Quantum field theory marries the ideas of other quantum theories to depict all particles as “excitations” that arise in underlying fields. The British physicist Paul Dirac started the ball rolling in the late 1920s with his equation describing how relativistic electrons – and with it most other matter particles – behave.
Standard quantum theory as developed by the likes of Niels Bohr and Werner Heisenberg in the 1920s is fine for describing the workings of individual particles in isolation and at slow speeds. But to explain their interactions in the real world, you need something more.
In particular, you need to marry quantum theory with special relativity, Einstein’s theory of how space and time warp for things travelling at high speeds. Special relativity says mass and energy are interchangeable, as embodied by the equation E=mc2. Heisenberg’s quantum uncertainty principle, meanwhile, says particles can borrow energy from the vacuum for a certain amount of time.
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The Dirac equation had a sting in its tail: it predicted the existence of a particle identical to the electron in every way, apart from the opposite electric charge. The positron, the first antimatter particle, was duly discovered in cosmic rays a few years later. It was the first of a whole new menagerie of particles that theorists proposed as quantum field theories evolved – and that later popped up in reality.
Two quantum field theories lie at the heart of the standard model of particle physics. The product of many decades of theoretical work, meticulously confirmed by experiment, this model covers the workings of three of the four forces of nature through interactions of force-carrying boson particles with matter-making fermions.
Quantum electrodynamics (QED) is the unified “electroweak” theory of electromagnetism and the weak nuclear force, which governs nuclear processes such as radioactive beta decays that are crucial, for example, in how the sun burns its fuel.
Quantum chromodynamics (QCD), meanwhile, is the theory of the strong nuclear force. Transmitted by bosons called gluons, this strong, very short-range force binds quarks together to make particles such as protons and neutrons.
The crowning glory of the standard model came in 2012, with the discovery of the Higgs boson, predicted almost five decades earlier. Mass is the most solid property of matter, and the mass of a fundamental particle is determined by its degree of interaction with the Higgs boson. According to a theory first proposed in 1964, the molasses-like field associated with the Higgs provides a drag that varies according to particle type.
What we’re still lacking, however, is a quantum field theory of gravity. Alone of the four forces, gravity has no particles attached to it, and is instead explained by Einstein’s general theory of relativity as the warping of space-time – a very different kettle of fish.
