The Biggest Thing in Physics
The Biggest Thing in Physics
Two teams of physicists compete to explain matter—and win a Nobel Prize
The CMS detector created by a vast collaboration of 2,000
scientists and engineers, will race ATLAS
to find the Higgs boson.
Image courtesy of © CERN
Near the west end of Lake Geneva in Switzerland, buried under the river plain of the Rhône, workers are fitting together the final pieces of the machine that hopes to unlock one of the biggest mysteries of the universe. It has taken over 20 years, $8 billion, and the combined efforts of more than 60 countries to create this extraordinary particle smasher, the Large Hadron Collider, or LHC, built and operated by CERN, the European physics consortium.
The “large” in Large Hadron Collider is something of an understatement. “Enormous” is closer: The collider’s underground tunnel carves a circle 17 miles in circumference, traversing the border between Switzerland and France. At four locations it passes through caverns crammed with detectors the size of buildings. In a deliberately constructed rivalry, two of these detectors—along with their armies of scientists, engineers, and technicians—will vie with each other to discover the obscure but wildly important particle known as the Higgs boson.
According to the most accurate scientific theory ever created—known as the standard model—all of space is filled with a mysterious stuff called the Higgs field. Unlike magnetic or gravitational fields, which vary from place to place (things weigh more here than on the surface of the moon, for instance), the Higgs field is exactly the same everywhere. What varies is how the different fundamental particles interact with it. That interaction, the theory goes, is what gives particles mass. In a nutshell, the Higgs field is what makes some particles (like protons and neutrons) relatively heavy, others (like electrons) subatomic lightweights, and still others (like photons) utterly massless. If photons weren’t so light, you’d be shredded by a photon hailstorm every time you lazed under a sunbeam. Then again, if protons and neutrons weren’t so heavy, you wouldn’t be there to sunbathe anyway: Without mass and its affinity for gravity, there’d be no galaxies, no stars, no us.
How does the Higgs work this magic? British theoretician John Ellis likens the Higgs field to a flat field of snow. Try to get across it in hiking boots and you will sink in and take forever. Snowshoes would be faster, and with skis you could glide across the field swiftly and easily. In the parlance of physics, “slow” is another way of saying “heavy.” So by analogy, your mass depends on some fundamental physics attribute, equivalent to snowshoes or skis, that affects how a particular type of particle passes through the Higgs field.
The Higgs boson is supposed to be the endower of this attribute; it is what determines if a particle can glide along effortlessly like a photon or if it must trudge like a hefty proton. The trouble is that nobody knows exactly what a Higgs boson is like or even if it really exists. It must be extremely heavy, or other lower-energy facilities, like Fermilab outside Chicago, would already have detected it. But it cannot be too heavy, or the theories that predict its existence would not work.
By design, the LHC is the first accelerator capable of exploring the full range of energies within which the Higgs boson is thought to exist. If the LHC finds the Higgs, it will verify the last, grandest aspect of the standard model and solve the ancient question of just what mass is. If the LHC fails to find the Higgs, the standard model will have to be reevaluated from the ground up. At stake is a fundamental part of our understanding of how the universe works.
Peter Limon, an American from Fermilab, hands me a hard hat and a metal box containing breathing apparatus. “You’re entering an industrial area,” he says. “Watch out for bicycles.”
We’re about to take an elevator more than 300 feet belowground, into a tunnel containing the biggest, most violently energetic particle collider the world has ever known.
The endless, gently curving tunnel is so crowded with massive high-tech equipment that there isn’t much room for any transportation other than a bike. “Best way of getting around down here,” Limon explains.
What’s filling the tunnel is the beam pipe: the hardware used to accelerate subatomic particles (protons, mostly) to 99.999999 percent of the speed of light. From the outside, the beam pipe looks like a series of huge steel barrels, connected end to end and brightly painted in reds, oranges, and blues; it stretches off into the distance like a giant oil pipeline. Many of the barrels bear a stenciled sign that betrays the international nature of the project. Some are from Italy, others from Japan or the United States. One of the barrels is cut away, and Limon shows me the complexity within. The beam pipe actually contains two beam lines, tubes just an inch and a half across, inside of which streams of particles will speed around the circuit of the LHC. Surrounding the beam lines is a forest of pipes, electronics, and ultrapowerful magnets. When the machine is switched on for the first time at the end of this year, particles will make a lap around the LHC in less than one ten-thousandth of a second.
Keeping those particles on track requires serious bending power from more than 1,200 superconducting magnets, each of which weighs several tons apiece. Each magnet must be kept at –456 degrees Fahrenheit—colder than the void between galaxies—requiring CERN to build the world’s biggest cryogenic system to handle the 185,000 gallons of liquid helium that will be used to chill the magnets.
Particles will circulate in opposite directions in each beam line—clockwise in one, counterclockwise in the other. The individual beam lines will keep the racing particle streams separated—except at four points around the ring where physicists will deliberately allow the streams to cross. At those spots, the LHC physicists will observe the resulting mayhem with detectors of staggering scale and complexity.
Standing at one of these collision points, I try to imagine the energy involved. “If I were down here when the beam was operating, would it be highly radioactive and dangerous?” I ask. “If you were down here when the beam is operating,” Limon replies, “it would be highly radioactive and fatal.” There will be 600 million particle collisions per second, and although the particles themselves are mere specks—less than a million millionth the size of a gnat—their collective energy will be that of an express train. Once set in motion, a stream of particles might circulate for 10 hours before needing to be refreshed. During that time, it would travel more than 6 billion miles, enough to get to the planet Neptune and back.
“I think this is the most complicated thing that humans have ever built,” Limon says, proudly.
(Cont'd on site)
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