When the Large Hadron Collider (LHC) began operations, a small but noisy group of people tried to stop it out of fear. Their reasoning: The energies produced as protons slammed into each other at close to the speed of light would be sufficiently high to create miniature black holes or other exotic, destructive things. The fruits of human curiosity would be the literal end of the world.
Those fears were unwarranted for a simple reason: Earth is bombarded by much higher-energy particles all the time, and we haven’t been eaten by a planet-munching black hole yet. In fact, the universe has many naturally-occurring particle accelerators that are far more powerful than the LHC, exceeding even anything we could build in the foreseeable future. Anything exotic we can create in our labs, the cosmos has beaten us to it.
Like colliders on Earth, astronomical accelerators use magnetic fields to whip particles up to nearly light speed. Additionally, the universe uses shock waves, the powerful compression of plasmas to speed particles and ram them into each other. Those intense, high-energy processes take place near black holes or neutron stars, during supernova explosions and in the hot remnants those explosions leave behind. This violence generates electrically charged particles—mostly electrons and protons, with a smattering of helium nuclei—neutrinos, and gamma ray photons. Of those, protons are the most interesting: they have an effective combination of high mass (in particle terms!) and fast speed. When they reach Earth’s atmosphere, they collide with air molecules and produce cascades of other particles, which we call “cosmic rays.”
“The energy for particle creation … in the upper atmosphere is more than a factor of twenty greater than at the LHC,” said Glennys Farrar, professor of physics at New York University. Most impressive of all are “ultra-high-energy” cosmic rays, which reach energies 100 million times greater than the fastest protons produced in the LHC.
While astronomical sources may be good accelerators, that doesn’t mean the cosmos is full of good colliders. Our Earth-bound experiments are designed to focus beams of protons or other particles into tight “bunches” (that’s the technical term!), then send those bunches slamming into each other from opposite directions. That sort of thing is rare in space: A cloud of gas left after a supernova may be full of fast-moving protons, but direct collisions are not very common.
However, Earth itself is part of the Universe’s collider, the accidental “target” for beams of cosmic rays from deep space. As Farrar pointed out, “Accelerators concentrate the beam so while the total collision rate of [cosmic rays] with atmospheric nuclei is higher than in accelerators, we only see a tiny fraction of the collisions.”
The key is energy: If two protons collide with enough velocity, some of the energy of their motion can be converted into mass, creating new particles in accordance with E = mc2. The result is a number of particles of many types, from ordinary stuff like electrons and photons to rarer specimens. Among the exotica are certain types of mesons, which are made of two quarks instead of the three that compose protons and neutrons; they were first discovered in cosmic ray detectors in 1936. Since they’re short-lived, these mesons must have been born in collisions in Earth’s atmosphere; otherwise they would have decayed during the long trip across space.
But even more exotic things could pop out of the froth of cosmic ray collisions: unusual quarks, Higgs bosons, or even as-yet unseen particles predicted by theories such as supersymmetry. However, most of these are even more short-lived than the mesons, decaying into bursts of more ordinary particles that might reach cosmic ray detectors on Earth’s surface. Collisions might make a few stable exotics such as hypothetical weakly-interacting massive particles (WIMPs), which are candidates for dark matter, but based on current experimental limits, their numbers must be few.
It’s unfortunate that the most interesting collisions take place high in Earth’s atmosphere, where we can’t observe them directly. Farrar said, “Even when we see them, e.g., with the Auger detector which is the largest in the world, the fact that Higgs [bosons] are produced is not evident because the Higgs just decays to ordinary particles which have no special character when they hit the detector.” That’s in contrast with human-built colliders: Researchers put a lot of effort into finding the telltale decay products from a Higgs particle, but the detectors have to sit near the point of collision to have any hope of doing so.
Nevertheless, the collisions in the upper atmosphere are still potentially helpful for particle physicists. Farrar noted that current observations of the cosmic ray spectrum “may indicate a transition to a new regime of [high-energy physics], not accessible at the LHC.” It’s tricky work, but what worthwhile scientific endeavor isn’t?
Researchers are never satisfied, especially when they consider what might be created in collisions inside supernova remnants and other distant events. Even if an individual collision that makes something exotic is extraordinarily rare, there are plenty of potential sources and lots of time. Yet we might never know, since we can’t observe these events closely enough to see anything but the decay products that eventually reach us.
Powerful as they are compared to the LHC, Farrar pointed out that even supernova remnants are “puny” particle accelerators compared with whatever makes the ultra-high-energy cosmic rays. If these are also protons from deep space, they must be from extremely powerful phenomena: highly magnetized neutron stars, the most energetic black holes, or maybe even something new we haven’t figured out yet.One thing we can say for certain, though, is that scientists are clever, and may yet figure out inventive ways to study cosmic colliders. It may turn out that the laboratory of the Universe will teach us things about fundamental particles and forces.
Editor’s picks for further reading
Ars Technica: Single hotspot may be the source of many ultrahigh-energy cosmic rays
John Timmer on the quest to understand where ultra-high-energy cosmic rays are coming from and how they are created.
CERN: Cosmic rays: particles from outer space
An introduction to the science and history of cosmic rays.
Live Science: Earth Is Safe: No Black Holes Spun Out of Atom Smasher, Yet
Science writer Charles Choi reports on the search for quantum black holes from the LHC.