Cosmic Rays

Cosmic rays are high-energy subatomic particles, originating in deep space, mainly composed of protons (around 90%) and fragments of atomic nuclei, such as alpha particles (9%) and heavier nuclei (less than 1%). Antimatter such as positrons and antiprotons also account for a very small proportion of cosmic rays.

As these particles strike the Earth, they interact with atoms high in the upper atmosphere, resulting in showers of secondary subatomic particles that are often detectable even at the Earth’s surface.

It has been determined that comic rays are produced in supernova explosions, although it is likely that cosmic rays also originate from other sources, such as active galactic nuclei, quasars and perhaps gamma-ray bursts or magnetic variable stars.

High energy photons (electromagnetic radiation) also strike the Earth from space, however, these particles are simply referred to as gamma-rays or x-rays, depending on their wavelength, rather than cosmic rays. (See gamma-ray bursts.)

Cosmic rays can cause damage to computer electronics, as well as DNA, which is a particular problem for spaceflight,  since the flux of cosmic ray particles is much higher above the Earth’s atmosphere.

Cosmic Ray Energies and the Oh-My-God Particle

Most cosmic rays have energies in the region of 10 mega-electronvolts (MeV) to around 10 giga-electronvolts (GeV).

However, on 15 October 1991, the University of Utah’s Fly’s Eye Cosmic Ray Detector recorded an ultra-high-energy cosmic ray, with an energy of 3×1020 eV, equivalent to the kinetic energy of a baseball travelling at about 60 mph. This cosmic ray (most likely a proton) has been nicknamed the ‘Oh-My-God particle’ (as a tongue-in-cheek reference to the nickname of the Higg’s Boson, which is often referred to as the ‘God Particle’). The particle would have been travelling at 99.99999999999999999999951 per cent of the speed of light.

Since then, several more of these ultra-high-energy cosmic rays have been detected, providing confirmation that the Oh-My-God Particle detection event was indeed the result of a cosmic ray interaction, rather than an equipment malfunction.

Cosmic Ray Tests of Relativity Theory

Particles known as muons, produced in the upper atmosphere via cosmic ray collisions,  have provided a useful test of the time-dilation and length-contraction effects predicted by Einstein’s special theory of relativity.

Muons decay quickly into other particles once they are created, and based on their short average lifetime, it would be expected that very few of these muons would be detectable at the Earth’s surface. However, the effect of relativistic time dilation means that, to an observer on the Earth’s surface, a clock moving through the atmosphere at close to the speed of light, would appear to run more slowly than a clock which is stationary relative to the observer. Since these muons are travelling at a high proportion of the speed of light when they are created in a cosmic ray collision, this means that the muon’s average lifetime appears, to an observer on Earth, to be extended, and, on average, the muons takes slightly longer to decay than they would if they were not moving at such high velocities. This allows the muon to travel further through the Earth’s atmosphere than a simple, non-relativistic calculation would suggest.

Many more muon’s are, indeed, detected at the Earth’s surface, than would otherwise be expected, without this relativistic time-dilation effect.

To a hypothetical observer travelling at the same velocity as these muons, their average lifetime would not appear to be extended. However, the distance measured by this second observer from the position in the upper atmosphere, at which the muons are created, to the Earth’s surface, would appear to be shorter than a measurement taken by the first observer on the Earth, between the same two points. This ‘length contraction’ is the same relativistic effect as time-dilation, but viewed from the moving observer’s perceptive, (frame of reference). Importantly, this means that, using the theory of relativity, both observers will still agree on the percentage increase in the numbers of particles detectable by the first observer on the Earth’s surface over the course of the experiment.

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Astronomy, Cosmology, Astrophysics and Space Exploration

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