by Harry Cliff
On a crisp February morning, I arrived at Imperial College’s Blackett Laboratory, a 1960s block plonked unselfconsciously amid the neighborhood’s Victorian grandeur, just across the road from the Royal Albert Hall. I was met in the foyer by Isabel Rabey and Sid Wright, two young postdoc researchers who have dedicated years of their lives to the experiment. Isabel was back in town to see her old team, having spent her PhD down in the basement improving the experiment before heading off for a position at the Max Planck Institute near Munic; while Sid, a relative newcomer, had taken on a lot of the work at the coal face following Isabel’s departure. They seemed pleased to have an opportunity to talk about what is obviously a labor of love. I explained how as an LHC physicist I had been keen to see the incredible experiment that was giving us a run for our money. Isabel laughed. “I think you’re in for shock.”
Down a couple of flights of an echoing stairwell and along a short corridor, they welcomed me into their laboratory. It really was tiny, hardly any bigger than the living room of my not-very-big London flat. Isabel told me that when they showed around representatives of the national agency that funds large particle physics and astronomy projects in the United Kingdom they had been taken aback by just how pint-sized their experiment was. “We got the sense that they were thinking, ‘If you made it a bit bigger, perhaps we could fund you.’ ”
To see the experiment properly we had to shuffle one by one between a wall and a heavy piece of shielding protecting the experiment from any stray magnetic fields that might interfere with the delicate measurements. To our right was a bank of oscilloscopes and electrical gizmos that were used to read out and monitor the experiment, on the left a large table covered with optical elements that glowed with lurid green laser light, and, in the center, was the business end of the experiment, which, I hope Isabel and Sid won’t mind me saying, looked to my untrained eyes like a metal dustbin.
It was certainly a far cry from the towering experiments of the Large Hadron Collider, which one journalist I showed around compared to the giant alien portals from the 1990s sci-fi show Stargate. If the LHC is Stargate then Isabel and Sid’s experiment were more like Doc Brown’s time-traveling DeLorean from Back to the Future, a little ramshackle looking perhaps, but remarkably effective.
Isabel and Sid gamely talked me through how the experiment worked. I will readily admit that my atomic and molecular physics is a bit rusty, and I felt like a slowpoke as I tried to get my head around the complex system of lasers and magnetic and electric fields used to measure the shape of the electron.
The first thing to understand is what we mean by the shape of the electron. Strictly speaking, the experiment measures something called an “electric dipole moment” (EDM), which is a measure of how the electric charge of the electron is spread out in space. An EDM of zero means that the electron’s charge is distributed in a perfectly symmetrical sphere, while a nonzero EDM would mean that the electron is like a cigar, more negatively charged at one end and more positively charged at the other. The electron’s EDM turns out to be extremely sensitive to whatever quantum fields are hanging around in the vacuum in the vicinity of the electron, which is why it’s such an interesting quantity to measure. If you do some serious number crunching using only the quantum fields that we already know about you find that the EDM of the electron should be stupendously tiny, with a value of 10-38 e cm (an e cm is the unit of the electric dipole moment, where e is the charge of the electron and cm is a centimeter, but don’t worry about the details; the main thing is that 10-38 is really, really small). This is so tiny that if there are no new quantum fields beyond the ones that we already know about, then the electron should appear perfectly spherical to any experiment that we can currently imagine.
However, many popular theories that attempt to explain dark matter and other mysteries introduce new quantum fields that dress the electron in a way that squashes it into a far more pronounced cigar shape, in some cases increasing its EDM by a factor of more than a trillion. Such huge enhancements would put it within reach of the Imperial experiment, allowing the team to potentially discover hints of new quantum fields—without the aid of a 27-kilometer particle collider.
But even with a huge boost from new quantum fields, the electron’s EDM would still be indescribably minute, and measuring it requires a correspondingly ingenious experiment. Rather than measure electrons directly, the team at Imperial study ytterbium fluoride, a molecule of the rare metal ytterbium and fluorine gas, carefully chosen thanks to its special sensitivity to the electron’s EDM. In particular, the outermost electron in an ytterbium fluoride molecule can exist in two different energy levels, one where the electron’s spin is pointing up and the other where it’s pointing down. The crucial point is that energies of these up and down levels are shifted in opposite directions by the EDM of the electron. In other words, if the electron has a large EDM, then one level gets shifted up in energy, while the other gets shifted down. So if you can measure the difference in energy between these two levels then you can indirectly measure the electron’s EDM. This property means ytterbium fluoride acts a bit like a magnifying glass that makes you a million times more sensitive to the electron’s EDM than if you tried to measure it using electrons flying around outside the confines of an atom or molecule.
Working with these molecules comes at a cost, including the fact that ytterbium fluoride is so unstable that you have to create it continuously inside the experiment. As we stood in the lab, Sid pointed out a continuous drr-drr-drr-drr-drr-drr, the sound of a laser striking a ytterbium metal target twenty-five times per second, vaporizing little puffs of ytterbium from a solid metal block, which then react with fluorine gas to form tiny clouds of ytterbium fluoride molecules. A clever system of lasers, radio waves, and microwaves then puts the molecules into a mixture of the spin up and spin down energy levels before they are allowed to drift upward through the metal cylinder (the thing I compared to a dustbin), which contains an electric field.
The up and down states get opposite energy shifts from the electric field, and the size of their shifts depends on the size of the electron’s EDM—the bigger the EDM the bigger the energy shift. Once the molecules exit the electric field at the top of the cylinder, they are measured using a laser, allowing Isabel, Sid, and the team to determine the energy shift and, after painstaking months of data collecting, measure the electron’s EDM.
That’s the idea at least. In practice making the measurement is extremely tricky. The instrument is so sensitive that it can be affected by all kinds of external influences. Particularly troublesome are stray magnetic fields. They once discovered that a problem they were having with the experiment was due to a powerful magnet being used by a different team two floors above them. A stern word from their boss and originator of the EDM experiment, Professor Ed Hinds, soon saw the other team agree to move their magnet to a higher floor. Sid also told me that they’d noticed that the magnetic interference gets much worse when London Underground trains were running.*7
The Imperial team led by Ed Hinds released their first measurement back in 2011, finding that the electron appeared to be exquisitely round, all the way down to a precision of 10-27 e cm. To give you a sense of just how spherical that is, if you were to blow up an electron to the size of the solar system, it would be spherical to within the width of a single strand of human hair!
Disappointingly (at least for particle physicists), the marvelous roundness of the electron ruled out the existence of a bunch of new quantum fields that I and my colleagues at the LHC were busily searching for at the time. However, the measurement itself was a real tour de force; not only was it the most precise in the world, it was also the first time that molecules, rather than atoms, had been used in an EDM measurement. Back then, every other experiment used single atoms, and many of the Imperial group’s rivals had thought they were wasting their time trying to make the delicate measurement work with relat
ively messier molecules. Nowadays, though, almost all their rivals are following the trail blazed by the Imperial team, thanks to molecules’ powerful EDM magnifying properties.
Indeed the ACME experiment, run by a joint Harvard-Yale team in the United States, has since leapfrogged the Imperial measurement, pushing the EDM down by another factor of a hundred, with a second team based in Colorado snapping at their heels. To catch up, the Imperial team are now developing a new secret weapon, an upgraded version of their experiment, which I was given a glimpse of in a neighboring, far more spacious lab. “This one’ll look more like what you’re used to at CERN,” Sid assured me as we entered. In front of us was a gleaming stainless-steel tube, not unlike a particle accelerator, which will eventually be extended to span the full length of the laboratory. A longer tube means that the molecules spend more time in the electric field, giving a bigger energy shift and a big boost to the experiment’s sensitivity. When their new instrument starts collecting data, it’ll effectively be probing quantum fields whose particles could have masses well above what the LHC is able to produce directly, providing a fantastic opportunity to discover more of nature’s fundamental ingredients.
* * *
—
We’ve certainly come a long way from that first apple pie experiment. Back then we were dealing with tangible things you could taste and touch: jagged lumps of black carbon, oily liquids, curling tendrils of acrid vapor. The ingredients that we’re now left with are about as far from tangible as you can get—invisible, ethereal, omnipresent quantum fields. The apparent solidity of the world turns out to be an illusion, a conjurer’s trick. There are no indivisible atoms as the ancients thought. Democritus, for all his ancient beardy wisdom, was wrong. Nature, deep down at its roots, is continuous, not discrete. I’ve used the phrase “building blocks” throughout this book to talk about the fundamental ingredients of nature, but in truth there are no such things. The apparent “blockiness” of matter dissolves when we look closely enough. Particles are not particles, they are passing disturbances in quantum fields, entities that strain the imagination and yet fill every last cubic centimeter of the cosmos. All objects—apple pies, humans, stars—are agglomerations of vast multitudes of these vibrations, moving together in a way that creates the illusion of solidity, of permanence. What’s more, since there is only one electron field, only one up quark field, and only one down quark field, you and I, dear reader, are connected to each other. Each of our atoms is a ripple in the same cosmic ocean. We are one with each other, and with all of creation.*8
At the start of the chapter, we said that the ingredients of an apple pie are electrons, up quarks, and down quarks. Quantum field theory tells us that these three particles are vibrations in three corresponding fields. However, as we just saw, even this is a gross oversimplification. An electron is not merely a ripple in the electron field, but a complicated mixture of distortions in every quantum field that we have ever discovered. The same goes for the up quarks and down quarks that make up protons and neutrons in the nucleus. This means that to fully understand the makeup of our apple pie we need to know about every last quantum field in nature, even those whose particles are too unstable or weakly interacting to bind together to make atoms.
Our current best description of the known quantum fields is the standard model of particle physics, an exceptionally boring name for one of the greatest achievements of human thought. We have already met many of its stars: the electron, the quarks, neutrinos, gluons. However, there is one key piece of this picture that was found only in the last few years, the final ingredient of our apple pie, and one that opens up a Pandora’s box of new problems and opportunities.
Skip Notes
*1 Another misused word. “Micro” refers to objects a millionth of a meter in size; however, the proton is around 10-15 meters across, so really the correct term should be “femtoscopic.”
*2 By comparison, mine was called “A measurement of the Bs0 to K+K-lifetime at the LHCb Experiment.” You can guess which one had more impact.
*3 Stars of many a physics analogy, who first appeared as fictional characters in Ron Rivest, Adi Shamir, and Leonard Adleman’s 1978 paper on cryptography.
*4 He hadn’t. Today there’s an entire community of researchers who spend their time thinking about this stuff.
*5 All matter particles, including the electron, have total spin ¹/₂, which can either be pointing “up” (spin +¹/₂) or “down” (spin -¹/₂).
*6 To be fair, this version of the equation uses a more compact notation than the slightly more intimidating version that Dirac first wrote down, but the physics and the structure of the equation are identical.
*7 Apparently, the Piccadilly line is the worst culprit.
*8 At the risk of getting a bit too Neil deGrasse Tyson, that also means that we are all one with lots of unpleasant stuff: the Ebola virus, dog shit, and Piers Morgan, for instance.
CHAPTER 10
The Final Ingredient
I first arrived at CERN, the European Organization for Nuclear Research on the outskirts of Geneva, on a sunny afternoon in July 2007. I was a fresh-faced, twenty-one-year-old undergraduate with an unspoiled enthusiasm for physics and what in hindsight was rather ill-advised shoulder-length hair. For a few weeks that summer I and more than a hundred other summer students from across Europe would get a taste of life at the cutting edge of particle physics.
The CERN of my imagination was a gleaming, futuristic place lifted straight from the pages of science fiction, where promethean scientists used gigantic subterranean machines to probe the very nature of reality. I was therefore a little taken aback to find myself deposited at the gates of what looked more like a scruffy 1960s university campus in need of some serious TLC: a chaotic jumble of shabby office buildings and dilapidated warehouses with peeling paintwork and rusty corrugated roofs.
Many first-time arrivals at CERN experience a similar culture shock, a milder cousin of so-called Paris syndrome, which affects some visitors to the City of Lights when they find themselves in a far grubbier, noisier, and ruder place than the fairy tale of books and movies. On the other hand, while Paris syndrome can apparently induce symptoms as extreme as paranoia, dizziness, and hallucinations, CERN syndrome only left me with a vague feeling of disappointment.
A lack of sci-fi wizziness notwithstanding, I had arrived at the lab at one of the most exciting possible moments. After three decades of planning, fundraising, and construction, the Large Hadron Collider, the world’s biggest machine, was just months from firing up for the very first time. I was to spend the summer working on the CMS experiment, one of the four cathedral-sized detectors whose job would be to scour the collisions produced by the LHC in search of new fundamental particles.
My specific project was to help get one of CMS’s subsystems ready to collect data. In practice this involved sitting in an office staring in puzzlement at reams of computer code, a task I was woefully unprepared for—no one had thought to teach us any coding at university.*1 To make matters worse, the person who was supposed to be supervising me was away for my first two weeks, which only compounded my sense of being lost in a strange, dreary world.
Then, two weeks in, everything changed. One afternoon, some fellow students and I were taken by minibus to the far side of the 27-kilometer LHC ring to visit the site of the CMS experiment. We pulled up at a fenced compound surrounded by peaceful French farmland, at the center of which was a large hangar-like building. What I saw inside took my breath away. Towering above us were huge sections of the detector, each more than three stories high, lined up ready to be lowered down an enormous concrete shaft at the far end of the hangar, which plunged vertically a hundred meters through the earth to the experimental cavern below.
At last, here was the sci-fi magic I had been waiting for. Most mesmerizing of all was the huge object farthest f
rom the access shaft, the piece of CMS that would be lowered down last and slid into place to complete the experiment—the so-called endcap. Unfortunately, there are no handy comparisons to help you picture what the endcap looked like—it was such an otherworldly thing—but imagine, if you can, a twelve-sided disk balancing on its narrow edge, 15 meters top to bottom and side to side, roughly the size of three double-decker buses stacked on top of one another. Its red surface was crisscrossed by bright blue cabling and at the center of the disk was a large black and silver cylinder protruding outward like the hubcap of some giant alien wheel.
Seeing these monumental slabs of the detector laid out waiting to go underground suddenly made the whole enterprise much more real to me. Standing beneath them, you began to properly appreciate the decades of work that were involved in making this experiment a reality. Every tiny component had been painstakingly researched, designed, built, and tested before being shipped to CERN from labs all over the world, and tested again before finally being installed.
But the best was yet to come. After several minutes wandering around the hangar, gawping at the huge slices of detector, we took a lift down to the experimental cavern itself. Standing on a metal gantry raised 10 meters above the cavern floor you could survey the near-completed experiment. CMS stands for Compact Muon Solenoid, which I’ve always thought is rather a strange use of the word “compact.” The detector is shaped like a giant barrel lying on its side, 15 meters high and 22 meters long and weighing in at a whopping 12,500 metric tons. The whole thing contains enough iron to build two Eiffel Towers.*2