Cobra Effect · Particles
The mirror image of the electron
How an equation predicted antimatter, and a cloud chamber photograph found it.
7 cards, read aloud in 2:53, with a test and sources.
In 1928, Paul Dirac wrote a new equation for the electron.
Dirac was a young British physicist at Cambridge. His equation joined quantum theory with Einstein’s theory of relativity. It worked beautifully, but it also allowed strange solutions with negative energy. Nobody knew what they could mean.
Dirac first guessed that the strange solutions were protons.
He imagined empty space as a sea, completely filled with unseen electrons of negative energy. A hole in that sea would behave like a positively charged particle. But other physicists showed that such a hole would have the same mass as an electron. In 1931, Dirac predicted a new particle, an anti-electron, with the electron’s mass and the opposite charge.
Meanwhile in California, Carl Anderson was photographing cosmic rays.
Anderson worked at Caltech, with a cloud chamber inside a powerful magnet. Charged particles left trails of tiny droplets, and the magnet curved each trail. Positive and negative particles curved in opposite directions. Some trails curved like positive particles, yet looked as light as electrons.
But which way were those particles travelling?
An electron moving up curves the same way as a positive particle moving down. To tell them apart, Anderson fixed a lead plate across the middle of the chamber. A particle loses energy passing through lead, so its track curls more tightly afterwards.
In the summer of 1932, one photograph settled it.
A particle crossed the plate and curled more tightly on the far side, which showed its direction. It was positively charged, yet far too light to be a proton. Anderson published the result that September, and the name positron soon stuck.
The positron was the first antimatter ever found.
At Cambridge, Patrick Blackett and Giuseppe Occhialini soon confirmed it. When a particle meets its antiparticle, the two can vanish in a burst of energy. Dirac shared the Nobel Prize in Physics in 1933, and Anderson shared it in 1936. Today, hospital PET scanners use positrons to make images of the body.
So when an equation gives a strange answer, ask whether it describes something real.
Dirac’s mathematics pointed to a particle no one had seen. At first, even he misread what it meant. The strange part of a theory can turn out to be the discovery.
Sources
- The production and properties of positrons, Carl Anderson, Nobel lecture, 1936. Anderson’s own account, with the famous photograph.
- Theory of electrons and positrons, Paul Dirac, Nobel lecture, 1933. Dirac explains the idea of the positron.
- Carl Anderson discovers the positron, CERN. A short timeline entry on the discovery.
Nearby ideas
- The radiation that came from the sky. How balloon flights showed that radiation pours in from space.
- The particle smaller than an atom. How a bent beam in a glass tube revealed the first particle smaller than an atom.
- The particle with no charge. How recoiling atoms revealed a particle that leaves no track.
- The ghost particle that took decades to catch. How a desperate guess about missing energy led to catching a ghostly particle.
- The particle that revealed a hidden field. How a 1964 prediction about mass was confirmed at CERN in 2012.