“On a scale from one to ten... I’d give it maybe a one.”
That was the gentle critique of physicist Dave Barney, rating my proposal for a Star Wars-themed analogy to explain the Higgs boson. I had travelled halfway across the world to meet with Dave and his colleague and fellow physicist Steve Goldfarb for a tremendous opportunity to visit the facilities of CERN--home of the Large Hadron Collider-- and learn about the discovery of the Higgs boson. But I also arrived with an ulterior motive: to pitch an analogy for the Higgs boson that would be palatable to non-physicists and lay-geeks. I had even stowed along props in the form of LEGO minifigs.
Unfortunately, Higgs boson analogies are to the scientific community what screenplays are to Hollywood--everyone has one to pitch and most are pretty bad. Mine was no exception; Steve and Dave were none too impressed with it, LEGO props and all.
But if the task was easy, then physicists wouldn’t be struggling to wrap their heads around it. Steve and Dave would know, being the respective Outreach Directors for CERN’s ATLAS and CMS experiments. The two collaborated on a fantastic TED-Ed video earlier this year to explain the Higgs boson, settling on the analogy of a cherry dropping into a milkshake (originally, their analogy was actually an olive dropping into a martini, but the milkshake was more family friendly). Their milkshake analogy is the latest in a long series of analogies educators have used to try to explain the Higgs boson to the world at large. There’s another involving skiing and snowflakes, and even one featuring Margaret Thatcher.
Finding ways to explain the Higgs boson has almost become a part-time job for CERN physicists after making the announcement last July that they had, with more than 99 percent certainty, found the elusive particle (they have since confirmed the discovery as the Higgs, but are unsure of what kind of Higgs). With that announcement came the attention of the world media, who struggled to explain to a curious public just what had been found within the detectors at the world’s largest particle accelerator. The “god particle” metaphor quickly took hold, but physicist detest the name, which, not the least being sensational, is just plain uninformative. That moniker was lifted from a 1994 book by physicist Leon Lederman, and the apocryphal story goes that the book’s publisher actually shorted his original title, “The Goddamn Particle.”
Misnomers and misconceptions tend to run rampant when you’re talking about particle physics and machines the mass of the Eiffel tower constructed 100 meters below ground. It doesn’t help that, until last year, the public’s most noteworthy exposure to the activities of CERN was Dan Brown’s 2000 pop thriller Angels & Demons in which a gram of antimatter was stolen from the fictional version of the facility. So let’s clear some things out of the way first: a gram of antimatter, at the current rate of detection, would take over 10 billion years to produce. And the LHC isn’t generating wormholes either--sorry Jamie and Adam.
So what is being produced with the Large Hadron Collider smashes protons together 600 million times a second?
Turns out, it’s other subatomic particles, but those that are much less stable than the ones that make up your typical atom. These unstable subatomic particles decay very quickly, so they exist only for a few moments before becoming or combining with stable particles. But according to current theories about the nature of the universe, these unstable particles existed in abundance immediately after the Big Bang. Finding evidence of them using the LHC helps confirm assumptions and theories that yet haven’t been proved in physics. And among these unstable subatomic particles: yep, the Higgs boson.
We’ve danced around the it long enough, so let me do my best as a layperson to explain what the Higgs boson is. (I’ll try to keep this as succinct and painless as possible, for the both of us.) The Higgs boson is one of the elementary particles defined by the current theories physicists use to explain physical attributes of the universe. Since we’re not all-knowing beings, we use theories to explain the existence of principles like gravity and electromagnetic forces. One of the principles that physicists want to explain is that of mass--or why some particles have the properties of mass while others don’t. The prevailing theory for why mass exists assumes the existence of something called a Higgs field, as theorized by six physicists in 1964 (including the eponymous Peter Higgs). The Higgs field pervades throughout the universe, and when certain particles move through it, they attain the attribute of mass. Under the Higgs field theory, the Higgs boson was the corresponding particle would explain mass. And since the Higgs field can’t be directly detected, finding the Higgs boson would be the best confirmation of that field.
(Steve Goldfarb emailed to clarify further."Quantum Field Theory--the mathematical basis for particle physics born of quantum mechanics and relativity--explains that there is a particle (or quantum) for every field and vice verse. The Higgs boson would be the quantum corresponding to the Higgs Field (just as the photon is the quantum for the electromagnetic field). Unlike an electromagnetic field, which can be detected directly with the eye or an antenna, the Higgs field cannot (yet) be directly detected. However, just as the photon is the quantum of an electromagnetic field, the Higgs boson is the quantum of the Higgs field. So, finding a Higgs boson is the best way to confirm the existence of the Higgs field.)
A universe-pervading field that can’t be detected but gives particles special properties? You can see why my Star Wars analogy sounded so good in my head.
So how is something as short-lived and elusive as the Higgs boson detected? With a special detection machine, of course. That’s what the ATLAS experiment is, along with five other experiments placed along the 27km circumference of the Large Hadron Collider. And that’s where I found myself last month, enthusiastically waving LEGO figurines in front of unamused physicists.
ATLAS is housed almost 100 meters underground in a cavernous concrete bunker. It’s difficult to get a sense of just how big the thing is (150 feet long by 80 feet tall and 80 feet wide) when standing in front of it--a four story mural painted on the visitor’s facility on the surface isn’t even 1:1 scale. Its tremendous size is impressive, but what you notice more is the sheer complexity of the machine. A colorful intermingling of cables, pipes, and walkways wrap around 500-ton magnets and exposed electronics that all play a role in detecting particles moving at near the speed of light. The wires give it a techno-organic aesthetic, and to me it felt a little like looking at the warp core of the starship Enterprise, complete with tubes of cryogenic cooling (for the superconductive magnets, natch).
Visitors are only allowed access along a short walkway, but that put us plenty close to the core of the detector. Peering over the blue railing, I could make out the cylindrical shape of the detector. And in front of that railing was a safety net, placed ostensibly to prevent equipment and visitor hardhats from accidentally falling and damaging the detector. But I also heard a rumor that the net was put in place after one visitor lost their dentures while leaning over the railing. ATLAS is literally jaw-dropping.
On that walkway, Steve, Dave, and ATLAS engineers explained how the detector works. Surrounding the collision point where the LHC sends trillions of protons are a series of sensors. While these are specially designed sensors to detect the aftermath of particle collisions, they are akin to familiar technologies. The first is a pixel detector, which is much like a camera sensor but assembled in the shape of a cylinder. These digital imaging sensors take pictures of the collisions with a resolution of about 80 megapixels and at an incredibly high rate. Next are a system of calorimeters, which absorb and measure particle energy as protons collide. Finally there’s a spectrometer for detecting the momentum of muon particles, which pass through the calorimeters without stopping but are then directed by ATLAS’s supermagnets.
ATLAS is one of CERN's two general-purpose detectors along the LHC. The CMS experiment, where Dave works, is the other, and lies on the opposite side of the accelerator. CMS also uses calorimeters to measure the energies of colliding particles, but its primary tracker relies on a massive solenoid magnet to direct the path of particles to measure their momentum. ATLAS and CMS have similar goals and capabilities, but were designed with different strengths and limitations and complement each other. That's important for discovering something like the Higgs boson because one experiment can validate the findings of the other. It's like the quote from Contact: "why build one when you can have two at twice the price?"
Detectors weren’t always built this way, of course. On the CERN campus is an exhibition of decommissioned detectors, including giant “bubble chambers” used in the 1970s to detect particle interactions. These chambers, filled with superheated liquid hydrogen, allowed physicists to “see” the movement of charged particles when they moved through the liquid and created a trail of tiny bubbles. Technicians manually took photographs of these vapor trails in a very analog way, a far cry from the petabytes of data produced by modern CERN detectors.
We were ushered out of the ATLAS cavern to conduct the rest of our interviews after our allotted time was up to make room for VIP visitors. Surprisingly, CERN actually accommodates a lot of visitors a year, including hundreds of journalists, scholars, and dignitaries. The general public can make reservations for tours and the high-tech Globe visitors center is regularly packed with students on field trips from Switzerland and France (the French border being less than a five minute drive away). I was impressed by the openness of ATLAS's visitors center lobby, which is directly connected to its control room. Visitors can not only learn from interactive displays and computers set up in the lobby, but get a direct view of the Mission Control-like room where ATLAS controllers monitor the experiment with multi-monitor computer setups and a wall of projected video feeds. Since ATLAS--and the rest of the LHC, for that matter--is undergoing maintenance for upgrades for the next two years, the control room was empty during our visit.
That's not to say work isn't still being done at CERN. We stopped by a few of the other facilities where engineers and physicists were buzzing around working toward the next stage of the LHC's operations. There was SM-18, a test facility in where the over 1700 superconducting cryo-magnets that line the tracks of the LHC were individually tested over the course of five years while the LHC and detector experiments were being constructed. Glyn Kirby, a lead engineer in the design of these superconducting magnets, explained with infectious excitement the precision with which these magnets have to be made, since they not only are responsible for sending protons around the LHC track at a rate of 11,000 revolutions per second, but have to also narrow that particle cloud into a 16 micron stream for collision. That's one fourth the thickness of human hair.
The planning, design, and construction of the LHC and CERN's previous particle colliders takes place over the course of years, so its creators have to plan for the future and project the availability of technologies and technological capabilities that aren't available when constructing their plans. That's most apparent in CERN's Computing Center, which was founded in the 1970s to take on the task of processing the data produced by CERN's experiments. As collider and detector ambitions scaled up, computer scientists had to plan for the task of ingesting and processing that data without knowing if technologies like network cable bandwidth would be able to keep pace. Today, the LHC experiments generate over a petabyte of data per second, the majority of which has to be discarded or is deemed uninteresting for study. The rest is processed through a worldwide grid computing system where data is backed up to tape and crunched. Fun fact: storage capacity at CERN has surpassed 200 Petabytes of data, split between tape and hard drives.
The CERN Computing Center was also where Tim Berners-Lee famously invented the web, formalizing the concept of hypertext documents that could be read by special software. We know those as web pages and web browsers. The Computing Center's lobby showcases a timeline of computing artifacts, from early magnetic storage drives to one of Berners-Lee's NeXT computers that was the first web server. That machine, unassumingly displayed in a glass cabinet, still bears a worn sticky label with a handwritten reminder not to turn off power to the server.
Our visit concluded in the CERN cafeteria, where we met up with Steve to catch up on what we had seen that day. He poked some more fun at my LEGO analogy, but also mentioned the ATLAS LEGO model that a researcher designed last year (a smaller model just reached 10,000 votes of support on LEGO's Cuusoo site). Steve wears both the hat of a physicist but also that of the Outreach Coordinator for ATLAS, and he was really pumped by the educational opportunities offered by the LEGO ATLAS model. And that was the coolest thing about being at CERN--seeing some of the smartest people in the world being really passionate about their work and yearning to share the fruits of their research with the rest of the world. That work being done at CERN doesn't just advance our understanding of the universe, it has practical applications in medical treatments, energy storage, and even vinyl record restoration.
Looking around that cafeteria really cemented in my mind just how much CERN felt like a university campus, with researchers excitedly discussing their latest findings over lunch or a cup of coffee. You could even spot a Nobel laureate if you waited long enough. Just like any respectable university, CERN also has a central campus library. And though its holdings are are almost all non-fiction works, it does make shelf space for the rare exception: three copies of Dan Brown's Angels & Demons.