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Mathematics for Physics

236 Four Fundamental Forces

A set of four basic interactions that is used to classify how matter and fields influence one another.

The four interactions. The four interactions are gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. Together they account for the observed phenomena of ordinary and nuclear matter. This principle is used to name every known force by one of these four.

The strength ordering. In decreasing strength the order is strong, electromagnetic, weak, then gravitational. Gravity is far feebler than the other three. This principle is used to decide which interaction dominates a given process.

The two ranges. Electromagnetism and gravity have infinite range. The weak and strong nuclear forces have extremely short range. Range is the distance over which the interaction remains appreciable. This principle is used to explain why nuclei bind at femtometer distances while gravity and electromagnetism act across the solar system.

Mediation by force carriers. At the quantum level each interaction is mediated by the exchange of a force carrier. A force carrier is a boson exchanged between charged particles. This principle is used to identify the photon, the \(W\) and \(Z\) bosons, the gluon, and the graviton as the mediators.

Charge matching. A particle feels an interaction only if it carries the matching charge: mass for gravity, electric charge for electromagnetism, weak isospin for the weak force, and color for the strong force. This principle is used to decide which particles participate in which interaction.

The gauge-theory description. Electromagnetism, the weak force, and the strong force are described by gauge theories. A gauge theory is a field theory whose interactions follow from a local symmetry. This principle is used to write the Standard Model. Gravity is the usual exception and is described by general relativity.

Note: Also called the four basic forces. Also called the four fundamental interactions.

236.1 References

  1. Jaffe, R. L., & Taylor, W. The Physics of Energy. Cambridge University Press, 2018. — coverage of phenomena; energy; carriers.
  2. Griffiths, D. J. Introduction to Elementary Particles. — strength ordering; ranges.
  3. Cheng, T.-P., & Li, L.-F. Gauge Theory of Elementary Particle Physics. — gauge structure.
  4. Schwichtenberg, J. Physics from Symmetry. — vector bosons; charges.
  5. Feynman, R. P., Leighton, R. B., & Sands, M. The Feynman Lectures on Physics, Vol. I. — weakness of gravity.