Cosmic ray

Cosmic rays can loosely be defined as radiation consisting of energetic particles originating outside of the Earth. The composition of such rays includes electrons, protons, gamma rays, and atomic nuclei from a large region of the periodic table. Though neutrons are not stable, the relativistic time dilation may allow them to reach Earth at very high energies.

History of cosmic rays

Cosmic rays, also known as cosmic particles, were initially believed to originate in radioactive isotopes in the ground. This theory was disproven in 1911 by Victor Hess, who in 1936 received the Nobel prize in physics for his work. Hess used electroscope measurements taken at different altitudes from a hot air balloon to conclude that the radiation was cosmic in origin. Hess further showed that the sun could not be the primary source of cosmic rays by taking balloon measurements during a 1912 solar eclipse.

Related Topics:
Victor Hess - Nobel prize in physics - Electroscope

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In 1938, Pierre Auger observed near-simultaneous cosmic ray events at widely separated locations. He concluded that they were due to incident particles whose energy was too high to penetrate the atmosphere. Such particles instead collide with nuclei in the atmosphere, initiating a particle cascade known as a cosmic ray air shower. The events Auger had observed were found to have energies of 1015 eV, 10 million times higher than had previously been known.

Related Topics:
Air shower - EV

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The measurement of high-energy cosmic rays via sampling of extended air showers was first implemented in 1954 at the Harvard College Observatory. From their work, and from the many ground-array experiments that followed it, the cosmic ray spectrum is now known to extend up to at least 1020 eV.

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In the past, it was believed that the cosmic ray flux has remained fairly constant over time. In fact, this is one of the fundamental assumptions behind radiocarbon dating.

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Recent research has, however, produced evidence for large century time-scale changes in the cosmic ray flux in the past 10000 years .

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Cosmic rays have been implicated in the triggering of electrical breakdown in lightning. It is now considered likely that essentially all lightning is triggered through a relativistic process, "runaway breakdown", seeded by cosmic ray secondaries. Subsequent development of the lightning discharge then occurs through "conventional breakdown" mechanisms.

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