- Person
- Steven Weinberg
- Lived
- 1933–2021
- Field
- Theoretical physics
- From
- American
The story
The strong nuclear force holds the nuclei of atoms together. In principle, its theory, quantum chromodynamics, allows it to treat matter and its mirror-image antimatter differently, a property physicists call CP violation. If that happened to any significant degree, the neutron would have a measurable electric dipole moment. Experiments find none, which means the relevant parameter in the theory must be extraordinarily close to zero, with no obvious reason why. This puzzle is known as the strong CP problem.
In 1977 Roberto Peccei and Helen Quinn, then working at Stanford, proposed an elegant solution: a new symmetry that would naturally drive the troublesome parameter to zero. Before publishing, they went to talk to Steven Weinberg, who was visiting Stanford. Quinn later recalled that a short impromptu lecture from him on how to think about CP violation showed them where their first ideas had gone wrong and what they needed to solve.
Peccei and Quinn published their theory without noticing one important consequence. Steven Weinberg did. In 1978 he saw that the new symmetry, for any model, predicted a light new particle. Frank Wilczek noticed the same thing independently, and each published a brief paper on it. Wilczek gave the particle its name, the axion. Quinn later recalled in Physics Today that before publishing, Weinberg called her to ask whether she was aware of this property of the theory, and that she was sure he would have offered to share the paper had she said yes.
The first axion models were soon ruled out, but physicists realised that much lighter, more weakly interacting invisible axions would also solve the problem. Such axions, if they exist in the right mass range, could make up the cold dark matter that accounts for most of the matter in the universe.
Axions have never yet been detected, but physicists around the world are searching for them, including with the Axion Dark Matter Experiment at the University of Washington and with helioscopes that look for axions produced by the Sun. Finding one would solve two problems at once: the puzzle about the strong force and the mystery of dark matter.
Why it matters
The axion has become one of the most important targets in the search for dark matter, which makes up most of the matter in the universe.
About Steven Weinberg
Physicist whose three-page 1967 paper unified electromagnetism and the weak nuclear force, a pillar of the Standard Model, and whose The First Three Minutes brought the Big Bang to millions.
More about Steven Weinberg
- Steven Weinberg changed how physicists view their theoriesIn 1979 Steven Weinberg's folk theorem launched the modern view that quantum field theories are effective theories, valid at particular energy scales, an idea now used across physics.
- Steven Weinberg unified two forces of nature in 3 pagesIn 1967 Steven Weinberg's three-page paper A Model of Leptons showed that electromagnetism and the weak nuclear force are two faces of one force, now among the most cited papers in physics.
- Steven Weinberg's theory predicted new particle processesSteven Weinberg's electroweak theory predicted a new kind of weak interaction, the neutral current, which was discovered in 1973, and the Higgs boson, found in 2012.
- Steven Weinberg explained the Big Bang to millionsSteven Weinberg's 1977 book The First Three Minutes explained to a wide public how physics can reconstruct the earliest moments of the universe, winning a science writing award.
- Steven Weinberg worked to curb nuclear dangersSteven Weinberg advised the US Arms Control and Disarmament Agency, wrote Glory and Terror on the nuclear danger, and served on the Bulletin of the Atomic Scientists' Board of Sponsors from 2009.
Sources
- Steven Weinberg (1933–2021) Physics Today
- Steven Weinberg Wikipedia
- Axion Wikipedia
Another act of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.