In this first of a series of informational briefs about the fundamental nature of our universe, I want to discuss the electron. This is the first of the current set of building blocks which was discovered, and is a key player in the field of high-energy physics to this day. I hope that this short essay will help you to gain a little knowledge about the electron, and prepare you for the discussions about the siblings and cousins of the electron which are to come.
The name, *electron*, derives from the Greek word for the stone *amber*. This name, given by G. Johnstone Stoney in 1894 [Stoney], was done in honor of the ancient Greek philosopher Thales. Thales was the first to observe the then strange properties of amber when friction is applied to it. The electron was the name for the “atom”, or fundamental constituent, of electricity.
It was experimentation by J. J. Thompson [Thompson] in 1897 using electrical discharges that led to the discovery of the electron. Thompson hypothesized that the discharges of particles in these experiments were actually small pieces of the chemical atom being ejected. The experimental correspondence between the presence of atoms in the material and the corpuscular nature of the discharges established their relation through the electron.
Since its discovery, we’ve come to know a lot about the electron. Every atom is composed of a core nucleus, surrounded by at least one electron. It’s the number of electrons that define the chemical properties of an atom. If we consider hydrogen, the simplest atom (one electron, one proton forming the nucleus), we learn that the electron is a small fraction of the total mass of an atom. In hydrogen, for instance, the electron amounts to *one-half of one-tenth of one percent of the atomic mass*. In kilograms, the electron’s mass can be expressed as 0.00000000000000000000000000000091 kg (where a kilogram is the same as 2.2 pounds)!
Modern particle physicists don’t like to use the kilogram to talk about fundamental particles. It’s an inconvenient unit, suited more to daily life than the quantum world. Instead, we rewrite the mass of the electron in terms of units of quantum energy, electron-volts (eV), and the speed of light (c). In these units, the mass of the electron is 511keV/c2. This is a much better number to throw around than the one in kilograms! It also lets us take advantage of Einstein’s realization that solid mass and fluid energy are two faces of the same thing. Mass and energy anre related by the speed of light, a relationship born out every time we write the mass of the electron in this way.
The electron is, so far as we know, indivisible. Unlike the atom, of which it is a part, the electron has never been broken into smaller pieces. People are often confused when we talk about particle decay, and mistake the decay of a particle as indication that the stuff into which the particles decay must be like little bricks rattling around inside the parent particle. Not so. Particle decay is when the solid mass of a heavy particle is transformed into the small masses of other particles, as well as fluid kinetic energy. These smaller particles weren’t inside the parent before they appeared; rather, they manifested from the energy of the parent, which was transformed into the daughters. It sounds philosophical, but this is observed all the time in particle physics experiments and therefore becomes fact and not opinion.
The electron is an indispensable tool, and player, in modern frontier particle physics. We use it, and its antimatter counterpart, the positron, to produce new particles with much higher masses than that of the electron. How is this possible? It’s Einstein all over again! We collide the electron and the positron in huge bunches millions of times per second. Each collision of one positron and one electron takes their mass, and all their kinetic energy, and makes it available for quantum mechanics to do what it will. For instance, collide electrons and positrons at an energy of 3.1 GeV and you will, quite often, produce the J/psi particle. The “quite often” part of that sentence derives from the probabilistic nature of quantum mechanics. I cannot tell you whether a single collision will make the J/psi, but with careful observation I can tell you how *likely* it is to happen with each collision. God does play dice, and he’s a high-roller.
The electron was the first of the known building blocks to be isolated. It, and the positron, will carry high-energy physics into the distant future. The “International Linear Collider”:http://www.interactions.org/linearcollider/ (ILC) is an advanced accelerator design concept that would bring electrons and positrons into collision at *unprecedented* energies. Doing this will allow particle physicists to probe, with high precision (thanks to the pointlike structure of the electron), the most minute and subtle of nature’s properties. From the identity of dark matter, to the matter/antimatter imbalance of the universe, to the origin of mass, the ILC will employ our trusted colleague, the electron, one more time in the ongoing quest to scrutinize our rationally intelligible universe.
.. [Thompson] “http://www.aip.org/history/electron/”:http://www.aip.org/history/electron/
.. [Stoney] “http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Stoney-1894.html”:http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Stoney-1894.html
