Meanwhile, a secondary detection method of 24 telescopes has sensed the shower's approach in the night sky. Detectors in the tanks register the light and send the information to the observatory headquarters through a cellular network. When the shower of particles drills through the pitch-black water in each detector, they create light. For the highest-energy cosmic rays, the showers consist of 10 billion particles spread over 20 kilometers, hitting about 65 detectors at once. The more energy carried by the original particle, the wider the shower. This shower of secondary particles falls toward Earth, distributing the energy of the original particle. When a cosmic ray particle of any energy level strikes air molecules high in the atmosphere, the collision triggers a chain reaction of collisions with other air molecules. Or rather, it detects evidence of the particles-a much easier task. This array detects about 30 high-energy particles per year. For such a cutting-edge experiment, the impression is surprisingly low-tech. The detectors, each about the size of a compact car, are hard plastic cylinders containing water and instruments. The Pierre Auger Observatory is not one instrument, but an array of 1,600 detectors evenly dotting 3,000 square kilometers. One of the 1,600 cosmic-ray detectors at the Pierre Auger Observatory. Science teams have spent $50 million and 17 years assembling the project in the hope that its data will finally pinpoint where ultra high-energy cosmic rays originate. Named after Pierre Auger, another pioneer in the field, it's the largest scientific instrument ever built. To find an explanation, an international team of physicists has built a giant catcher's mitt, of sorts on the vast, flat plains of Argentina. "It's very difficult to find an explanation of their existence-a physical process that either generates or accelerates particles at this enormous energy." "You would have to wait 10,000 years to have one land in a ball field," says Paolo Privitera, a physicist from the University of Chicago working on the Pierre Auger project. Unlike lower-energy cosmic ray particles, the higher-energy particles are rare. Yet no ordinary exploding star could propel particles to the energies seen in the higher-energy cosmic rays. Particles from the original star or the region around it are likely accelerated by this expanding cloud, and some hit Earth. The expanding cloud of gas that remains after a supernova lasts for thousands of years. The likeliest sources are violent explosions called supernovas, which occur when stars use up their fuel. This suggests that they are relatively easy to accelerate and probably originate from a short distance away-within in our own galaxy. But what?Īfter decades of research, physicists have some idea of where lower-energy cosmic rays come from. Some mechanism in the Universe is accelerating particles of space stuff at an incredible velocity. ![]() Even the Large Hadron Collider, the world's largest and fastest particle accelerator, can only hurl particles fast enough to reach energies at the lower end of that spectrum. Cosmic rays measure anywhere from 1 billion eV to 100 billion-times that much. A medical X-ray machine emits about 150,000 eV. ![]() Energy is measured in electron volts, or eV. The energy of cosmic ray particles is staggering. Instead, they are the particles that make up atoms, such as protons. The particles themselves are very ordinary-the kind that all matter is made of. Instead, cosmic rays are made of atomic particles moving toward Earth at near light speed. ![]() ![]() Nor are they a form of electromagnetic radiation, the most typical radiation we encounter, which includes sunlight, X-rays, and gamma rays. The Pierre Auger Project, a huge experiment now underway in Argentina, hopes to finally answer this most basic question.Ĭosmic rays are not a form of radioactivity. In the decades since Hess flew his balloon, physicists have learned much about this energy, now called "cosmic rays." But even today, scientists don't know where the highest-energy cosmic rays come from in space. It was so energetic, he said, that it must have been able to travel a very long distance indeed. Given the signal direction, Hess suggested that this mysterious energy did not come from Earth. In fact, it was far stronger than known radioactive substances could release. The higher he flew, the stronger the energy was. Hess's electroscopes picked up a pervasive signal. He rose five kilometers skyward with three electroscopes, devices that measure radiation emitted by radioactive elements such as radium. Austrian physicist Viktor Hess decided to take his own measurements in a hot-air balloon. In 1913, there was talk among scientists that an unusual type of radiation had been discovered high in the atmosphere. Cosmic-ray pioneer Viktor Hess after his 1912 balloon flight.
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