Cosmic Fireworks

Somewhere deep in our universe, infinitesimal particles with enormous energies are being created. These sparks are sent flying across galaxies by some hellish dynamos deep within some of the most intensely warped regions of space. These particles, flashing along within a tiny fraction of the speed of light, are woven throughout the complex magnetic fields of space for many thousands, perhaps millions of years.

A single particle may have a kinetic energy of up to about 50 J. This is enough to light a standard light bulb for 1 second. While this doesn�t seem much, it must be remembered that this energy is carried by a single subatomic particle. In a lab we are used to handling small quantities of say, one mole, or about 10²⁴ atoms at a time. Here we have only one small part of one of those tiny atoms.

Frequently, one of these particles weaves its way through the much more intense magnetic fields near our sun and especially the Earth. After wandering for all those long years in nearly empty space, it suddenly encounters the rapidly thickening atmosphere. By about 15 km of altitude this cosmic bullet has smacked into one thing or another in our air. It breaks into many other particles; these in turn break or decay into many others, creating a cascade, in fact a veritable shower of particles. These particles rain down upon us constantly, accounting for about half of all the natural radiation background that our bodies must constantly endure.

One of the most interesting of these "secondary" particles that rain down upon us, is the muon. It is a lepton like the electron. However, it is much more massive and has a very short mean lifetime. In fact, its life is so short that it shouldn�t be able to make it to the surface. Yet it is observed to be the most common high-energy particle to be seen here at the ground. This is an opportunity to see a real life example of Time Dilation.

The famous "Twin Paradox" was interesting but totally impractical. Especially the idea of catching a peek at a clock onboard a relativistic spaceship, as it flashes by, and comparing it to a clock beside you, was ridiculous. Yet here we have the opportunity to do just that. These tiny particles carry a built in clock telling them (statistically) when to decay. Yet they are traveling at very nearly the speed of light, therefore our perception is that their clocks are running slow. We can compare their lifetimes to identical particles in the lab and measure the amount of time dilation by measuring the increase in their range before decaying.