Force carrier

In particle physics, quantum field theories such as the Standard Model describe nature in terms of fields. Each field has a complementary description as the set of particles of a particular type. A force between two particles can be described either as the action of a force field generated by one particle on the other, or in terms of the exchange of virtual force carrier particles between them.

Contents

Particle and field viewpoints

The energy of a wave in a field (for example, electromagnetic waves in the electromagnetic field) is quantized, and the quantum excitations of the field can be interpreted as particles. The Standard Model contains the following particles, each of which is an excitation of a particular field:

In addition, composite particles such as mesons can be described as excitations of an effective field.

Gravity is not a part of the Standard Model, but it is thought that there may be particles called gravitons which are the excitations of gravitational waves. The status of this particle is still tentative, because the theory is incomplete and because the interactions of single gravitons may be too weak to be detected.[1]

Forces from the particle viewpoint

A Feynman diagram of scattering between two electrons by emission of a virtual photon.

When one particle scatters off of another, altering its trajectory, there are two ways to think about the process. In the field picture, we imagine that the field generated by one particle caused a force on the other. Alternatively, we can imagine one particle emitting a virtual particle which is absorbed by the other. The virtual particle transfers momentum from one particle to the other. This particle viewpoint is especially helpful when there are a large number of complicated quantum corrections to the calculation since these corrections can be visualized as Feynman diagrams containing additional virtual particles.

The description of forces in terms of virtual particles is limited by the applicability of the perturbation theory from which it is derived. In certain situations, such as low-energy QCD and the description of bound states, perturbation theory breaks down.

Examples

History

The concept of messenger particles dates back to the 18th century when the French physicist Charles Coulomb showed that the electrostatic force between electrically charged objects follows a law similar to Newton's Law of Gravitation. In time, this relationship became known as Coulomb's law. By 1862, Hermann von Helmholtz had described a ray of light as the "quickest of all the messengers". In 1905, Albert Einstein proposed the existence of a light-particle in answer to the question: "what are light quanta?"

In 1923, through the work of the physicist Arthur Holly Compton at the Washington University in St. Louis, the existence of Einstein's light-particle was made undeniably plain to physicists. Lastly, in 1926, one year before the theory of quantum mechanics was published, Gilbert N. Lewis introduced the term "photon", which soon became the name for Einstein’s light particle. From there, the concept of messenger particles developed further.

See also

References

  1. ^ Rothman, Tony; and Stephen Boughn (November 2006). "Can Gravitons be Detected?". Foundations of Physics 36 (12): 1801–1825. arXiv:gr-qc/0601043. Bibcode 2006FoPh...36.1801R. doi:10.1007/s10701-006-9081-9. 

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