The world’s most epic physics experiment will flip back on as early as Saturday. After a two-year tuneup, the Large Hadron Collider (LHC) will run at twice the power it needed in 2012 to find the Higgs boson, the long-theorized particle that confers mass onto matter.
As monumental as the Higgs discovery was — its theorists won the Nobel Prize in Physics the next year — physicists still have very little idea what’s going on in the universe, beyond the stuff we can see, touch, and smell. A big question concerns “dark matter,” what scientists call the stuff that makes up 80 percent of galaxies but that doesn’t interact with light, atoms, and molecules. They know it’s there, but it’s hiding from us.
With the Higgs in hand, finding traces of dark matter is the next big hunt in high-energy physics.
The Standard Model of physics is what scientists consider their working picture of how fundamental particles behave and interact. But it “has some holes in it,” says Verena Martinez Outschoorn, an assistant professor of physics at the University of Illinois at Urbana-Champaign. “We know that our worldview, our model, our understanding of particles and their interactions is kind of a subset of a bigger picture,” she says. “We have reason to believe there are other particles out there.”
The LHC is located at CERN, the scientific research juggernaut in Meyrin, Switzerland. It’s a network of superpowered, supercold, super empty magnet-driven beam pipes that zip protons around a 17-mile loop. Some circle the ring in one direction; some trace the opposite path.
How high-powered? Ultimately, 14 tera-electron volts, or 14 trillion electron volts (eVs). That’s a lot of anything. Neutrons popping out of a radioactive nucleus — nuclear fission — have about a million electron volts. Medical X-rays have about 200,000 eVs. Electrons hit old-fashioned cathode-ray television screens with about 20,000 eVs.
How cold? At 1.9 kelvins (-456F), the LHC magnets are colder than outer space.
How empty? The vacuum beam pipes that carry the particles around in circles are so empty they make the moon’s atmosphere look like a choking smog.
Protons loop around and around at 11,000 laps per second, until they’re steered into each other. The collision frees energy and many kinds of particles, which are just as soon collected by detectors, including four massive ones underground. The Atlas detector (see above) is 46 meters long, 25 meters high, and 25 meters wide. At 7,000 metric tons, its the largest one ever by sheer volume. (Atlas is outweighed, however, by the Compact Muon Solenoid detector.)
From the data these and other detectors collect, scientists try to piece together what happened in the proton collision, and what particles it released.
If there’s one rule in the universe that’s unbreakable, it’s the law of conservation of energy: Energy cannot be created or destroyed. So when physicists add up the energy of all the particles that come out of a collision, they must total the known energy level of the experiment. If it doesn’t add up, that may indicate that some energy was siphoned away in the generation of dark matter particles. And by definition, those can’t be detected by us.
It’s not an insane way to find new particles. This process is essentially how scientists found the neutrino, a fundamental particle that’s shot out of radioactive elements and passes right through us all the time. “This sounds like kind of a funny way of doing a measurement, by saying what’s not there,” says Jesse Thaler, an assistant professor of physics at MIT. “But actually, historically, we’ve been quite successful in this.”
For April Fools’ Day, CERN published a faux press release claiming that researchers confirmed the existence of “the Force,” the all-powerful field linking living things in the Star Wars universe. Closer to the truth may be Force skeptic Han Solo, who says in the original Star Wars:
I’ve seen a lot of strange stuff, but I’ve never seen anything to make me believe that there’s one all-powerful Force controlling everything.
Physicists have four forces they can account for: gravity, the weak and strong nuclear forces, and electromagnetism. Plus, there’s the basket of stuff they don’t understand, such as dark matter and dark energy. Minimalists might prefer that they find, if there’s one waiting to be found, an even simpler understanding of the universe, one that reconciles the four fundamental forces and the dark stuff. For scientific accounting — Han Solo be damned — it just might be neater and easier to have one all-powerful force controlling everything.