Matter With the Charges Flipped
Every particle in ordinary matter has a partner with the same mass but the opposite electric charge. That partner is called an antiparticle. The partner of the electron carries a positive charge instead of a negative one, and goes by the name positron. The partner of the proton is the antiproton, and it carries a negative charge.
These partners can join up the same way normal particles do. A positron and an antiproton can bind together into an atom of antihydrogen. Scientists have also built anti-nuclei of helium, which are the most complicated ones made so far. In theory there could be an anti-version of every element on the periodic table.
What makes antimatter awkward to keep is what happens when the two sides meet. A particle and its partner destroy each other on contact, an event called annihilation, and the mass turns into a burst of energy. The energy that comes out is huge for the amount of stuff involved.
Predicted on Paper First
The word antimatter was first used in 1898 by Arthur Schuster, in two playful letters to the journal Nature. He imagined whole anti-solar systems, but his idea was speculation rather than serious physics.
The real story starts in 1928 with a paper by Paul Dirac. His version of the wave equation for electrons allowed for a positive twin of the electron, though Dirac at first missed what his own maths was telling him. He gave credit for the insight to J. Robert Oppenheimer, whose 1930 paper argued that this twin must have the same mass as the electron. Dirac then wrote about the anti-electron directly in 1931.
The prediction held up. Carl Anderson found the positron in 1932 and gave it its name. Then came a long wait. The antiproton was not reported until 1955, and the antineutron a year after that, both found at the Bevatron in Berkeley, California.
A Few Nanograms, at a Staggering Price
All the antimatter ever made by people adds up to a few nanograms. Nobody has assembled a visible lump of it, and the reason is cost and difficulty rather than physics.
The numbers are eye watering. In 1999 NASA put the price of antihydrogen at about 62.5 trillion dollars per gram. A 2006 estimate for positrons worked out at roughly 25 billion dollars per gram. CERN has said that producing the tiny amount used so far in collisions cost a few hundred million Swiss francs. At current rates, making a single gram of antihydrogen would take something like 100 billion years.
Storing it is its own puzzle, since it destroys any ordinary container it touches. Charged particles can be held in place with electric and magnetic fields in a device called a Penning trap. In 2010 a CERN team held 38 antihydrogen atoms for about a sixth of a second. By 2011 they were holding 309 atoms for roughly 1,000 seconds.
Already Working in Hospitals
Antimatter is not only a laboratory curiosity. Nature makes it constantly. Positrons come out of the decay of everyday radioactive isotopes such as potassium-40, and cosmic rays hitting the upper atmosphere produce small amounts that vanish almost at once. Research published in 2011 found positrons forming above thunderstorm clouds, and antiprotons have been detected in the Van Allen belts that circle the Earth.
The most familiar use is medical. A PET scan works by feeding the patient a tracer that emits positrons, then picking up the pair of gamma rays that come from each annihilation. Antiprotons have also shown promise in the lab for treating some cancers.
The bigger question is why there is so little of it. The visible universe looks to be made almost entirely of ordinary matter, and physicists still cannot fully explain how that imbalance came about.
