The 1928 relativistic wave equation whose negative-energy solutions demanded a mirror particle (1928)
Dirac's equation; he shared the 1933 Nobel Prize with Schrödinger
Science · Physics
Antimatter
predicted 1928; positron found 1932
Physics — particle physics
Matter made of antiparticles: counterparts with the same mass but opposite electric charge and other quantum numbers. Paul Dirac's 1928 relativistic equation for the electron had negative-energy solutions he eventually interpreted as a positively charged twin, and Carl Anderson found exactly that in cosmic-ray tracks in a cloud chamber in 1932, naming it the positron. When a particle meets its counterpart both are annihilated into energy, a process used every day in PET scanning. Why the universe is made almost entirely of one kind and not the other — the baryon asymmetry — remains one of physics' great unsolved problems.
How to spot them
- ▸Predicted by a 1928 equation with awkward negative-energy solutions
- ▸First seen in 1932 as an upward-curving cosmic-ray track in a cloud chamber
- ▸Meets its ordinary counterpart and both vanish into gamma rays
- ▸The P in PET scanning; and the great unexplained imbalance in the universe
Key facts
The 1932 cloud-chamber track that curved the wrong way, and its discoverer's coinage (1932)
Carl Anderson's positron; Nobel Prize 1936
Made at the Bevatron in Berkeley in 1955, earning a Nobel Prize four years later (1955)
The antiproton, found by Emilio Segrè and Owen Chamberlain
Nine atoms of the simplest kind were made at CERN in 1995 and lasted 40 billionths of a second (1995)
Antihydrogen, produced by the PS210 experiment
CERN's ALPHA experiment confirmed in 2023 that it falls downwards (2023)
Settling a long-standing question about how it responds to gravity
The scan in which a fluorine-18 tracer emits particles that annihilate into back-to-back gamma rays
Positron emission tomography (PET)
The 1964 discovery of CP violation, a clue to why the universe is lopsided (1964)
Cronin and Fitch, working on neutral kaons; Nobel Prize 1980
Key ideas
- Every particle has a counterpart with identical mass and spin but opposite charge and other additive quantum numbers; some neutral particles, such as the photon, are their own counterpart.
- Annihilation converts the entire rest mass of both particles into energy, following E = mc² exactly, which makes it the most energy-dense reaction known.
Quick-fire facts
- •Paul Dirac predicted it from his 1928 equation, publishing the hole interpretation in 1931; he shared the 1933 Nobel Prize with Erwin Schrödinger
- •Carl Anderson discovered the positron in cosmic rays in 1932 and named it; he shared the 1936 Nobel Prize with Victor Hess
- •The antiproton was created at Berkeley's Bevatron in 1955 by Emilio Segrè and Owen Chamberlain, who won the 1959 Nobel Prize; the antineutron followed in 1956
- •The first antihydrogen atoms — nine of them — were made at CERN in 1995; ALPHA first trapped them in 2010
- •In 2023 CERN's ALPHA-g experiment showed antihydrogen falls down, as general relativity requires
- •Electron–positron annihilation produces two 511 keV gamma rays travelling in opposite directions, the signal a PET scanner detects
- •Naturally occurring sources include potassium-40 in bananas and the human body, and cosmic rays
- •CERN has only ever produced a few nanograms in total; making a gram would take longer than the age of the universe at current rates
- •The excess of matter over its counterpart, the baryon asymmetry, requires the Sakharov conditions, including CP violation — discovered by Cronin and Fitch in 1964
- •A common misconception is that it has negative mass or antigravity; its mass is positive and it falls like anything else
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Sources
- CERN — Antimatter
- The Nobel Foundation — The Nobel Prize in Physics 1936 Carl Anderson, for the discovery of the positron
- The Nobel Foundation — The Nobel Prize in Physics 1959 Segrè and Chamberlain, for the discovery of the antiproton