Quantum Field Theory

Quantum Field Theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics.

  • Quantum fields are thought to be the underlying reality of all particles and forces. Quantum field theory has developed the Standard Model to describe all the known fundamental particles and force carriers as fields.
  • Quantum field theory uses mathematical formulas to represent things that are often too small or transient to observe. The exact behaviour of forces, particles and waves can often only be inferred and so must be described in terms of a mathematical probability of different events and outcomes.
  • Quantum fields serve as the comprehensive framework that encompasses the entirety of matter-energy and can be thought of as a dynamic medium that permeates all of space. It is a unified theory of spacetime and matter in which particles are simply localized excitations of this medium. This means that when a particle appears, it is a localized disturbance in a quantum field. When a particle is destroyed, the disturbance in the quantum field disappears.
  • In quantum field theory, a quantum field is an entity whose fundamental element is a “quantum” of energy. A quantum of energy is the smallest possible unit-quantity that can be contained within a field. This is the smallest possible unit that can be used to describe the behaviour of the quantum field in question.
  • So, a quantum field can be thought of as a field that may be made up of a single quantum of vibrating energy-matter – a particle. However, a quantum field can include more than one unit. The number of particles in a quantum field is determined by the state of the field so can be between zero to an infinity of quanta.
  • A field containing a single particle and a field containing groups of particles are related in the sense that they are both manifestations of the same underlying quantum field. The difference is that the field containing a single quantum is a localized disturbance, while the field containing groups of particles is more spread out (diffuse).
  • The electromagnetic field (EM field) is an example of a quantum field. Disturbances in the EM field create photons. Photons are responsible for all forms of electromagnetic radiation from radio waves through visible light to gamma rays. It is also responsible for the force of electromagnetism, which is one of the four fundamental forces of nature.
Quantum fields & fundamental forces
  • There is a quantum field corresponding with each fundamental force:
    • The electromagnetic force is mediated by the exchange of photons between charged particles. The EM field is the underlying reality that allows this exchange to take place.
      • Using the term mediated in this context is a way of explaining how a force can act at a distance. It is a way of saying that the force is not transmitted directly from one object to another. Instead, it is transmitted through a medium and that medium is the electromagnetic field.
      • A simple example of this is when two charged particles interact with each other and exchange photons. The photons carry momentum and energy, which can cause the charged particles to accelerate or change direction.
  • The weak nuclear field is the field associated with the weak nuclear force, one of the four fundamental forces of nature. It is responsible for radioactive decay and other processes that involve the nucleus of an atom.
  • The strong nuclear field is the field associated with the strong nuclear force, one of the four fundamental forces of nature. It is responsible for binding protons and neutrons together in the nucleus of an atom.
  • The graviton field is the field that is theorized to be responsible for gravity, one of the four fundamental forces of nature. The graviton is a hypothetical particle that is thought to be the carrier of the gravitational force.
  • Meanwhile, The Higgs field is the field that is responsible for the Higgs boson, which is a fundamental particle discovered in 2012. The Higgs boson gives mass to other particles, such as electrons and quarks.
Clouds of gas and beams of light
  • A cloud of gas made up of countless vibrating subatomic particles, or a beam of light in which all units are free to interact are both quantum fields. The particles in a cloud of gas are held together by the electromagnetic force, while the photons in a beam of light are the force carriers of the electromagnetic force.
  • The matter and energy of a particle or group of particles are manifestations of the underlying quantum field. When a particle is created, it is a localized disturbance in the field. When a particle is destroyed, the disturbance in the field disappears.
  • Quantum fields are said to persist, even when particles or energy are absent. Classical physics fails to comprehensively explain this. In quantum mechanics (quantum physics), particles lack a definite location until observed. This doesn’t imply electrons can manifest anywhere at random. Instead, they are generated and annihilated in alignment with the principles of quantum mechanics.
Properties of quantum fields
  • Quantum fields have:
    • Continuous Nature: Quantum fields extend continuously throughout space and time, imbuing every point in the cosmos with their presence. This means that they are fields of energy that permeate all spacetime. Quantum fields can be described by mathematical statements used to describe the behaviour of the particles and forces within them. This is possible even though quantum fields do not contain simple objects with identifiable locations or masses.
    • Quantized Energy: Quantum fields exhibit quantization, meaning they can only assume specific and discrete values of energy. This property underpins the discrete nature of particles and their interactions within the quantum realm.
    • Dynamic Fluctuations: Quantum fields are inherently dynamic, subject to fluctuations and oscillations over time. These fluctuations give rise to the spontaneous creation and annihilation of particles, contributing to the intricate tapestry of matter and energy in the universe.
    • Interactions: Quantum fields possess the capacity to interact with one another, giving rise to the fundamental forces that govern the cosmos.These interactions are responsible for the forces that we experience in the world, such as gravity, electromagnetism, and the strong and weak nuclear forces.
  • An unresolved idea to grapple with is that quantum fields do not denote bounded spatial and temporal regions where forces act. In this sense, quantum fields are the fundamental entities that make up the universe and may play a fundamental role in the structure of space-time. Evidence is emerging that everything we see and experience is a manifestation of quantum fields. So, the energy and matter that make up the universe, the forces that govern its behaviour, and even the laws of physics themselves are all emanations of quantum fields.
  • In theoretical physics, Quantum Field Theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics.  QFT is used in particle physics to construct physical models of subatomic particles and in Condensed Matter Physics to construct models of quasiparticles.