- Person
- Steven Weinberg
- Lived
- 1933–2021
- Field
- Theoretical physics
- From
- American
The story
Physics works at many scales at once. Chemists can predict reactions without knowing how quarks behave inside protons, and engineers can design bridges without quantum mechanics. Yet for much of the twentieth century, physicists building quantum field theories hoped that each successful theory would be a final, complete description of nature, valid at every energy. Steven Weinberg offered a more practical and powerful perspective.
In 1979, while a professor at Harvard, he set out what he called a folk theorem. It states that if you write down the most general theory consistent with the symmetries of a system at low energies, and respect basic principles such as unitarity and cluster decomposition, you obtain a well-behaved effective field theory. With it he pioneered the modern view that all quantum field theories should be understood as effective field theories.
An effective theory describes nature accurately within a particular range of energies or distances, while ignoring the details of what happens at much smaller scales. The classic example is Enrico Fermi's theory of beta decay, which treated the weak interaction as a point-like contact between particles and worked remarkably well. It was later understood to be the low-energy approximation of the electroweak theory that Weinberg himself had helped build. This way of thinking explains why physics works at many different scales, and how precise predictions are possible even when the deepest laws remain unknown.
The approach opened the way to an effective theory of quantum gravity, to low-energy descriptions of the strong force such as chiral perturbation theory, to heavy quark effective theory and to many other advances, and it remains a topic of considerable interest in current research. Effective field theories are now used in particle and nuclear physics, condensed matter physics, hydrodynamics and even the calculation of gravitational waves from pairs of compact objects spiralling together. General relativity itself is expected to be the low-energy effective theory of a deeper theory of quantum gravity.
Physics Today counted effective field theory among Weinberg's most important contributions, one of many groundbreaking results that do not carry his name.
Why it matters
Effective field theory is now a standard tool in particle, nuclear and condensed matter physics, used to make precise predictions about complex systems.
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 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 helped predict the axionIn 1978 Steven Weinberg, and separately Frank Wilczek, realised that a proposed symmetry implied a new particle, the axion, now a leading candidate for the universe's dark matter.
- 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 Wikipedia
- Steven Weinberg (1933–2021) Physics Today
- Effective field theory Wikipedia
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