Electron
Restless Spark
Behind Almost Everything
The electron is the lightest particle that carries an electric charge, and nearly everything you see and touch comes down to it. Every atom holds its electrons in a cloud around its nucleus: a wave held in place, not a small ball circling on a path. Chemistry is those electrons shared and traded between atoms. In every metal wire, it is electrons that carry the current. Nearly all the light around you comes from electrons too: jostled in the hot gas of the Sun’s surface, dropping to a lower energy in the crystal of an LED, or falling to a lower pattern in an atom, as in a neon sign. And a fridge magnet is the spins of some of its electrons lined up.
For all that, an electron weighs very little: about one part in 1,836 of a proton. All the electrons in your body come to about three hundredths of a percent of your weight; the rest is in the nuclei of your atoms. In the Standard Model, the theory of the known particles, its mass comes from a weak grip on the Higgs field. That grip has been measured for heavier particles, but the electron’s is too weak for any collider yet to see. Small as its mass is, together with the strength of the electric pull it sets the size of every atom. Were electrons heavier, their clouds would be drawn in tighter and every atom would be smaller.
Its charge is the proton’s, with the opposite sign: tests on gases of neutral molecules find the two equal to within about one part in a billion trillion. And every electron in universe is exactly like every other, with the same mass, the same charge and the same amount of spin. Each is a ripple in one field that fills all of space, so there is nothing that could tell one from another.
No Size at All
How big is an electron? As far as anyone can tell, it has no size at all. Smashed head on into positrons, its antimatter twins, or into protons in the most powerful colliders built for them, electrons bounce off just as points would, as far as the measurements reach: down to distances a thousand times smaller than a proton, 10⁻¹⁸ meters, and below. Shot at in the same way, a proton shows a size and parts inside it. An electron shows neither.
Yet its electric field reaches out every way without end, falling to a quarter each time the distance doubles. That field is how an electron pulls on protons and pushes other electrons away.
Spin, and a Tiny Magnet
Every electron also carries a fixed amount of turning, the kind a spinning wheel has, called its spin. Nothing can speed it up or slow it down. It is not a ball spinning on its axis: the electron shows no size, and even a ball as wide as a proton would need its surface to race round hundreds of times faster than light to carry that much turning. The turning is real all the same, and it makes every electron a tiny magnet.
Ask a spin which way it points along a line, and it gives one of only two answers: up or down, never anything between. Fire silver atoms, whose magnet is almost all one electron’s spin, between the poles of a magnet whose field is much stronger near one pole than the other. Tossed out of an oven pointing every which way, they should smear into a line. Instead they land in two separate spots.
For its turning, the electron is twice as strong a magnet as a spinning ball would be with its charge spread like its mass. It is about a tenth of a percent stronger still, from the electron acting on itself through its own field. One electron held for months in a trap has had its magnet measured to 0.13 parts in a trillion, as a multiple of the strength its charge and mass alone would give. The theory of light and matter predicts the same value to about one part in a trillion, one of the closest checks any theory has passed. The prediction needs one more measured number, the strength of the electric force. Its two best measurements, made with different atoms, disagree with each other, and until they are settled the test can go no closer.
The spin and the magnet lie along one line through the electron, and that line lets us ask a sharper question: is the electron round?
Rounder Than Anything Measured
Even a point can be lopsided. Its electric field could be a hair stronger at one end of the line of its spin than at the other, as if its charge sat a little off center. The known particles give the electron such a tilt too, but far too small to measure. Many theories that add new, heavier particles predict a much larger one: the fields of those particles, trembling in the space around every electron, would tilt it a little to one side. A tilt would also mean that nature does not run quite the same backward in time, one of the things needed to explain why universe holds more matter than antimatter.
The most sensitive test held charged molecules in a trap and lined them up with a rotating electric field. Next to a molecule’s heavy nucleus, an electron feels an electric field of tens of billions of volts per centimeter, far beyond anything a laboratory can apply directly. Its spin turns round an applied magnetic field, and the molecule’s field can be set to point along the magnetic field or against it. A lopsided electron would turn a little faster one way than the other. The physicists watched the spins turn for up to three seconds at a time, and saw no difference. If the electron’s charge sits off its center at all, it is by less than 4 × 10⁻³² meters. That rules out many of the proposed particles, even some too heavy for any collider to make.
It Never Wears Out
An electron left alone does not decay. It is the lightest particle with a charge, so there is nothing lighter it could turn into that would carry its charge away, and electric charge is never made or destroyed. Nature does make two heavier copies of the electron, with the same charge and spin: the muon, 207 times heavier, and the tau, 3,477 times. On average they last only two millionths of a second and three tenths of a trillionth. A muon ends as an electron and two neutrinos. A tau is heavy enough to make other particles too, and most often ends as a neutrino and one or more pions, short-lived particles made of quarks.
The best test of the electron’s own life watched the quiet middle of a tank of liquid deep under a mountain, 75 tonnes holding some 2.5 × 10³¹ electrons, for more than a year. The liquid flashes all the time, from traces of radioactive atoms in it and from neutrinos from the Sun. An electron decaying into a neutrino and a particle of light would add flashes of one fixed energy, half the energy locked in its mass, and no such extra flashes appeared. By that route, an electron lasts on average at least 6.6 × 10²⁸ years, some five billion billion times the age of universe. Decay into neutrinos alone would carry the electron’s energy away unseen, since neutrinos almost always pass straight through. It would leave an atom with a gap in one of its inner clouds, and as the gap is filled the atom gives off X-rays and electrons of set energies. Searches for those put that route beyond 10²⁷ years.
Electrons are still made and unmade every second, but only together with a charge that balances theirs. An electron and a positron are born together from enough energy and vanish together into light, and a neutron that turns into a proton, alone or inside a nucleus, throws out a new electron, and an antineutrino with it.
A Wave, Caught Whole
Like every particle, an electron is a ripple in a field, so it is not a tiny ball that sometimes acts like a wave. It always travels as a wave, and it is always caught whole, in one spot. Send a beam of electrons through a thin film of tiny crystals and they land in sharp rings, the pattern only waves make: the waves scattered by the rows of atoms add up in some directions and cancel in others. Yet each electron lands in one place. Sent one at a time, each makes a single dot, at random, and only the dots of thousands draw the rings. The wave sets where an electron may be found: the bigger the wave at a place, the likelier it turns up there. It is not a wave of stuff, as a water wave is; what spreads is the chance. Caught, the electron shows no size at all.
The faster an electron moves, the shorter its wave. One moving at a hundredth of the speed of light has a wavelength of about a quarter of a nanometer, the size of an atom. In an electron microscope the waves are hundreds of thousands of times shorter than light’s, short enough to show single atoms.
Two to a Place
Electrons are all alike and carry half a unit of spin, and for such particles that has a strict consequence: no two can be in the same state. Two can share one pattern in an atom only if their spins point opposite ways. So an atom’s electrons cannot all crowd into the lowest pattern. They fill the patterns from the bottom up, two to each, and stack outward in layers. That stacking gives the periodic table its rows. And when two atoms are pressed together, their electrons cannot crowd into each other’s filled patterns: they are forced into costlier ones, and that cost is what stops your hand on a table.
Electrons on the Move
In a copper wire, about one electron from each atom is not tied to any atom. These free electrons move through the metal at more than a thousand kilometers a second, as many one way as any other. That speed does not come from warmth. A piece of copper holds so many free electrons that, since no two can share a state, most of them are pushed up into fast ones, and they would move as fast near absolute zero. A battery adds only a slow drift on top: in a phone’s charging cable, about a millimeter every few seconds.
Yet a lamp lights the moment the switch closes, because the push travels along the wire at a large share of the speed of light, and the energy itself runs to the lamp through the space beside the wires.
From a wall socket the push swings back and forth fifty or sixty times a second, and the electrons only rock in place: in the cord of a kettle by a few thousandths of a millimeter, and in the cord of an LED lamp far less. The sea of electrons as a whole goes nowhere. Only the push runs along the wires, and the energy it brings flows through the space beside them.
