The Story of High Energy Cosmic Rays

In a balloon at an altitude of 5,000 meters, Victor Hess, the pioneer of cosmic ray research, discovered "penetrating radiation" coming form space. His was the first of many adventurous journeys made by physicists to study cosmic rays.

Using the newly invented cloud chamber, Dimitry Skobelzyn observed the first ghostly tracks left by cosmic rays.

A debate raged over the nature of cosmic rays. Accourding to a theory of Robert Millikan, they were gamma rays from space hence the name"cosmic rays." But evidence was mounting that cosmic rays were, in fact, mostly energetic particles.

While watching the tracks of cosmic rays passing through his cloud chamber, Carl Anderson discovered antimatter in the form of the anti-electron, later called the positron. A positron is a particle exactly like and electron, but with an opposite, positive charge.

Seth Neddermeyer and Carl Anderson discovered the elementary subatomic particle called the muon in cosmic rays. The positron and the muon were the first of series of subatomic particles discovered using cosmic rays. These discoveries helped givve birth to the science of elementary particles physics. Particle physicists used cosmic rays for their research until the advent of particle accelerators in the the 1950's.

Pierre Auger, who had positioned particle detectors high in the Alps, noticed that two detectors located many meters apart both signaled the arrival of particles at exactly the same time. Auger had discovered "extensive air showers," showers of secondary subatomic particles caused by the collision of primary high-energy particles with air molecules. on the basis of his measurements, Auger concluded that he had observed showers with energies of 1015 eV-ten million times higher than any known before.

Enrico Fermi put forth an explanation for the acceleration of cosmic rays. In Fermi's cosmic ray "shock" accelerator, protons speed up by bouncing off moving magnetic clouds in space. Exploding stars (supervovae) aer believed to act as such cosmic accelerators, but they alone cannot account for the highest energy cosmic rays.

In the early 1960's, Arno Penzias and Robert Wilson discovered that low-energy microwaves permeate the universe. Kenneth Greisen, Vadem Kuzmin and Georgi Zatsepin point out that high energy cosmic rays woud interact with the microwave background. The interactionwould reduce their energy, so that particles traveling long intergalactic distance could not have energies greater than 5 x 1019 eV.

The Fly's Eye cosmic ray research group in the U.S. observed a cosmic ray event with an energy of 3 x 1020 eV. Events with energies of 1020 eV had been reported in the previous 30 years, but this was clearly the most energetic.

The AGASA Group in Japan reported an event with an energy of 2x 1020 eV. The Fly's Eye and AGASA events are higher in energy than any seen before. Where did these two high-energy cosmic rays come from? Neither seems to point back to an astrophysical object that could accelerate particles to such enormous energies.

An international group of researchers begins design studies for a new cosmic ray observatory, the Pierre Auger Project, named in honor of the discoverer of air showers. The new observatory will use a giant array of detectors to detect and measure large numbers of air showers from the very highest-energy cosmic rays. Tracing high-energy cosmic rays to their unknown source will advance the understanding of the origin and evolution of the universe.