Cosmic rays could offer evidence of the validity of string theory. Scientists are unlikely to see some improbable events in laboratories on Earth, at least without a lot of work, so sometimes they look where they’re more likely to find them. Because cosmic rays contain very high energies and take so long to reach us, scientists hope they can observe these hard-to-see events by studying the cosmic happenings.
Cosmic rays are produced when particles are sent out by astrophysical events to wander the universe alone, some traveling at close to the speed of light. Some stay bound within the galactic magnetic field, while others break free and travel between galaxies, traveling billions of years before colliding with another particle. These cosmic rays can be more powerful than our most advanced particle accelerators.
First of all, cosmic rays aren’t really rays. They’re stray particles in mostly three forms: 90 percent free protons, 9 percent alpha particles (two protons and two neutrons bound together — the nucleus of a helium atom), and 1 percent free electrons (beta minus particles, in physics-speak).
Astrophysical events — everything from solar flares to binary star collisions to supernovae — regularly spit particles out into the vacuum of space, so our planet (and, in turn, our bodies) are constantly bombarded with them. The particles may travel throughout the galaxy, bound by the magnetic field of the galaxy as a whole, until they collide with another particle. (Higher energy particles, of course, may even escape the galaxy.)
Fortunately for us, the atmosphere and magnetic field of Earth protect us from the most energetic of these particles so we aren’t continuously dosed with intense (and lethal) radiation. The energetic particles are deflected or lose energy, sometimes colliding in the upper atmosphere to split apart into smaller, less energetic particles. By the time they get to us, we’re struck with the less intense version of these rays and their offspring.
Cosmic rays have a long history as experimental surrogates. When Paul Dirac predicted the existence of antimatter in the 1930s, no particle accelerators could reach that energy level, so the experimental evidence of its existence came from cosmic rays.
As the cosmic ray particles move through space, they interact with the cosmic microwave background radiation (CMBR). This microwave energy that permeates the universe is pretty weak, but for the cosmic ray particles, moving at nearly the speed of light, the CMBR appears to be highly energetic. (This is an effect of relativity, because energy is related to motion.)
In 1966, Soviet physicists Georgiy Zatsepin and Vadim Kuzmin, as well as the independent work of Kenneth Greisen of Cornell University, revealed that these collisions would have enough energy to create particles called mesons (specifically called pi-mesons, or pions).
The energy used to create the pions had to come from somewhere (because of conservation of energy), so the cosmic rays would lose energy. This placed an upper bound on how fast the cosmic rays could, in principle, travel.
In fact, the GZK cutoff energy needed to create the pions would be about 1019 eV (about one-billionth of the Planck energy of 1019 GeV).
The problem is that, while most cosmic ray particles fall well below this threshold, some very rare events that have had more energy than this threshold — around 1020 eV. The most famous of these observations was in 1991 at the University of Utah’s Fly’s Eye cosmic ray observatory on the U.S. Army’s Dugway Proving Ground.
Research since then indicates that the GZK cutoff does indeed exist. The rare occurrence of particles above the cutoff is a reflection of the fact that, very occasionally, these particles reach Earth before they come in contact with enough CMBR photons to slow them down to the cutoff point.
Still, the occasional existence of such energetic particles provides one means of exploring these energy ranges, well above what current particle accelerators could reach, so string theory may have a chance of an experimental test using high-energy cosmic rays, even if they are incredibly rare.