Chapter 21. The Field Concept in Physics

21.1 Fields and forces

Field theories describe how forces interact with matter. American physicist Richard Feynman described a field as something that has the potential to produce a force.[1] British natural philosopher James Clerk Maxwell described the electromagnetic field in 1864[2] (discussed in Chapter 5). The electromagnetic field describes the electromagnetic force, which is felt by all objects with a charge.

Feynman described how in the case of the electromagnetic field, where positive charges repel negative charges, the positive charge creates a “condition” where the negative charge “feels” a force.[1]

German-Swiss-American physicist Albert Einstein’s theory of general relativity described the gravitational field in 1916[3] (discussed in Book I). The gravitational field describes the gravitational force, which is felt by all objects with mass. Einstein showed that the force of gravity travels at the speed of light, and this led to the prediction that the gravitational field carries gravitational waves, just as the electromagnetic field carries electromagnetic waves.

21.1.1 Quantum field theories

Quantum field theories were developed to explain how forces work, taking into account both quantum mechanics (discussed in Chapter 17) and Einstein’s theory of special relativity (discussed in Book I). In the 1900s, it was shown that there are at least four fundamental forces:[4]

  • The electromagnetic force, which is described by quantum electrodynamics (QED) (discussed in Chapter 22).
  • The strong nuclear force, which is described by quantum chromodynamics (QCD) (discussed in Chapter 23).
  • The weak nuclear force, which is described by electroweak theory (EWT) (discussed in Chapter 24).
  • The force of gravity, which is the least understood force, but may be described by string theory or loop quantum gravity (discussed in Chapter 25).
Illustration of magnetic field lines created by a bar magnet.

Figure 21.1
Image credit

Iron filings in a magnetic field.

Artist’s impression of magnetic field lines around a neutron star.

Figure 21.2
Image credit

Artist’s impression of the magnetic field around a magnetar - a type of highly magnetic neutron star.

It was also shown that forces are actually ‘transmitted’ by particle-like objects, the fundamental bosons: photons, gluons, and the Z and W bosons.[4]

In 2015, physicists from the Hungarian Academy of Sciences reported evidence of a new boson that may transmit a fifth fundamental force with an extremely short range.[5] Further evidence came in 2016,[6] and research is currently being conducted at CERN and at the Thomas Jefferson National Accelerator Facility in the United States.[7]

Quantum mechanics shows that at high energies, electromagnetism and the weak force combine to form a single force, known as the electroweak force.[8-10] It’s generally expected that at even higher energies, the strong force may combine with the electroweak force. Theories that suggest this are known as grand unified theories (GUT).[11]

At higher energies still, the force of gravity might combine with this other force, so that all the forces can be described by a single theory. Theories that suggest this are sometimes referred to as theories of everything (TOE).[12]

21.2 References

  1. Feynman, R. P., Leighton, R. B., Sands, M., The Feynman Lectures on Physics, Volume I, Basic Books, 1965.

  2. Maxwell, J. C., Proceedings of the Royal Society of London 1865, 13, 531–536.

  3. Einstein, A. in The principle of relativity; original papers, The University of Calcutta, 1920 (1916).

  4. Salam, A., Unification of Fundamental Forces: The First 1988 Dirac Memorial Lecture, Cambridge University Press, 2005.

  5. Krasznahorkay, A. J., Csatlós, M., Csige, L., Gácsi, Z., Gulyás, J., Hunyadi, M., Kuti, I., Nyakó, B. M., Stuhl, L., Timár, J., Tornyi, T. G., Physical Review letters 2016, 116, 042501.

  6. Feng, J. L., Fornal, B., Galon, I., Gardner, S., Smolinsky, J., Tait, T. M., Tanedo, P., “Evidence for a Protophobic Fifth Force from Be 8 Nuclear Transitions”, arXiv preprint arXiv:1604.07411, 2016.

  7. Cartlidge, E., Has a Hungarian physics lab found a fifth force of nature?, Nature News, 2016.

  8. Glashow, S. L., Nuclear Physics 1961, 22, 579–588.

  9. Weinberg, S., Physical Review letters 1967, 19, 1264–1266.

  10. Salam, A., Elementary Particle Physics: Relativistic Groups and Analyticity, (Ed.: Svartholm, N.), Almquvist & Wiksell, 1968.

  11. Georgi, H., Glashow, S. L., Physical Review letters 1974, 32, 438–441.

  12. Davies, P. C. W., Brown, J., Superstrings: A Theory of Everything?, Cambridge University Press, 1992.

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