Electron

• 1. Introduction

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  The electron carries a negative charge of exactly one elementary charge, and it is one of the building blocks of all ordinary matter in the universe, alongside up and down quarks. It is also extraordinarily light. Its mass is roughly 1,836 times smaller than that of a proton. This tiny particle hides inside every atom, where it occupies a kind of standing wave around a positively charged nucleus. How did anyone come to know it exists, when it can never be seen directly? How does something so faint account for electricity, magnetism, chemistry, and the heat that flows through metal? And how did a particle no one could touch end up at the center of welding torches, microscopes that resolve single atoms, and theories about the birth of the cosmos? The answers begin with rubbed amber and end inside black holes.

• 2. From Amber To A Name

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  The ancient Greeks noticed that amber, when rubbed with fur, would pull small objects toward it. Along with lightning, this counts as one of humanity's earliest recorded brushes with electricity. The Greek word for amber was ἤλεκτρον, ēlektron, and through the Latin ēlectrum it gave us both electric and electricity.

  In his 1600 treatise De Magnete, the English scientist William Gilbert coined the Neo-Latin term electrica for substances that behaved like amber, attracting small objects after being rubbed. More than a century of puzzling followed about what this attraction actually was.

  In the early 1700s, the French chemist Charles François du Fay found that a charged gold leaf repelled by glass rubbed with silk was instead attracted by amber rubbed with wool. He concluded that electricity came in two fluids, a vitreous fluid and a resinous fluid, which could neutralize one another. Ebenezer Kinnersley later reached the same conclusion independently.

  Benjamin Franklin proposed a single fluid instead, present in excess or deficit, and gave us the words positive and negative. He guessed the charge carrier was positive, but he did not correctly identify which case was a surplus and which was a shortfall.

  George Johnstone Stoney, the Irish physicist, studied electrolysis in 1874 and argued for a single definite quantity of electricity, the charge of a monovalent ion. He estimated this elementary charge using Faraday's laws of electrolysis. Stoney first coined the term electrolion in 1881, then switched to electron, writing in 1894 of his wish to suggest the name electron for this fundamental unit. The suffix that later named the proton and neutron traces back to this very word.

• 3. Rays In The Dark

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  In 1859, the German physicist Julius Plücker watched radiation from a cathode make a glass tube glow with phosphorescent light, and found a magnetic field could move where that glow appeared. A decade later his student Johann Wilhelm Hittorf placed a solid object between the cathode and the glow and saw it cast a shadow, suggesting straight rays traveled from the cathode.

  Eugen Goldstein showed in 1876 that these rays left the cathode at a right angle to its surface, and he named them cathode rays. During the 1870s the English chemist and physicist Sir William Crookes built the first cathode-ray tube with a high vacuum inside. In 1874 he showed the rays could spin a small paddle wheel, proving they carried momentum, and a magnetic field bent them as if they were negatively charged.

  Arthur Schuster, the German-born British physicist, set metal plates parallel to the rays and applied a voltage. The rays bent toward the positive plate, and in 1890 he estimated their charge-to-mass ratio. His figure came out more than a thousand times larger than expected, so few believed him. The trouble was the assumption that the carriers were heavy hydrogen or nitrogen atoms. Schuster's estimates would later prove largely correct.

  In 1897 the British physicist J. J. Thomson, working with John S. Townsend and H. A. Wilson, showed cathode rays were genuine particles rather than waves or atoms. By 1899 he found their charge-to-mass ratio did not depend on the cathode material. He measured the corpuscles to be roughly 1,400 times less massive than hydrogen, the lightest ion then known. That same year Emil Wiechert and Walter Kaufmann also calculated the ratio, but stopped short of declaring a new particle.

• 4. Weighing The Invisible

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  The American physicists Robert Millikan and Harvey Fletcher pinned down the electron's charge in their oil-drop experiment of 1909, publishing the results in 1911. They used an electric field to hold a charged droplet of oil against gravity, measuring charges from as few as 1 to 150 ions with an error margin under 0.3 percent.

  Oil drops earned their place by accident of chemistry. Earlier attempts by Thomson's team had used clouds of charged water droplets from electrolysis, and in 1911 Abram Ioffe got the same result with charged metal microparticles, publishing in 1913. Oil simply evaporated more slowly than water, staying stable long enough for precise work.

  Charles Wilson, around the turn of the century, noticed that a fast-moving charged particle could trigger condensation of supersaturated water vapor along its path. In 1911 he turned this into the cloud chamber, which let him photograph the tracks of charged particles such as fast electrons. For the first time, the trail of a single particle could be captured on film.

  Beta rays helped seal the case. Henri Becquerel, studying naturally fluorescing minerals in 1896, found they emitted radiation with no external energy source. Ernest Rutherford, the New Zealand physicist, named the emitted particles alpha and beta by how well they penetrated matter. In 1900 Becquerel showed radium's beta rays could be bent by an electric field, with the same mass-to-charge ratio as cathode rays.

• 5. Shells, Spin, And Exclusion

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  By 1914, work by Ernest Rutherford, Henry Moseley, James Franck and Gustav Hertz had established the atom as a dense nucleus of positive charge ringed by lighter electrons. In 1913 the Danish physicist Niels Bohr proposed that electrons sat in quantized energy states, set by the angular momentum of their orbits, and could jump between them by emitting or absorbing photons. This explained the spectral lines of hydrogen, though it failed for more complex atoms.

  Gilbert Newton Lewis explained the chemical bond in 1916, proposing that a covalent bond is held by a pair of electrons shared between two atoms. Walter Heitler and Fritz London gave the full quantum-mechanical account of electron pairing in 1927. In 1919 the American chemist Irving Langmuir arranged electrons in concentric spherical shells of equal thickness, divided into cells each holding a pair, and used this to explain the repeating chemistry of the periodic table.

  Wolfgang Pauli, the Austrian physicist, saw in 1924 that the atom's shell structure followed from four parameters defining each quantum state, provided no state held more than one electron. This became the Pauli exclusion principle. The fourth parameter, with its two possible values, was explained by the Dutch physicists Samuel Goudsmit and George Uhlenbeck. In 1925 they proposed that the electron carries an intrinsic angular momentum, called spin, likened to the Earth turning on its axis as it circles the Sun. Spin accounted for the fine-structure splitting of spectral lines that had baffled observers.

• 6. Waves And The Dirac Sea

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  In his 1924 dissertation, the French physicist Louis de Broglie proposed that all matter could be described as a wave, much like light. Under the right conditions, electrons would show the properties of either particles or waves. In 1927 George Paget Thomson and Alexander Reid produced interference when a beam of electrons passed through thin foils, and Clinton Davisson and Lester Germer did the same by reflecting electrons off a nickel crystal. Reid, who ran the first experiments as Thomson's graduate student, died soon after in a motorcycle accident and is rarely mentioned.

  Erwin Schrödinger answered de Broglie's idea with a wave equation for electrons under the pull of the nucleus. In 1926 his equation described how electron waves propagate, yielding not a fixed location but the probability of finding an electron near a position. This was the second formulation of quantum mechanics, after Heisenberg's of 1925, and it reproduced the hydrogen energy states Bohr had derived in 1913.

  Paul Dirac built on Pauli's work in 1928 to produce a model of the electron consistent with relativity. To fix problems in his equation, in 1930 he pictured the vacuum as an infinite sea of negative-energy particles, the Dirac sea, which led him to predict the positron. Carl Anderson found this antimatter twin in 1932, suggesting that standard electrons be called negatrons.

  Willis Lamb, with graduate student Robert Retherford, found in 1947 that two hydrogen states which should share the same energy were shifted apart, the Lamb shift. Around the same time Polykarp Kusch and Henry M. Foley measured the electron's magnetic moment as slightly larger than Dirac predicted, the anomalous magnetic moment. Both puzzles were resolved by quantum electrodynamics, built in the late 1940s by Sin-Itiro Tomonaga, Julian Schwinger and Richard Feynman.

• 7. A Point With No Size

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  Electron-electron scattering shows no deviation from Coulomb's law, which means that experimentally the electron is structureless and point-like. Observation of a single electron in a Penning trap places an upper limit on its radius. Theoretical ideas of its size stay ambiguous, since the Dirac equation treats it as a point charge while the equivalent Newton-Wigner form does not.

  A spinning electron would break the speed limit. A mechanically rotating electron with the classical electron radius and the observed gyromagnetic ratio would have a surface speed exceeding the speed of light. The classical electron radius of 2.8179, larger than the radius of the proton, serves as a physical constant but is not a measure of the particle's structure.

  Within the Standard Model the electron is considered stable, because it is the least massive particle carrying charge, so its decay would violate charge conservation. Many experiments have hunted for that decay anyway. The experimental lower bound on the electron's mean lifetime is 6.6 years, at a 90 percent confidence level.

  Strange things happen in certain materials. In condensed matter physics, spin-charge separation can split an electron into three independent quasiparticles. The spinon carries the spin, the orbiton carries the orbital degree of freedom, and the holon, or chargon, carries the charge. The electron can always be treated as a bound state of all three.

• 8. From The Big Bang To The Welding Torch

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  For the first millisecond of the Big Bang, temperatures topped 10 billion kelvins and photons carried mean energies above a million electronvolts, energetic enough to form pairs of electrons and positrons. After 15 seconds the universe cooled below that threshold. Most electrons and positrons annihilated, but for reasons still uncertain about one electron survived for every billion pairs, a leftover that matched the excess of protons over antiprotons.

  Stars kept the reckoning going. Stellar nucleosynthesis produces positrons that annihilate with electrons, steadily reducing electron numbers while neutron numbers rise. Some isotopes then undergo negative beta decay, emitting an electron and an antineutrino, as when cobalt-60 decays to nickel-60. At a black hole's event horizon, a virtual electron-positron pair can be torn apart, one member radiating away as Hawking radiation while the hole loses mass-energy.

  On Earth the electron became a tool. Electron beams weld conductive materials in a vacuum without filler, and electron-beam lithography etches semiconductors down to about 10 nanometers. Linear accelerators aim electron beams at superficial tumors in radiation therapy, penetrating up to 5 centimeters for energies of 5 to 20 MeV, useful against skin lesions like basal-cell carcinomas.

  The electron also let us see the atom. Optical microscopes in blue light stop at a resolution near 200 nm, but the electron microscope is limited instead by the de Broglie wavelength, which falls to 0.0037 nm for electrons accelerated across a 100,000 volt potential. The Transmission Electron Aberration-Corrected Microscope reaches sub-0.05 nm resolution, fine enough to resolve individual atoms. The cathode-ray tube that once defined the particle now sits supplanted by the transistor, even as the electron still does all the work inside it.