1912 — Hess discovers cosmic rays

In a balloon at an altitude of 5000 meters, Victor Hess, the father of cosmic ray research, discovered "penetrating radiation" coming from space. His was the first of many adventurous journeys made by physicists to study cosmic rays.

1927 — Cosmic rays seen in cloud chamber

Using a cloud chamber, Dimitry Skobelzyn photographed the first ghostly tracks left by cosmic rays.

1932 — Anderson discovered antimatter. Debate over cosmic rays

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

A debate raged over the nature of cosmic rays. According 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.

1937 — Discovery of muon

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 a series of subatomic particles discovered using cosmic rays - discoveries that gave birth to the science of elementary particle physics. Particle physicists used cosmic rays for their research until the advent of particle accelerators in the 1950s.

1938 — Auger discovered extensive air showers

At the Jean Perrin's laboratory on a ridge of the Jungfrau (Switzerland) Pierre Auger and his students discovered extensive air showers. They showed the presence in primary cosmic rays of particles of a million Gigavolts - a million times more energetic than accelerators of that day could produce.

1946 — First air shower experiments

Groups led by Bruno Rossi in the USA and Georgi Zatsepin in Russia started experiments on the structure of Auger showers. These researchers constructed the first arrays of correlated detectors to detect air showers.

1949 — Fermi's theory of cosmic rays

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 (supernovae) are believed to act as such cosmic accelerators, but they alone cannot account for the highest-energy cosmic rays.

1962 — First 10²⁰ eV cosmic ray detected

John Linsley and collaborators discovered the first cosmic ray with an energy of about 10²⁰ eV in the Volcano Ranch array in New Mexico, USA.

1966 — Proposal of GZK cutoff energy for cosmic rays

In the early 1960s, Arno Penzias and Robert Wilson discovered that low-energy microwaves permeate the universe. Kenneth Greisen, Vadem Kuzmin and Georgi Zatsepin pointed out that high-energy cosmic rays would interact with the microwave background. The interaction would reduce their energy, so that particles traveling long intergalactic distances could not have energies greater than 5x10¹⁹ eV.

1967 — Haverah Park cosmic ray detector begins operations

An array of over 200 water-Cherenkov detectors covering 12 km² was operated for over 20 years from 1967 at Haverah Park in England. Although designed before the GZK prediction, the data have contributed in a major way to our understanding of cosmic rays at the highest energies. For the Auger Observatory, the demonstration that water could be kept bacteria-free in a sealed container for over 25 years was of major importance as were early studies of inclined showers and of the time structure of the shower front.

A tank was opened at the 'end of project' party on 31 July 1987. The water shown in the photo had been in the tank for 25 years but was quite drinkable!

1991 — Fly's Eye detected highest-energy cosmic ray

The Fly's Eye cosmic ray research group in the USA observed a cosmic ray event with an energy of 3x10²⁰ eV. Events with energies of 10²⁰ eV had been reported in the previous 30 years, but this was clearly the most energetic.

1994 — AGASA high-energy event

The AGASA Group in Japan and the Yakutsk group in Russia each reported an event with an energy of 2x10²⁰ eV. The Fly's Eye event and these events are higher in energy than any seen before. Where did these three high-energy cosmic rays come from? None seem to point back to an astrophysical object that could impart such enormous energies.

1995 — Pierre Auger Project begun

An international group of researchers began 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.

1999 — Groundbreaking for Pierre Auger South

With a spade full of Argentinean soil, scientists from the 19-nation Pierre Auger Project came an important step closer to understanding the mystery of one of nature's most puzzling phenomena, the origin of highest energy cosmic rays. Dr. Arturo Lafalla, Governor of the Province of Mendoza, served as the official host of the March 17 groundbreaking ceremony for the new cosmic ray observatory located near the cities of Malargüe and San Rafael in Mendoza Province, Argentina.

2005 — Celebration and first physics results from Auger

The Pierre Auger Observatory is designed to study the highest energy cosmic rays with unprecedented statistics and precision. To celebrate the imminent completion of the construction of the southern site, along with the presentation of the first science results in the summer of 2005, we celebrated a ceremonial event at the Observatory, in Malargüe, Province of Mendoza, Argentina in November 2005. We were especially pleased to mark our scientific launch during the World Year of Physics.

2007 — Auger discovers extragalactic origin of highest-energy cosmic rays

Scientists of the Pierre Auger Collaboration announced on 8 Nov. 2007 that active galactic nuclei are the most likely candidate for the source of the highest-energy cosmic rays that hit Earth. Using the Pierre Auger Observatory in Argentina, the largest cosmic-ray observatory in the world, a team of scientistsfrom 17 countries found that the sources of the highest-energy particles are not distributed uniformly across the sky. Instead, the Auger results link the origins of these mysterious particles to the locations of nearby galaxies that have active nuclei in their centers. The results appear in the Nov. 9 issue of the journal Science.

2008 — Inauguration of completed Pierre Auger South

Scientists of the Pierre Auger Observatory, a project to study the highest-energy cosmic rays, celebrated the inauguration of the southern site of their observatory in Malargüe, Argentina, on November 14, 2008. The event marked the completion of the first phase of the Observatory construction and the beginning of the project’s second phase, which includes plans for a northern hemisphere site in Colorado, USA, and enhancements to the southern hemisphere site.

It was as if they went out to catch butterflies, and caught the ISS - the International Space Station. It wasn't supposed to happen. Cosmic ray researchers were dumbfounded when their "Fly's Eye" detector in the high Utah desert in the western USA turned up an incoming particle from space with an energy six times higher than their theory allowed. Two years later, on the other side of the world, a Japanese detector recorded another of these "impossible" events. These two carefully documented cosmic rays, whose energy is so high it defies explanation, have spurred the effort to build a new detector big enough to capture and study many more of these high-energy particles, and to try to discover where they came from.

The Fly's Eye Event

The highest-energy cosmic ray ever detected was observed on October 15, 1991 by the Fly's Eye cosmic ray detector in Utah, USA. The detector is located in the desert in Dugway Proving Grounds 75 miles southwest of Salt Lake City. The Fly's Eye detects cosmic rays by observing the light that they cause when they strike the atmosphere. When an extremely high-energy cosmic ray enters the atmosphere, it collides with an atomic nucleus and starts a cascade of charged particles that produce light as they zip through the atmosphere. The charged particles of a cosmic ray air shower travel together at very nearly the speed of light, so the Utah detectors see a fluorescent spot move rapidly along a line through the atmosphere. By measuring how much light comes from each stage of the air shower, one can infer not only the energy of the cosmic ray but also whether it was more likely a simple proton or a heavier nucleus.

The Utah researchers measured the energy of the unusual cosmic ray event in 1991 to be 3.2x10²⁰ eV. They were stunned by their observation. They had previously believed that such energetic particles could not exist in the universe, because theory said the particles should rapidly lose their energy in collisions with the universal microwave radiation left over from the Big Bang. Thus, very high-energy particles now pose a cosmic mystery that has inspired a worldwide collaboration to begin planning the vast new detector called the Pierre Auger Cosmic Ray Observatory.

The AGASA Cosmic Ray Event

Akeno, Japan, a village about 120km west of Tokyo, was the home of the world's largest surface array for detecting very high-energy cosmic ray air showers, until overtaken by the Pierre Auger Observatory in 2004. The Akeno Giant Air Shower Array (AGASA) consists of 111 particle detectors spread about a kilometer apart over an area of 100 square kilometers. Each detector occupies a small hut 2.2 square meters in area. Construction of the array began in 1987; it has been measuring cosmic ray air showers ever since its completion in 1991.

On December 3, 1993, the AGASA array recorded a very large air shower. This very special event was particularly well measured because the air shower fell completely inside the detector array and arrived from a nearly vertical direction. This air shower was produced by a cosmic ray with an energy of about 2x10²⁰ eV. This is the highest-energy cosmic ray observed at AGASA; and, like the Fly's Eye event in Utah, it has an energy well above that expected from any known source.

Scales of Energy

Scientists measure the energies of fast-moving particles like those in cosmic rays and particle accelerators in units called electron volts, abbreviated eV. An electron volt is the amount of energy that one electron gains when it is accelerated by an electrical potential of one volt. (A flashlight battery has about 1.5 volts.) Electrons in a television set are accelerated by the picture tube to an energy of about 50,000 electron volts. When they strike the screen, they make it glow.

The most powerful man-made particle accelerator, Fermilab's Tevatron, can accelerate protons to nearly one trillion electron volts. The highest-energy cosmic ray particle ever observed had an energy 300 million times higher than the protons at the Tevatron. Scientific notation, shown below, saves writing out the many zeros required for such large numbers.

The history of cosmic ray research is a story of scientific adventure. For nearly a century, cosmic ray researchers have climbed mountains, ridden hot air balloons, and traveled to the far corners of the earth in the quest to understand these fast-moving particles from space. They have solved some scientific mysteries - and revealed many more. With each passing decade, scientists have discovered higher-energy, and increasingly more rare, cosmic rays. The Pierre Auger Project is the largest scientific enterprise ever conducted in the search for the unknown sources of the highest-energy cosmic rays ever observed.

Scientists love a mystery, because solving a mystery in nature means the opportunity to learn something new about the universe. High-energy cosmic rays are just such a mystery. Something out there - no one knows what - is hurling incredibly energetic particles around the universe. Do these particles come from some unknown superpowerful cosmic explosion? From a huge black hole sucking stars to their violent deaths? From colliding galaxies?

We don't yet know the answers, but we do know that solving this mystery will take scientists another step forward in understanding the universe.

What Are Cosmic Rays?

Cosmic rays are fast-moving particles from space that constantly bombard the earth from all directions. Most of the particles are either the nuclei of atoms or electrons. Of the nuclei, most are single protons - the nuclei of hydrogen atoms - but a few are much heavier, ranging up to the nuclei of lead atoms. Cosmic ray particles travel at nearly the speed of light, which means they have very high energy. Some of them, in fact, are the most energetic of any particles ever observed in nature. The highest-energy cosmic rays have a hundred million times more energy than the particles produced in the world's most powerful particle accelerator.

Where Do Cosmic Rays Come From?

Lower-energy cosmic ray particles that strike the earth come from within our own Milky Way Galaxy. They may originate, directly or indirectly, from the supernova explosions that mark the deaths of many stars. These explosions throw out fast-moving magnetic fields which reflect charged particles. Cosmic ray nuclei gain energy when they collide with such a moving reflector. At a magnetic shock, where the magnetic field slows abruptly, particles can become trapped between two reflectors. Like a ping-pong ball caught between two converging paddles, the nuclei make many reflections, and the energy gained in each reflection grows as their energy increases. This "magnetic shock acceleration" model was first proposed by the great physicist Enrico Fermi as an explanation for the acceleration of most cosmic rays. The process has been observed in magnetic shocks in the solar wind that flows out from our sun, producing cosmic rays of modest energy. The stronger moving magnetic fields produced in supernova explosions could provide the energy for most other cosmic rays.

Even these shocks are not strong enough, however, to accelerate the highest-energy cosmic rays. While no one knows their source, there are compelling reasons to believe that they must originate outside our Milky Way galaxy - but where?

Where Do They Get Their Energy?

Wherever they come from, the highest-energy particles hold secrets to the origin of their enormous energies, many millions of times greater than any earthbound particle accelerator can create. Fermi's acceleration mechanism provides an explanation for cosmic ray energies perhaps as high as 10¹⁵ eV. Acceleration mechanisms for cosmic rays of higher energies are not understood.

Observational evidence supports the view that cosmic rays with energies up to about 3 x 10¹⁸ eV originate within our galaxy. Above this energy, most cosmic rays may be coming from outside the Milky Way. The highest-energy cosmic rays are not deflected much by the weak magnetic fields in our Galaxy, yet they do not arrive preferentially from the disk of the Milky Way or the side of the sky toward the center of the Galaxy. This strongly suggests an extragalactic origin. Although we have not confirmed any source in the cosmos that can produce such energies, several hypotheses have been proposed. These include radio galaxy hot spots and active galactic nuclei (AGN) jets.

Cosmological Questions

Because the highest-energy cosmic rays are deflected very little by the magnetic fields in our galaxy - and even less by the much weaker fields in intergalactic space - we ought to be able to look back in the direction of the cosmic rays to find their origin. So far, however, none of the cosmic ray events with energies above 10²⁰ eV point back to a possible source in the cosmos! Where have they come from? The mystery deepens when we realize that, unless the source is fairly close to our Milky Way Galaxy (within 100 million light years or so), collisions with the low-energy microwaves that pervade the universe would reduce cosmic ray energies to levels below 10²⁰ eV before they ever reached Earth. The sources must be relatively nearby, but the arrival directions do not point to any known astrophysical powerhouses.

Cosmologists, who study the structure and dynamics of the universe, offer another possible explanation for the mysterious source of the highest-energy cosmic rays. Cosmologists postulate a universe filled with relics left over from the Big Bang - hypothetical objects, called topological defects, with names like "cosmic strings," "domain walls," and "monopoles." Although these strange objects figure prominently in theories of the evolution of the universe, we have no experimental evidence to show that they really exist. However, if they did exist, and if they sometimes collapsed, their collapse could produce enough energy to create very high-energy cosmic rays. If we could make the connection between high-energy cosmic rays and the collapse of topological defects, it would provide experimental evidence for these topological defects and a great step forward in understanding the early universe.

How Do We Learn About Cosmic Rays?

To learn about the nature of high-energy cosmic rays, scientists measure their energy and their direction as they arrive from space. Cosmic rays of modest energy are measured directly by sending detectors to heights above most of the earth's atmosphere, using high-flying balloons and satellites. For high-energy cosmic rays, however, it is more efficient to exploit the atmosphere, measuring each cosmic ray indirectly by observing the shower of particles it produces in the air.

An air shower occurs when a fast-moving cosmic ray particle strikes an air molecule high in the atmosphere, creating a violent collision. Fragments fly out from this collision and collide with more air molecules, in a cascade that continues until the energy of the original particle is spread among millions of particles raining down upon the earth. By studying the air showers, scientists can measure the properties of the original cosmic ray particles.

Frequently Asked Questions

• What Are Cosmic Rays?
• Where Do Cosmic Rays Come From?
• How And When Were Cosmic Rays Discovered?
• How Much Energy Do Cosmic Rays Have?
• How Many Cosmic Rays Strike The Earth's Atmosphere Each Second?
• How Do We Observe Cosmic Rays?
• Are Low Energy Cosmic Rays Produced Inside Our Galaxy?
• Where Are Very High Energy Cosmic Rays Produced?
• How Does The Composition Of Cosmic Rays Change With Energy?
• What Can We Learn From The Arrival Direction Of Cosmic Rays?
• Is There A Maximum Energy For Cosmic Rays?
• What Is The Highest Energy Ever Seen In A Cosmic Ray?
• What Does It Mean For A Cosmic Ray To Have An Energy 1020 Ev?
• Why Should Society Support Cosmic Ray Research?

What Are Cosmic Rays?

These are very energetic charged particles that continually bombard the earth. These particles are usually protons, but can also be larger nuclei. When such a particle strikes the earth's atmosphere, it creates a shower of lower energy secondary particles, and these are observed to reach the ground. In fact, about a hundred of these secondary particles pass through our bodies every second. Exposure to cosmic rays is even greater at high altitudes.

Where Do Cosmic Rays Come From?

The answer depends largely on the energy of the particle, but the short answer is that we still don't know. Part of the problem is that unlike light, which travels directly from a star to us, cosmic rays are charged particles, and so they are influenced by magnetic fields which extend throughout space. The magnetic fields cause the lower energy cosmic rays to swerve along complicated paths, and in most cases we can't determine their point of origin. Low to medium energy cosmic rays, up to energies of about 1018 eV, are thought to originate within the Milky Way galaxy. Higher energy cosmic rays may have an extragalactic origin. More information on this topic is found here.

How And When Were Cosmic Rays Discovered?

In 1912 a scientist named Viktor Hess carried an instrument called an ionization chamber in a balloon to high altitudes. An ionization chamber is a device that records the passage of charged particles. As Hess made his ascent in the balloon, the ionization chamber recorded fewer particles, up to an altitude of 2,000 meters. The interpretation is that some of this ionization is due to the natural radioactivity of the earth, and its influence decreases with altitude. Above 2,000 meters, however, he recorded more particles, and the increase in particles became even more rapid as his balloon reached its maximum altitude of 5,350 meters. Hess correctly guessed that this increase was due to radiation entering the atmosphere from space. On one occasion he rode the balloon during a solar eclipse, and found no decrease in ionization. From this he concluded that the radiation was coming from somewhere other than the sun. We now know that much of this cosmic radiation originates far outside the solar system.

How Much Energy Do Cosmic Rays Have?

They have a very broad range of energies. The weakest ones have an energy of about 1,000,000,000 electron volts , which is about the minimum energy needed for a particle to get from beyond the solar system through the magnetized solar wind. (It is much easier to work with very large numbers by writing them in scientific notation. For example, we can write 1,000,000,000 as 1 x 109.) The highest energy cosmic rays ever recorded had energies of about 1 x 1020 eV. In contrast, the highest energy man-made particles, produced by very expensive machines called accelerators, have energies of about 1 x 1012 eV.

How Many Cosmic Rays Strike The Earth's Atmosphere Each Second?

The rate of cosmic rays reaching us falls off rapidly as the cosmic ray energy increases. For 1 GeV particles, the rate is about 10,000 per square meter per second. At 1000 GeV (or 1012 eV), the rate is only 1 particle per square meter per second. The rate starts to decrease even more rapidly around 1016 eV, where there are only a few particles per square meter per year. The highest energy particles, above 1019 eV, arrive only at a rate of about one particle per square kilometer per year.

How Do We Observe Cosmic Rays?

The technology of recording cosmic ray showers has improved over the years. At first, they were studied using instruments such as ionization chambers, Geiger counters, and cloud chambers. These instruments recorded a signal when an energetic charged particle passed through them. In the late 1920s, the French scientist Pierre Auger discovered the phenomenon of extensive air showers using these techniques. What he found was that very energetic cosmic rays were capable of producing showers of secondary particles which spread over a large area up to hundreds of meters. These methods only detect particles that reach the ground, but do not tell us about how a cosmic ray shower develops in the atmosphere.

A new technique was developed in the 1980s based on the phenomenon of atmospheric fluorescence. When a charged particle passes close to molecules in the atmosphere, it transfers some energy to the molecules, in effect "shaking up" the electrons inside. The molecules respond by emitting light as their electrons return to their normal arrangement, and this light is known as fluorescence. Nitrogen molecules, which make up most of the air, make blue fluorescent light. This light can be detected by sensitive instruments called photomultipliers. Even so, the light is so faint that it can only be observed on moonless nights without clouds. This technique has been successfully used by the Fly's Eye experiment in Utah, and will also be used by future experiments including HiRes and the Pierre Auger Observatory.

Another technique, useful for measuring cosmic rays that reach the ground, uses a phenomenon called the Cerenkov effect. In transparent materials, the speed of light is less than its value in vacuum (300,000 kilometers/second). In water, for example, light travels at 70% of its speed in vacuum. When a high energy charged particle, such as a cosmic ray, passes through the water at speeds greater than this, it creates a shock front of light that spreads out in a cone around the particle. Photomultiplier tubes placed in the water detect the Cerenkov light. An array of these detectors was used in an experiment in Haverah Park, England, for more than 20 years until 1991. Tanks of water using photomultipliers to see Cerenkov light are also being used by the Pierre Auger Observatory.

Are Low Energy Cosmic Rays Produced Inside Our Galaxy, The Milky Way?

We think this may be true. We know that very low energy cosmic rays are produced by the sun. We believe, however, that the vast majority of cosmic rays come from outside the solar system. Results from the Compton Gamma Ray Observatory satellite tell us information about the distribution of gamma rays (very high energy photons) in the sky. We expect that gamma rays are produced when cosmic rays interact with the diffuse gas in our galaxy, the Milky Way. The satellite data show that the intensity of these gamma rays falls off with increasing distance from the galactic center. This would happen if lower energy cosmic rays were produced in the central bulge of the galaxy. Exploding stars, called supernovae, may be responsible for producing many of the cosmic rays within our galaxy.

Where Are Very High Energy Cosmic Rays Produced?

Nobody knows for sure. We believe that cosmic rays with energies up to about 1017 or 1018 eV are trapped within our Milky Way galaxy by magnetic fields. Cosmic rays with higher energies would escape from the galaxy and wander through the vast distances of intergalactic space. Very high energy cosmic rays produced by other galaxies would be able to travel to us as well.

How Does The Composition Of Cosmic Rays Change With Energy?

When we ask about composition, we are asking about what cosmic rays are made of. Are they protons, electrons, or something else? As discussed above, low energy cosmic rays consist of mainly protons and light nuclei. Measurements taken in a high altitude balloon, the Japanese-American Cooperative Emulsion Experiment (JACEE), show that as the cosmic ray energy increases, the proportion of heavier nuclei also increases. This suggests that as the energy reaches around 1015 eV, heavy nuclei become the dominant component. It is very difficult, however, for a satellite or balloon experiment like JACEE to study particles at these high energies. This is because the flux is very low, and the detector area that can be carried aboard a satellite is so small. A better alternative is to use a ground based detector to sample the energies from extensive air showers and infer the particle energies indirectly. The situation is complicated because there is a lot of fluctuation in the way a shower develops, but in general, a heavy nucleus will start to shower higher up in the atmosphere than a light nucleus. Observations suggest that at the highest energies, there are very few heavier nuclei, and the cosmic rays are mostly protons. This question is still a subject of considerable research.

What Can We Learn From The Arrival Direction Of Cosmic Rays?

Because cosmic rays are deflected by magnetic fields, we expect to see them arriving from all directions. We do, and in fact the deviation from directional uniformity, or anisotropy, is less than 1%. Because the earth, along with the solar system, is moving through the galaxy at 200 kilometers/second, we expect a small anisotropy due to this motion. This is called the Compton-Getting effect: we should see slightly more cosmic rays in the direction we're moving. As yet, we have not yet observed this effect even though some experiments should be sensitive enough.

Cosmic rays with the highest energies are deflected the least by magnetic fields. Based on current understanding of magnetic fields in galaxies, these ultra-high energy rays may only be deflected by less than 3 degrees. Thus, it may be possible to correlate the arrival directions with known astronomical objects, such as nearby galaxies. Establishing this correlation would usher in the era of cosmic ray astronomy.

Is There A Maximum Energy For Cosmic Rays?

There is a predicted maximum energy 6 x 1019 eV, which was calculated by Kenneth Greisen in the United States and G.T. Zatsepin and V.A. Kuz'min in the Soviet Union in 1965. It is called the GZK cutoff after the three scientists who discovered it. Space isof filled with microwave radiation, called the cosmic microwave background, which is leftover radiation from the Big Bang. While a microwave photon doesn't have much energy, a sufficently energetic cosmic ray would see the photon's wavelength to be compressed due to the Doppler effect. From the cosmic ray's perspective, the microwave photon would appear to be a gamma ray. Collisions between protons (cosmic rays) and gamma rays have been studied in accelerators, and these collisions often result in the production of particles called pions, which cause the proton to lose energy. A collision in space between a cosmic ray proton and a microwave photon would result in the same production of pions. With each collision, the proton would lose roughly 20% of its energy. This only happens for cosmic rays that have at least 6 x 1019 eV of energy, and this is the predicted GZK cutoff. So if cosmic rays were given an initial energy greater than that, they would lose energy in repeated collisions with the cosmic microwave background until their energy fell below this cutoff. This is expected to happen by the time a cosmic ray has traveled about 150 million light years. To summarize, we expect to see very few cosmic rays above the GZK cutoff energy, unless there are "nearby" sources.

What Is The Highest Energy Ever Seen In A Cosmic Ray?

There are several events worth mentioning. In the 1960's, a ground array of 19 detectors spread over 8 square kilometers was built at Volcano Ranch, New Mexico, by a team led by John Linsley. In 1963, his team reported an observation of a cosmic ray with an energy greater than 1020 eV. Since then, several large detector arrays have been built to search for very high energy csomic rays. One such detector, called the Fly's Eye, and built in the Utah desert, observed a cosmic ray shower in 1991 that at it's maximum contained 200 billion particles in the shower. The energy of the primary particle was 3 x 1020 eV, the highest energy cosmic ray ever observed. While the composition of the primary particle isn't known with certain, the best guess is that it was a moderate mass nucleus (something like oxygen).

What Does It Mean For A Cosmic Ray To Have An Energy 1020 eV?

Physicists have learned that one way of understanding nature is by classifying it by sizes. Everyone sees small things and big things, but scientists quantify these notions. Here's an example: A mouse is about 10 cm long. The Sun is about 10,000 million mice across. Now imagine if one day you ran across a mouse that was the size of the Sun. You might say: Cheese, I wonder how it got that big!

Physicists are asking the same question when it comes to cosmic rays. On Earth we know what "everyday" fundamental particles, such as protons, are like. (We quantify them by measuring their mass-energy.) But when we study radiation from space, we find a few of them with as much more energy as the everyday ones on Earth, as the size of a mouse when compared to the size of the Sun. The curiosity of the situation is that cosmic rays and particles we find on Earth are both the same type of object. How is it that nature can produce such a variety?

Why Should Society Support Cosmic Ray Research?

Indeed, why should society support basic research at all? Our colleagues at Fermilab deal with this question a lot; they have created a web page summarizing the benefits of supporting basic science: Why Support Science?

Ref: Cosmic Bullets, written by Roger Clay and Bruce Dawson.
 

The Pierre Auger Observatory experiment was named after Pierre Victor Auger (1899 - 1993), who can be considered the discoverer of giant airshowers generated by the interaction of very high-energy cosmic rays with the earth's atmosphere. Most of his professional life was devoted to the following fields of experimental physics:

• Atomic physics (photoelectric effect)
• Nuclear physics (slow neutrons)
• Cosmic ray physics (atmospheric air-showers)

During the Second World War, he joined the Free French Forces, and partipated in the creation of a French-British-Canadian group on atomic energy research, becoming the head of this department in Montreal. After the war, he became Director of the Department of Sciences for the UNESCO. He strongly campaigned for the creation of international research organizations.

More information on the life and work of Pierre Auger can be found at the CNRS website.

Pierre Auger on Cosmic Rays:

"In his excellent paper, Louis LePrince-Ringuet, citing a remark of Powell's at the Conference of Bagneres-de-Bigorre in 1953, declared that from that date on, particle accelerators took the place of cosmic rays, which more or less faded into the background. And yet, even today accelerators have not caught up with cosmic rays. For in 1938, I showed the presence in primary cosmic rays of particles of a million Gigavolts - a million times more energetic than accelerators of that day could produce. Even now, when accelerators have far surpassed the Gigavolt mark, they still have not attained the energy of 10²⁰ eV, the highest observed energy for cosmic rays. Thus, cosmic rays have not been dethroned as far as energy goes, and the study of cosmic rays has a bright future, if only to learn where these particles come from and how they are accelerated. You know that Fermi made a very interesting proposal that particles are progressively accelerated by bouncing off moving magnetic fields, gaining a little energy each time. In this way, given a certain number of "kicks," one could perhaps account for particles of 10¹⁸ - 10²⁰ electron volts. As yet, however, we have no good theory to explain the production of the very-high-energy particles that make the air showers that my students and I discovered in 1938 at Jean Perrin's laboratory on a ridge of the Jungfrau." — Pierre Auger, Journal de Physique, 43, 12, 1982

Pierre Auger sur Rayons Cosmiques:

"Dans son bel exposé, Louis LePrince-Ringuet a déclaré, en citant une remarque de Powell à la Conférence de Bagnères-de-Bigorre en 1953 qu'à partir de cette époque, les accélérateurs de particules prenaient la relève des rayons cosmiques, qui passaient plus ou moins au second plan. Et pourtant même actuellement, les accélérateurs n'ont pas encore rattrapé les rayons cosmiques. Car en 1938, j'ai pu démontrer la présence dans le rayonnement cosmique primaire, de particules d'énergie supérieure à 1 million de fois le Gigavolt, par conséquent un million de fois supérieure à ce que pouvait produire les accélérateurs à cette époque. Et même maintenant que les accélérateurs dépassent de beaucoup le Gigavolt n'atteignent tout de même pas 10²⁰ eV qui est l'énergie la plus élevée qui ait été évaluée pour ces rayons cosmiques. Par conséquent, le rayonnement cosmique n'est pas détroné en ce qui concerne l'énergie des particules et il a encore devant lui un bel avenir, ne serait-ce que pour savoir d'où proviennent ces particules et comment elles ont été accélérées. Vous savez que Fermi a fait une proposition très intéressante qui est que ces particules sont accélérées progressivement par des chocs sur des champs magnétiques variables donnant chaque fois un petit supplément d'énergie. Et par conséquent avec un certain nombre de ces chocs on pourrait peut être construire des particules de 10¹⁸ - 10²⁰ électron-volts. On n'a donc pas encore, actuellement de bonne théorie de la production des particules de très haute énergie qui sont responsables des grandes gerbes de l'atmosphère, celle que, avec mes élèves, nous avons découvert en 1938 au laboratoire de Jean Perrin au Jungfraujoch." — Pierre Auger, Journal de Physique, 43, 12, 1982

A computer is used to construct a model of what happens in a high energy cosmic ray airshower. By comparing the data collected by the observatory to this computer models, we improve our understanding of the nature of cosmic rays.

This picture shows an animation of a shower caused by a proton with an energy of 10¹⁹ electron volts. The colors represent different kinds of particles (photons, electrons, muons) contained in the shower.

This animation was created by the Cosmus group at the University of Chicago.