Positron
Mirror Spark
Matter Seen in a Mirror
Every particle has a twin. An exact mirror image with opposite charge. For the electron, this twin is the positron. Same mass. Same spin. But where the electron carries negative charge, the positron carries positive. It is the electron’s antimatter counterpart, the very first antiparticle ever discovered. In 1932, the physicist Carl Anderson spotted a strange track in a cloud chamber. It curved the wrong way. It looked like an electron moving backward through a magnetic field.
Knowing it was not moving backward took one piece of hardware. Anderson had run a lead plate across the middle of his chamber. Any particle crossing it loses energy, and a slower particle curls tighter in a magnetic field, so the half of the track with the smaller radius is always the half that came second. That fixed the direction of travel. With the direction fixed, the way the track curved could only mean one thing: positive charge, and a mass far too small for a proton. It was a brand new particle, and Paul Dirac’s equation had implied its existence four years earlier. Anderson shared the Nobel Prize in 1936.
Born From Light
A photon has no mass. It is a lump of ripple in the electromagnetic field, and it carries energy and momentum. But when a sufficiently energetic photon passes close to an atomic nucleus, something extraordinary can happen. The photon vanishes. In its place, two particles with mass appear: an electron and a positron, matter and antimatter in exactly equal amounts. The energy the photon carried is all still there, now held in the two particles’ masses and their motion.
This is pair production, perhaps the most vivid demonstration of E=mc² in all of physics. The photon needs at least 1.022 MeV of energy to create a pair, exactly twice the rest mass energy of an electron. A nearby nucleus absorbs a tiny bit of recoil momentum, which is what makes the process possible at all: in empty space, energy and momentum cannot both balance, so a photon on its own can never do this. The newborn electron and positron spiral away from each other in opposite directions, curving under any nearby magnetic field.
Total Annihilation
What happens when a positron meets an electron? Both particles vanish entirely. In their place, most often, two gamma-ray photons fly off in opposite directions. The energy is kept: each photon carries 511 thousand electron-volts, the energy that one of the two particles held as its mass, give or take a little from the pair’s own motion. All of the pair’s energy leaves as light. Nothing is left behind.
This is not like a chemical explosion, where atoms rearrange and give up less than a billionth of the energy their mass holds. Here all of it goes. If you could annihilate one gram of matter with one gram of antimatter, the energy released would equal roughly 43 kilotons of TNT, nearly three times the Hiroshima bomb. For an electron and a positron the process is clean: no radioactive waste, no leftover particles, just light carrying away all the energy. Protons and antiprotons are messier, annihilating into showers of short-lived particles, and some of their energy escapes with neutrinos.
Practical Application
PET Scanner
Antimatter is not just a curiosity. We use it in hospitals every day. Positron Emission Tomography uses radioactive tracers that emit positrons inside the body. Each positron travels a millimeter or two, meets an electron and annihilates, producing two gamma rays shooting off in exactly opposite directions. Because their energy is fixed at 511 thousand electron-volts, the detectors can be tuned to that one number and ignore everything else the body is doing.
A ring of detectors surrounding the patient catches both gamma rays simultaneously. By drawing a line between the two detection points, a computer pinpoints exactly where the annihilation happened. Millions of these events build a detailed 3D map of metabolic activity inside the body. Cancer cells consume more sugar than normal cells. The tracer concentrates in tumors, making them glow bright on the scan. Antimatter annihilation happening inside your body, saving lives.
The First Twin, and the Question It Left
The positron was the first evidence that every particle has a mirror twin. Its discovery confirmed Dirac’s equation and opened the door to antimatter physics. But the deeper question it raises remains unanswered: the Big Bang should have produced equal amounts of matter and antimatter, and they should have annihilated completely. Something gave matter a tiny edge – a process known as baryogenesis – leaving a slight excess of matter that survived the initial annihilation to form everything we see today. Understanding why you exist and antimatter does not is one of the most fundamental open problems in physics. Every positron annihilation in a PET scanner is a small echo of that cosmic imbalance.
