Sunday, April 6, 2008

Electrons and Their Siblings

Electrons are the little points of charge that are racing right now through the logic board of your computer, just so you can read this blog. They are tiny. In fact, they are too small to measure. We know that they are smaller than 1/1000th the size of a proton†. They weigh 1/1800th as much as the proton. They have one unit of electric charge.

The photo is of J. J. Thompson, who used the pictured Cathode Ray Tube to discover the electron in 1897.

There are two siblings of the electron which are just like electrons, except heavier. They are called the muon and the tau.

[first of three posts designed to explain what this giant thing is.]

Muons (μ) weigh about 200 times as much as the electron. They are not stable, and decay in about 2 millionths of a second, on average. That's actually very long-lived for an unstable particle. They are produced in the upper atmosphere by cosmic rays, and because they are going near the speed of light, they live until they reach the ground (I'll explain that later). There are hundreds of them going through you every second. Don't worry, we have spent our entire time as a species bathed in a background of particles going through us. Most of them pass straight through, and our bodies are well equipped to deal with a small amount of radiation.

Taus (τ) weigh about 3500 times as much as the electron (they are almost twice as heavy as a proton). They are even less stable than muons, and decay in about a trillionth of a second. Since they are shorter-lived and produced less frequently in cosmic rays, there are far fewer taus going through you than muons.

[next Neutrinos and Their Siblings]

† 'Size' becomes less well defined for elementary particles. What do use to measure it? Here I mean that electrons don't seem to be made of other particles and so are pointlike, as far as we can tell. (Protons and neutrons are made of quarks, and they do have a size, a quadrillionth of a meter, better known as a femtometer.)

[Note: The numbers for mass in the table are actually in units of Gev/c2, and the proton weighs 0.938 Gev/c2, not 1, but one shouldn't worry about such small differences when one is getting a sense of scale.]
[confidence: established, my qualifications: trained]