for doubt; the effect was real. What’s more, there was an unmistakable linear relationship between the speed of a galaxy and its distance—in other words, a galaxy twice as far away from the Earth would be moving away twice as quickly. The controversial bit was how to interpret it. Many physicists, including Einstein, had been wedded to the idea of a static, unchanging, eternal universe. To admit that the universe might be expanding opened up the possibility that it might have had a beginning, and that idea made many physicists and astronomers feel unwell.

  One man who felt no such queasiness was the Belgian physicist and Catholic priest Georges Lemaître. Not only did Lemaître argue that the universe was expanding, he took the argument to its logical extreme: if the universe was getting bigger, then in the past it must have been smaller, and if you keep winding the clock back then eventually you arrive at a point where everything in the universe was squashed into a single unimaginably dense object, what Lemaître called the “primeval atom.”

  Taking his inspiration from radioactivity, Lemaître imagined the primeval atom as an atomic nucleus, just a really, really heavy one weighing as much as the entire universe. According to Lemaître the universe began when his cosmic nucleus suddenly exploded like a firework, shattering into star-sized atoms, which carried on breaking apart into smaller and smaller pieces, eventually giving rise to everything we see around us.