Matt Strassler [January 21, 2013]
Electrons, the tiny objects that inhabit the outskirts of atoms, play the leading role in chemistry, carry electric current across our electrical grid and inside strokes of lightning, and make up the “cathode rays” that were used to create the pictures on 20th century televisions and computer screens. They are the quintessential example of (apparently)-elementary particles.
By “elementary” I mean that electrons are indivisible, and are not made from still smaller particles. By “apparently” I remind you that they are only elementary as far as we currently know — what we know about electrons comes from experiments, and our experiments are not infinitely powerful. If electrons are not elementary, but are so small that our current experiments cannot break them apart, then they will appear elementary to all experiments we’ve done in the past and present, but not to all future experiments. So someday — just as 80 years ago people thought protons might be elementary but had incomplete knowledge, and 150 years ago people thought that atoms might be elementary but had incomplete knowledge — someday we may discover electrons are not elementary after all. But for now, since all experiments we can perform show them as elementary, we will assume, provisionally, that this is the case — remembering that this is partly an experimental fact and partly an assumption!
The electron was the first sub-atomic particle to be identified (i.e., the first object whose size is smaller than an atom). At the time of its discovery in the 1890s (the usual date is given as 1897, but the discovery was, in some senses, gradual) the scientific debate as to whether matter was made from atoms, or whether atoms were simply convenient fictions useful for describing how matter behaves, was nearing an end. But even those who believed atoms existed were not necessarily sure whether or not atoms were indivisible (as their name, derived from Greek for “not cuttable”, would imply). A generation later, by the mid-1930s, physicists had confirmed the existence of atoms, understood their basic structure, and learned how to calculate their properties with high precision. They did these calculations using the equations of a 1920s theory of how matter behaves, called “quantum mechanics”, which was needed because Newton’s famous equations were not able to describe how atoms work. Many of the key tests of the accuracy of quantum mechanics involved measuring carefully were how electrons behave, within and outside of atoms.
All electrons are identical and indistinguishable; if I switched two of them you wouldn’t be able to tell. [How do we know this? I'll explain later.] So I can write about the property of “an electron”, and you can be sure that those properties are true of all electrons. What are these intrinsic properties?
An electron has a mass — its mass is small compared to the mass of any atom, so you can usually forget about it in a beginning chemistry class, but not so small that you can forget about it in particle physics, or even in understanding what atoms are. (Although the electrons in an atom don’t contribute much to the atom’s mass, the electron mass is essential in determining the size of an atom, something which you can read about here. In fact, this is part of why the Higgs field and particle are important.) How big the mass is can be written in several ways, each of which gives a different perspective:
- It is about 9 × 10-31 kilograms = 0.000,000,000,000,000,000,000,000,000,000,9 kilograms (i.e. 0.000,000,000,000,000,000,000,000,000,002 pounds), or about a one (two) millionth(s) of a millionth of a millionth of a millionth of a millionth of a kilogram (pound).
- It is about 0.05% (more precisely, about 1/1838) of the mass of a hydrogen atom, which is the lightest atom in nature; most of an atom’s mass sits in its nucleus.
- The energy stored in an electron’s mass, E = mc², is 0.000,511 GeV, (learn about GeV here). This is about 200,000 times larger than the energy carried by a single photon of green light [a photon is the particle out of which light and other electromagnetic waves are made.] Often in particle physics we write the mass of a particle using the inverse energy-mass relation: for a particle that is stationary, m = E/c². In this language, the electron’s mass is 0.000511 GeV/c² .
An electron has an electric charge — which means that it is affected by electric (and magnetic) fields. An electrically charged particle, in the presence of an electric field, will be pushed on by an electric force. Indeed such forces keep electrons bound inside their atoms.
How big is the electron’s electric charge? Well, think about static electricity — you walk across a rug in your shoes, and then, when you touch your finger to a doorknob, another person, or (!!!) your computer, you feel a spark. That spark carries charge from one place to another — typically 10 million million times more charge than carried by an electron. Physicists measure charge using an arbitrary unit called a Coulomb (just as they measure time using seconds and length using meters). A typical static electricity charge is about 1 millionth of a “Coulomb”, give or take quite a bit. The size of the charge on an electron is traditionally called e, where e is about 1.6 × 10-19 Coulombs (that is, 0.16 millionths of a millionth of a millionth of a Coulomb.)
It was decided historically to define the charge on an electron to be negative (and the charge on an atomic nucleus is therefore defined to be positive) so the electron’s charge is actually -e.
An electron does not have a known size; it might be a point object with no size at all, or it might have a extremely tiny size, smaller (in radius) than about 10-18 meters — a millionth of a millionth of a millionth of a meter (and a meter is about 3 feet). It’s at least 100,000,000 times smaller in radius than an atom. If this weren’t the case, we’d have seen signs of the electron’s size in experiments.
Hmmm… Actually this is a tricky point (which you can skip if it’s too confusing), because what does an electron look like? As I discussed in my article on atoms, defining what you mean by the size of an elementary particle is complicated by the fact that an electron, despite the name “particle”, isn’t really like a dust-particle or a grain of sand or salt. It also has wave-like properties. In an atom, electrons in some sense spread out around the atom, like a sound wave spreads out on a drum. In that limited sense, within an atom they are roughly as big as the atom itself.
But this size is contextual, and not intrinsic to the electron — in fact, I’ll call this the electron’s “contextual size”. Change the context — take the electron out of its atom and put it inside a tiny metal box, for instance — and the degree to which the electron spreads out (in this limited sense) may grow or shrink. In contrast, a proton has an intrinsic size, about 100,000 times smaller than an atom; you cannot, in any sense, make the proton smaller than its intrinsic size without breaking the proton apart. In short, the contextual size cannot be smaller than the intrinsic size. By shrinking the contextual size of electrons as much as we can, mainly through scattering high-energy electrons off of other particles, we’ve looked for signs of an intrinsic size. So far, none have ever been found.
So we may say, more accurately, that experiments show that the intrinsic size of an electron is smaller than about a millionth of a millionth of a millionth of a meter. How far the electron spreads out, wave-like, depends on context.
You may or may not want to know about this one! It will bend your brain (at least, it bends mine!)
Among the strange features of our quantum mechanical world is the very weird fact (initially discovered in the 1920s by Goudsmit and Uhlenbeck, who were trying to make sense of data from measurements of electrons inside atoms) that elementary particles can spin — even if they have no size! This is impossible to visualize; or at least, I certainly can’t do it. Let us say this as a practical matter: electrons and many other particles of nature act as though they are little rotating tops, in that if they are absorbed by another object, they will cause that object to rotate a little. [Imagine dropping a spinning chunk of soft clay onto a table that can rotate; the chunk will stick to the table, and the table will then rotate a little bit, proving the chunk was rotating to start with.]
And even more weirdly, each type of particle always spins at exactly the same rate! We say electrons have spin 1/2; this is the smallest non-zero rate of spin that a particle can have. We also know other types of elementary particles with spin 1/2, 1 and (we think) 0, and non-elementary particles with spin 0, 1/2, 1, 3/2, 2, 5/2, and on up to very large values.
A rotating ball of electric charge would behave like a magnet, and you might guess that because electrons have electric charge and spin, they behave like tiny magnets. You’d be right! The fact that electrons act like little magnets helps confirm that they really do spin. Ordinary daily-life magnets, made from (say) iron, get their magnetism from their electrons; lots and lots of electrons, with their spinning all nicely aligned, can make a big magnet out of lots and lots of tiny ones!
Are You Sure Electrons Really Exist?!
Isn’t it time in this article to show a picture of an electron?
Unlike molecules and atoms, which are large enough that we can make pictures of them using special microscopes, there is no way to make any image of an electron. It’s simply too small and too elusive. We can make images of tracks of electrons as they pass through matter, as in the figure at right (which actually shows an anti-electron [``positron''], but an electron would look almost the same), but we don’t image the electrons directly.
But our confidence that electrons exist is very high, and our knowledge of their properties is very precise. Where does that confidence come from?
This is an important issue, because one of the most common questions that particle physicists get asked is whether we really know all these particles exist, or whether we’re just fooling ourselves (and/or everyone else!) and wasting lots of money on stuff that is just hot air coming out of our heads (or worse.)
Well, yes, we know what we’re doing. And we have for over a hundred years. Part of our confidence comes from pictures such as shown in the photograph at right. But there are many other sources of confidence, which I’ll write about in a later article.
[How we know electrons exist: Coming Soon]
[How we know electrons are indistinguishable and all have the same properties: Coming Soon]