More doom hit the news this past month; the newest particle accelerator, the Large Hadron Collider (LHC) of CERN, was fired up this past week (on September 10, 2008), for the first time accelerating protons for a full revolution around its 26.7 km circumference ring, after overcoming lawsuits by those who thought it could bring about the end of Earth, and even the universe. The alarmists needn't worry, because nature has particle accelerators in space that are still more powerful than the LHC; the Earth is bathed by the cosmic rays created by these accelerators, and yet both the Earth and the universe survive.
The purpose of a particle accelerator is simple enough: drive particles such as protons, electrons, and atomic nuclei to nearly the speed of light, smash them together, and see what comes out. This research involves several fundamental physical processes. Because energy and mass are equivalent, the collisions convert the kinetic energy of the colliding particles into new and often unstable particles of very greater mass. The types of particles created in a collision are dependent on how the colliding particles exert force on each other. A commonplace example is the generation of light by hot electrons and ions; a collision between a fast-moving electron and an ion creates photons (light) through the electromagnetic force the particles exert on each other. In the extreme collisions generated by particle accelerators, other forces are involved, such as the “strong force,” which holds the proton together (it is actually composed of three quarks). Accelerator experiments has been going on for over half a century, and through it physicists have developed insights into the fundamental particles and the forces acting on them.
The LHC is designed to accelerate hadrons, which are particles composed of quarks. The most common hadrons are protons and neutrons, which compose the nuclei of all atoms. The LHC accelerates protons or atomic nuclei in two counter-rotating streams in a giant ring. These streams cross at a single point inside a shell of particles detectors. At this point, some of the counter-streaming protons or ions collide, creating sprays of particles that interact with the detectors. Protons in the LHC are accelerated to an energy of 7 TeV (7×1012 eV, compared to the 938 MeV [9.38×108 eV] mass of a proton), so the collisions release 14 TeV of energy, or 15,000 times the rest-mass energy of a single proton.
The particle collisions within the LHC are the most energetic created in the laboratory. A handful of very heavy particles—the Higgs boson and a number of other particles that are hypothesized to exist—may manifest themselves for the first time in these collisions. By accelerating and colliding the nuclei of lead, the experiment hopes to produce a quark plasma, which is believed to be a state the universe was in very early in its evolution; this is the reason the accelerator has been tagged as “The Big Bang Machine.”
Those people who fear the LHC, and a handful of scientists are among their numbers, believe these collisions would cause something exotic and catastrophic to happen. Some point to the speculative ideas that the vacuum can have energy states like an atom, with a ground state and more energetic states; they worry that if the vacuum of our local universe is in an energetic state, called a “false vacuum,” the particle collisions within the LHC may cause the false vacuum to decay, destroying the local universe. Others speculate that collisions within the LHC can create miniature black holes what would destroy the Earth.
But man can seldom rival the power of nature, and the Galaxy is filled with particles naturally accelerated to energies far greater than those created in the Earth-bound accelerators. These natural fast particles are called cosmic rays. The least energetic of the cosmic rays are observed by spacecraft in orbit around Earth, and the most energetic are observed with ground-based detectors. The Earth's atmosphere acts as a target for cosmic rays, and the most energetic cosmic rays collide with the atomic nuclei in the atmosphere, creating cascades of fundamental particles that are detected at the ground. The most energetic of these cosmic rays have energies exceeding 1020 eV. The most energetic cosmic ray, with an energy of 3.2 1020 eV, was seen on October 15, 1991 by the Fly's Eye in Utah.
A cosmic-ray proton with an energy of 1.04×1017 eV striking a proton at rest is equivalent to the laboratory head-on collision of two 7 TeV protons (this equivalence comes from the relationships of special relativity). For a collider to create the equivalent of a 1020 eV proton striking a stationary proton, it would need to accelerate protons to 217 TeV. Cosmic ray collisions are therefore far more energetic than the collisions within the LHC.
The cosmic ray flux at the high end of the energy range is very low. The flux of cosmic rays more energetic than 1020 eV is only 1 cosmic ray per square kilometer per century. But with a surface area of 510 million square kilometers, the Earth is hit by about 10 of these energetic cosmic rays every minute. The cosmic rays with energies above 1018 eV have a flux of 1 cosmic ray per square kilometer per week, so that 8 of these cosmic rays strike the Earth every second. With its larger surface area, the Sun receives many times more cosmic ray hits than Earth. About 2,000 cosmic rays with energies above 1020 eV and 100,000 cosmic rays with energies above 1018 eV strike the Sun every second.
In contrast, the LHC creates 600 million collisions per second. The collider therefore produces in one second a number of collisions that take a week to occur in Earth's atmosphere. On the other hand, to produce a number of collision that would equal the number of cosmic ray strike over the lifetime of Earth would require the LHC to run continuously for over 7,000 years. To equal the number of cosmic rays with energies above 1018 eV that have struck the Sun over its lifetime, the LHC would need to run continuously 750,000 years. Clearly, if something bad could happen through an LHC-generated particle collision, it would already have happened through a cosmic ray collision.
The man-made colliders have a ways to go before they create collisions that exceed the energies of collisions occurring right now in Earth's atmosphere, in the atmospheres of other planets, the atmosphere of the Sun, and the atmosphere of stars throughout the Galaxy, and they need to run for hundreds of years before they produce a number of events that comes close to the number of cosmic ray hits that Earth has sustained over its lifetime. The cosmic rays haven't killed us, so the Large Hadron Collider won't kill us.