History of classical field theory

This article is about classical field theory. For quantum theory, see history of quantum field theory.

Iron filings used to show the magnetic field lines of a bar magnet.

In the history of physics, the concept of fields had its origins in the 18th century in a mathematical formulation of Newton's law of universal gravitation, but it was seen as deficient as it implied action at a distance. In 1852, Michael Faraday treated the magnetic field as a physical object, reasoning about lines of force. James Clerk Maxwell used Faraday's conceptualisation to help formulate his unification of electricity and magnetism in his field theory of electromagnetism.

With Albert Einstein's special relativity and the Michelson–Morley experiment, it became clear that electromagnetic waves could travel in vacuum without the need of a medium or luminiferous aether. Einstein also developed general relativity, in which spacetime was treated as a field and its curvature was the origin of the gravitational interactions, putting an end to action at a distance.

In quantum field theory, fields become the fundamental objects of study, and particles are excitations of these fields.

Early mechanical explanations of forces

Magnetism

René Descartes drawing of a magnetic field from 1644. It shows the magnetic field of the Earth (D) attracting several round lodestones (I, K, L, M, N) and illustrates his theory of magnetism.

The first record of explanations of how magnets works comes from ancient Greece. Thinkers like Thales of Miletus, Aristotle and Diogenes Laertius considered that magnets were animated and should have a soul in order to move. Empedocles tried to provide a mechanical explanation of why magnets could influence each other by introducing the concept of "effluences" emanated by magnets. According to book Quaestiones by Alexander of Aphrodisias from about 200 AD, this was Empedocles view:

On the reason why the lodestone attracts iron. Empedocles says that the iron is attracted to the stone by the effluences which issue from both, and because the pores of the stone are commensurate with the effluences from the iron. The effluences from the stone stir and disperse the air lying upon and obstructing the pores of the iron and when this is removed the iron is drawn on by a concerted outflow. As the effluences from the iron travel towards the pores of the stone, because they are commensurate with them and fit into them the iron itself follows and moves together with them.

Democritus had a similar view as Empedocles but added that the effluences created a void. Metals and rocks could also contain void in order to be less or more attracted to magnets.

This idea survived up to the Scientific Revolution. In 1664, René Descartes produced his theory of magnetism, in which the flow of effluences or effluvia rarified the air, creating differences in air pressure. According to Descartes, these effluvia circulated inside and around the magnet in closed loops.

Gravitation

Further information: Mechanical explanations of gravitation

In ancient times, Greek thinkers like Posidonius (1 BC) noticed a relation between the tides and the position of the Moon in the sky. He considered that light from the Moon had an influence on the tides.

In the 9th century, Abu Ma'shar al-Balkhi (Latinized as Albumasar) wrote his book on The Great Introduction to the Science of Astrology (Kitāb al-madkhal al-kabīr) recorded the correlation between the tides and the Moon, noticing that there were two tides in a day. As there is no moonlight when the Moon is the opposite side of Earth, he proposed that the Moon had some intrinsic virtue that attracted the water. The Sun would have some of that virtue but less than the moon. This book was translated to Latin and was a reference for European medieval scholars. One writer that rejected this astrological reading was Robert Grosseteste who wrote On the Ebb and Flow of the Sea (Latin: Questio de fluxu et refluxu maris), written around 1227, in which he insisted that light from the Moon rarefied the air producing the tides. He explained the tides when the Moon is below the horizon as reflections from the celestial sphere. Two theories coexisted, the idea of light influencing the tides and Albumasar' virtue. Roger Bacon supported the idea of Grosseteste, Albertus Magnus supported a mix of both, and others like Jean Buridan hesitated between the two.

In 17th century, Johannes Kepler who came up with the Kepler's laws of planetary motion, proposed the idea that the Sun emitted some sort of invisible "species" that traveled instantaneously and acted more strongly depending on the distance, size and density of the planet. Kepler considered that if the Sun rotated, it would create a whirlpool of species that drags all planets to orbit around it. The idea of the rotation of the Sun was confirmed by Galileo Galilei, but the frequency did not match Kepler's calculations. To explain the tides, Kepler considered that the species would behave similar to magnetic forces.

Descartes rejected Kepler's theory and also constructed also a mechanical explanation of gravitation based on the ideas vortices, considering space continuous.

Newtonian gravitation

In Isaac Newton's classical gravitation, mass is the source of an attractive gravitational field.

Newtonian mechanics

Before Newton, only a few mechanical explanations of gravity existed.

In 1687, Newton's Principia in 1687 provided a framework with which to investigate the motion and forces. He introduced mathematical definition of gravitational force with his law of universal gravitation, in which the gravitational force between two bodies is directed along the line separating the bodies and its magnitude is proportional to the product of their masses, divided by the square of their distance apart.

While Newton explanation of gravity was very successful in astronomy, it did not explain how it could act at a distance and instantaneously. Newton, considered action at a distance to be:

so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it.

— Isaac Newton, Letters to Bentley, 1692/3

Gottfried Wilhelm Leibniz complained that Newtonian mechanics violated the metaphysics of continuity, in which every cause and effect should be connected to one another. Roger Joseph Boscovich rejected Leibniz take considering that bodies would have discontinuous changes in density at the boundaries and that if came into contact their velocities would change discontinuously.

British empiricist like John Locke, George Locke and David Hume regarded Newton's second law of motion as sufficient, as it establishes a causal relation between force and acceleration.

Beginning of aether theories

Main article: Aether theories

To solve the issue of action at a distance, aether theories were developed. The aether was considered as a yet undetected medium and responsible agent for conducting the force. In a letter to Robert Boyle in 1679 Newton proposed an "aethereal substance" to explain gravity. Later in his work Opticks of 1717 he considered the aether to be made of impenetrable corpuscules. Newtonian aether was very dilute and elastic. Immanuel Kant considered Newton's aether inconsistent as requiring additional forces between corpuscles. Leibiniz on the other hand considered a continuous medium.

Eulerian fluid dynamics

An important development of field theories appeared with Leonhard Euler who expanded Newtonian mechanics in his work Mechanica of 1736. Euler work expanded on how to deal with rotations of rigid bodies, elasticity and fluid mechancics. To describe fluids he considered a flow velocity function (today called velocity field) defined at every point in space. However this function was for a long time considered significantly different from that of the forces of gravitation as it was only defined inside a medium and thus was considered a real quantity.

Forces of electricity and magnetism

Charles-Augustin de Coulomb showed in 1785 that the repulsive force between two electrically charged spheres obeys the same (up to a sign) force law as Newton's law of universal gravitation. André-Marie Ampère showed in 1823 that the force between infinitesimal lengths of current-carrying wires similarly obeys an inverse-square law such that the force is directed along the line of separation between the wire elements. These law suffered from the same problem of action-at-a-distance.

Luminiferous aether

In 1800, Thomas Young proved the wave nature of light using the double-slit experiment. This discovery led him in 1802 to consider the existence of luminiferous aether in which light traveled. Augustin-Jean Fresnel considered it to be an elastic medium. The motion of this aether were described mathematically by scientist like Claude-Louis Navier (in 1821) and Augustin-Louis Cauchy (in 1828) as discrete medium. About 1840, George Stokes and Lord Kelvin extended the formalism to describe a continuous aether using the idea of a potential function (like the gravitational potential and electrostatic potential) introduced by mathematicians like Pierre-Simon Laplace, Siméon Denis Poisson and George Green. This development was important as it allowed to describe any deformable medium in terms of continuous functions.

Introduction of fields

Main article: History of electromagnetic theory

Faraday's lines of force

Magnetic field lines of a magnetic dipole.

Michael Faraday coined the term "magnetic field" in his Researches when postulating, after discovering that all the constituent materials of a human are diamagnetic, that if a human were set in a sufficiently strong magnetic field then they too would align with the field. Faraday did not conceive of this field as a mere mathematical construct for calculating the forces between particles—having only rudimentary mathematical training, he had no use for abstracting reality to make quantitative predictions. Instead he conjectured that there was force filling the space where electromagnetic fields were generated and reasoned qualitatively about these forces with force lines:

"Important to the definition of these lines is that they represent a determinate and unchanging amount of force. Though, therefore, their forms, as they exist between two or more centers or sources of power, may vary greatly, and also the space through which they may be traced, yet the sum of power contained in any one section of a given portion of the lines is exactly equal to the sum of power in any other section of the same lines, however altered in form or however convergent or divergent they may be at the second place."

Faraday's insights into the behavior of magnetic fields would prove invaluable for the development of electromagnetism, in terms of the relations between magnetic and electric fields.

Formal definition

In 1845, Lord Kelvin formalized the mathematical similarities between the fields of electromagnetic phenomena and Joseph Fourier work on heat; and in 1947 between electric conduction and elasticity. These similarities led Lord Kelvin to propose a formal definition of field in 1851:

Any space at every point of which there is a finite magnetic force is called ‘a field of magnetic force’ or (magnetic being understood) simply ‘a field of force,’ or sometimes ‘a magnetic field’.

— Lord Kelvin, On the theory of magnetic induction in crystalline and non-crystalline substances,

Maxwell electromagnetism

In 1865, James Clerk Maxwell compiled all known equations of electromagnetism. Maxwell's equations together led to a wave equation that propagated at the speed of light. Thus explaining electromagnetic radiation in terms of the same electric and magnetic fields. In order to explain this wave phenomena, Maxwell had to settle for the idea of a luminiferous aether. He wrote

"Another theory of electricity which I prefer denies action at a distance and attributes electric action to tensions and pressures in an all-pervading medium, these stresses being the same in kind with those familiar to engineers, and the medium being identical with that in which light is supposed to be propagated."

Maxwell was conflicted on the idea on the nature of the fields, he considered the aether to a mechanical medium in order to carry energy. In 1868 Carl Neumann discussed the idea of the electromagnetic field being an independent energy field.

In 1887, Heinrich Hertz published his experimental evidence of the existence of electromagnetic waves.

Relativity

Main articles: History of general relativity and History of special relativity

The 1887 Michelson–Morley experiment attempted to prove that electromagnetic radiation were oscillations of a luminiferous aether, however the result was negative, indicating that radiation could travel in vacuum. To explain this phenomenon, Albert Einstein developed his theory of special relativity (1905) which resolved the conflicts between classical mechanics and electromagnetism.

Einstein also developed general relativity in 1915, consistent with special relativity and that could explain gravitation in terms of a field theory of spacetime.

Quantum fields

Main article: History of quantum field theory

Fields become the fundamental object of study in quantum field theory. Mathematically, quantum fields are formalized as operator-valued distributions. Although there is no direct method of measuring the fields themselves, the framework asserts that all particles are "excitations" of these fields. For example: whereas Maxwell's theory of classical electromagnetism describes light as a self-propagating wave in the electromagnetic field, in quantum electrodynamics light is the massless gauge boson particle called the photon. Furthermore, the number of particles in an isolated system need not be conserved; an example of a process for which this is the case is bremsstrahlung. More detailed understanding of the framework is obtained by studying the Lagrangian density of a field theory which encodes the information of its allowed particle interactions.