Physics 1933

The Positive Electron

Carl D. Anderson

A single curved cosmic-ray track revealed antimatter: a particle as light as the electron, but positively charged.

Choose your version

In depth · the introduction

A single glowing trail, bent the wrong way, was the first sign that for every kind of matter there is a mirror-image anti-matter.

The big idea

In 1932 Carl Anderson was photographing cosmic rays — particles raining down from space — as they streaked through a cloud chamber, a tank of vapour where a charged particle leaves a thin trail of droplets. A magnet bent each trail into a curve, and which way it curved told him the particle's electric charge.

One trail bent the way a positive charge should — yet it was thin and gently curved like a lightweight electron, not heavy like the only positive particle then known, the proton. Anderson had found something new: a particle with the electron's tiny mass but the opposite, positive charge. He called it the positron. It was the first piece of antimatter ever seen.

How it came about

Four years earlier the theorist Paul Dirac had written an equation for the electron that, to everyone's puzzlement, also described a particle just like the electron but positively charged. Most physicists, Dirac included at first, suspected this was a mathematical artefact rather than a real object.

Anderson, a young experimenter in Robert Millikan's lab at Caltech, was not chasing Dirac's idea at all — he was simply studying cosmic rays. The trick that nailed the result was a sheet of lead laid across his chamber: a particle crossing it lost energy and curved more tightly afterwards, which revealed which way it was travelling. From that he could be sure the curve meant a positive charge on a light particle. Within months, Patrick Blackett and Giuseppe Occhialini in England confirmed the particle and tied it firmly to Dirac's prediction. Anderson received the Nobel Prize in 1936.

Why it mattered

The positron showed that antimatter is real, not a quirk of algebra — that nature comes in matter and anti-matter mirror pairs. It confirmed that Dirac's marriage of quantum theory and relativity was telling the truth about the world, and it opened the era of discovering particles by reading their tracks. It also posed a riddle we still cannot answer: if matter and antimatter are made in equal amounts, why is the universe almost all matter?

A way to picture it

Imagine rolling balls past a magnet that always nudges them sideways. A heavy ball barely swerves; a light ball curves sharply; and a ball with opposite charge curves the other way entirely. Anderson saw a trail that curved sharply (so, lightweight) but in the 'wrong' direction (so, opposite charge). A light ball curving the wrong way could not be any known particle — it had to be a new one, the electron's mirror twin.

Where it sits

Dirac's 1928 equation set the stage; Anderson's 1932 track is its confirmation — a theory-and-experiment pair, much like the prediction and proof that run through the rest of physics. From here the trail leads to the antiproton in 1955, to the realisation that antimatter and matter annihilate in a flash of light, and to today's experiments at CERN that bottle antihydrogen atoms to ask whether matter and its mirror really obey the same laws.

The original document

Original source text

C. D. Anderson · Physical Review 43 (1933): 491–494 · received February 28, 1933

Abstract

Out of a group of 1300 photographs of cosmic-ray tracks in a vertical Wilson chamber 15 tracks were of positive particles which could not have a mass as great as that of the proton. From an examination of the energy-loss and ionization produced it is concluded that the charge is less than twice, and is probably exactly equal to, that of the proton. If these particles carry unit positive charge the curvatures and ionizations produced require the mass to be less than twenty times the electron mass. These particles will be called positrons. Because they occur in groups associated with other tracks it is concluded that they must be secondary particles ejected from atomic nuclei.

The decisive photograph

The paper's evidence centres on a single track photographed on August 2, 1932. A particle crosses a 6 mm lead plate set across the chamber, which sits in a magnetic field of about 15,000 gauss. Above the plate the track is more sharply curved than below it: the particle had less energy there (about 23 MeV, against 63 MeV below), having lost energy passing through the lead. Because the curvature is tighter on the upper side, the particle must have been travelling upward — and the direction in which it bends, in a known field, then fixes the sign of its charge as positive.

Ruling out the proton

A positive particle of that curvature might have been a proton, but Anderson argues from the density of ionization along the track and from the range a proton of such momentum would have had that the mass is far too small. If the charge is one electronic unit, the measurements bound the mass below twenty times that of the electron — comparable to the electron itself.

These particles will be called positrons.

[ … ]

Interpretation

Anderson reports the positrons appearing in groups alongside other tracks and reads them as secondary particles ejected from nuclei. The full paper gives the curvature and ionization measurements, the reproductions of the cloud-chamber photographs, and the argument against the proton; it is four pages and is available in full at the source below. The deeper meaning — that the positron is the anti-electron predicted by Dirac, produced together with an electron when a photon converts near a nucleus — was made explicit shortly afterward by Blackett and Occhialini.

Norman Bridge Laboratory of Physics, California Institute of Technology · 1933