Fields and Forces

1. Fields

Field theories describe how forces interact with matter. American physicist Richard Feynman described a field as something that has the "potentiality for producing a force"[1a].

British natural philosopher James Clerk Maxwell described the electromagnetic field in 1864[2]. 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:

"[T]he existence of the positive charge, in some sense, distorts, or creates a 'condition' in space, so that when we put the negative charge in, it feels a force"[1b].

Illustration of magnetic field lines created by a bar magnet.

Iron filings in a magnetic field. Image credit: James David McCoy/Public domain.

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

Artist’s impression of the magnetic field around a magnetar - a type of highly magnetic neutron star. Image credit: ESO/L. Calçada/CC-A.

German-Swiss-American physicist Albert Einstein's theory of general relativity described the gravitational field in 1916[3]. 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.

1.1 Quantum field theories

Quantum field theories were developed to explain how forces work, taking into account both quantum mechanics and Einstein's theory of special relativity. In the 1900s, it was shown that there are at least four fundamental forces[4a]:

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

Quantum mechanics shows that at high energies, electromagnetism and the weak force combine to form a single force, known as the electroweak force[5][6][7].

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)[8].

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)[9].

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[10]. Further evidence came in 2016[11], and research is currently being conducted at CERN and at the Thomas Jefferson National Accelerator Facility in the United States[12].

2. References

  1. (a, b) Feynman, R. P., Leighton, R. B., and Sands, M., 1965, 'The Feynman Lectures on Physics, Volume I', Basic Books.

  2. Maxwell, J. C., 1865, 'A Dynamical Theory of the Electromagnetic Field', Proceedings of the Royal Society of London, 13, pp.531-536.

  3. Einstein, A., 1916, 'The foundation of the generalised theory of relativity', Annalen der Physik, 354, pp.769-822, reprinted in 'The principle of relativity; original papers', 1920, The University of Calcutta.

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

  5. Glashow, S. L., 1961, 'Partial-symmetries of weak interactions', Nuclear Physics, 22, pp.579–588.

  6. Weinberg, S., 1967, 'A Model of Leptons', Physical Review Letters, 19, pp.1264–1266.

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

  8. Georgi, H. and Glashow, S. L., 1974, 'Unity of all elementary-particle forces', Physical Review Letters, 32, pp.438-441.

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

  10. Krasznahorkay, A. J., et al, 2016, 'Observation of Anomalous Internal Pair Creation in Be 8: A Possible Indication of a Light, Neutral Boson', Physical Review Letters, 116, pp.042501.

  11. Feng, J. L., et al, 2016, 'Evidence for a Protophobic Fifth Force from Be 8 Nuclear Transitions', arXiv preprint arXiv:1604.07411.

  12. Cartlidge, E., 2016, 'Has a Hungarian physics lab found a fifth force of nature?', Nature News, last accessed 25-05-16.

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