Quantum electrodynamics (QED)

Quantum Electrodynamics (QED) is a quantum field theory that describes how electromagnetic interactions work at the quantum level. As a fundamental theory in physics, it specifically deals with the interactions between light (electromagnetic radiation) and matter.

  • QED is an integral part of the Standard Model of particle physics, a theoretical framework that encompasses the fundamental particles and their interactions. It constitutes the electromagnetic sector of the Standard Model, working alongside the weak nuclear force and the strong nuclear force.
  • As a quantum field theory, QED describes particle and field behaviour in terms of probabilities and wave functions by accounting for the principles of quantum mechanics that distinguish it from classical physics.
  • QED’s primary focus is on the electromagnetic force, encompassing interactions involving charged particles (such as electrons and protons) and photons (particles of light).
  • In QED, interactions between charged particles are described in terms of the exchange of virtual photons. These are not “real” photons in the sense that they cannot be observed directly but are mathematical constructs that represent the invisible intermediary particles in an interaction.
  • QED calculations frequently employ Feynman diagrams, graphical representations of particle interactions that offer a visual means to comprehend and compute the probabilities of likely scenarios.
  • In QED, electromagnetic fields are not considered fundamental entities. Instead, they emerge from the interactions of photons. It is photons, elementary particles of light, That serve as the carriers of the electromagnetic force and generate the electromagnetic fields.
  • QED explains a wide range of phenomena related to light, including:
    • Why light travels at a constant speed in a vacuum.
    • Why light is absorbed and emitted by matter (such as electrons) in discrete units or ‘quanta’ (photons).
    • Why light undergoes scattering when it interacts with matter.
    • How light causes interference and diffraction at the sub-atomic scale.
    • The photoelectric effect, which is the emission of electrons from a material’s surface when it is illuminated by light.
    • The Compton effect, which is the scattering of light by electrons.
  • Quantum electrodynamics represents a more complete and accurate theory of electromagnetism than had been developed previously. It was formulated in the 1940s by a team of physicists including Richard Feynman, Julian Schwinger, and Tomonaga Shin’ichiro, all of whom were awarded the Nobel Prize in Physics in 1965 for their contributions to this theory.
  • QED is based on the idea that when electric and magnetic fields are measured, the range of energy values they can hold corresponds directly with the energy of the photons they produce. Quantized fields refer to this range of specific and discrete values. This is in contrast to classical fields, which can theoretically take on any energy value.
  • To understand this, imagine a ruler marked in centimetres. The ruler meaures the field, and the centimetre marks represent the different values that the field can take. In classical physics, the fields can theoretically take any value. However, in quantum field theory, the values must align exactly with the marks because each one corresponds to a quantum of photon energy.
  • Taking this a step further, consider an electron moving around an atom at the moment it emits a photon of light. The electron can only exist at certain energy levels, and these correspond to the orbits the electron follows around the nucleus. The electron will be in one orbit or another when it emits the photon, and the energy level of the photon corresponds to the energy carried by the electron in that particular orbit. For example, if an electron transitions from the third orbit to the second orbit from the nucleus, it will emit a photon with an energy equal to the difference in energy carried by the electron in the third and second orbits. The resulting discrete range of energy levels provides the quantized energy values.