Quantum Field Theory - A logical extension of quantum mechanics
Dirac’s discoveries had other repercussions. With the success of Quantum Mechanics and (Special) Relativity, physicists set out to create a fundamental ‘theoretical structure’ for the electromagnetic filed and the two nuclear interactions (know as the strong and the weak) involving the fundamental constituents of matter.[0]
Such a theory involves the quantisation of fields such as the electric field (a reflection on wave-particle duality), and this area became known as Quantum Field Theory (QFT), so-called because it is the field which is quantised, of which the paradigm is Quantum Electrodynamiics (QED).
QFT is a theoretical framework for constructing quantum mechanical models of subatomic particles in particle physics by treating a particle as an excited state of an underlying physical field[1], the modern successor of classical fields such as the magnetic field.
QFT provides an alternative to the Standard Model and was developed between the late 1920s and the early 1950s by merging the earlier theory of quantum mechanics with Einstein’s special theory of relativity. It provides the conceptual underpinnings of the Standard Model of particle physics, which describes the fundamental building blocks of matter and their interactions in one common framework. In terms of empirical precision, it has been described as the most successful theory in the history of science: scientists turn to it every day to calculate the aftermath of particle collisions, the synthesis of matter in the big bang, the extreme conditions inside atomic nuclei and much besides[2].
In conjuring up a mental impression of what constitutes a particle, we naturally come up with ideas of little billiard balls bouncing off one another. However, this notion is misleading. It implies something that exists in a certain location, but the particles of QFT do not have well defined locations. So, many physicists have now come to think that particles are not things, but excitations in a quantum field, the modern successor of classical fields such as the magnetic field. But fields too are paradoxical, we are told. A point in space does not take on a specific physical quantity, merely a spectrum of possible quantities. So if then neither particles nor fields are fundamental, what is?
QFT asserts that the world, at root, does not consist of material things, but the relations in which those things stand, and that relations between things is all there is. Alternatively, what we call a thing may be just a bundle of properties: colour, shape, consistency, mass, charge, spin and so on[3]. The conception of properties as particulars rather than universals is referred to by philosophers as ‘tropes’. In other words, a thing’s properties or tropes are the one and only fundamental category.
Particles cannot be pinned down to any particular location, and in quantum fields, a point in space does not take on a specific physical quantity, merely a spectrum of possible quantities. The value that is actually chosen depends on a separate mathematical construct known as the state vector, which represents the configuration of the system in question. It is not assigned to any specific location; it spans all of space[4]. So, what we call an electron is in fact a bundle of properties of tropes: three fixed essential properties (mass, charge, spin), as well as numerous changing, non-essential properties (position and velocity). The trope conception helps makes sense of QFT theory, which predicts that elementary particles can pop into and out of existence very quickly.
The behaviour of the vacuum in QFT is particularly mind-boggling: the average value of the number of particles is zero, yet the vacuum seethes with activity. In a particle ontology, this activity is paradoxical. If particles are fundamental, then how can they materialise, and what do they materialise out of? In the trope ontology, this situation is quite natural. The vacuum, though empty of particles, contains properties. A particle is what you get when those properties bundle themselves together in a particular way[5].
QFT combines both physics and philosophical metaphysical ways of thinking in order to provide a comprehensive picture of the world. The two disciplines are complementary. . Metaphysics supplies various competing frameworks for the ontology of the material world, but it cannot decide amongst them. Physics lacks a coherent account of fundamental issues, such as the definition of objects, the role of individuality, the status of properties, the relation of things and the significance of space and time.
Physicists are not even sure what the ontology or basic physical picture of this theory is. However, metaphysical thinking is influencing many of those who are trying to unite QFT with Einstein’s theory of gravitation, and the alternatives to the standard particle and field views may inspire physicists in their struggle to achieve the grand unification[6].
[0] Michael Box, "The Fundamental Nature of Light", WEA course, 26 August 2019, 4.3.
[1] http://en.wikipedia.org/wiki/Quantum_field_theory Quantum field theory is also a foundation stone of Feynman's sum over paths
[2] Meinhard Kuhlmann, “What is real”, Scientific American, August 2013, 32-39.
[3] On these ideas, see also Anthony Quinton, The Nature of Things, (London, 1973).
[4] A state vector is not a property of a physical system.. It does not evolve continuously between measurements, nor suddenly ‘‘collapse’’ into a new state vector whenever a measurement is performed. Instead it represents a procedure for preparing or testing one or more physical systems:
© 1984 American Association of Physics Teachers (edited).: http://scitation.aip.org/content/aapt/journal/ajp/52/7/10.1119/1.13586
[5] This is a summary drawn from Meinhard Kuhlmann, “What is real”, Scientific American, August 2013, 32-39.
[6] Ibid at 39.
Such a theory involves the quantisation of fields such as the electric field (a reflection on wave-particle duality), and this area became known as Quantum Field Theory (QFT), so-called because it is the field which is quantised, of which the paradigm is Quantum Electrodynamiics (QED).
QFT is a theoretical framework for constructing quantum mechanical models of subatomic particles in particle physics by treating a particle as an excited state of an underlying physical field[1], the modern successor of classical fields such as the magnetic field.
QFT provides an alternative to the Standard Model and was developed between the late 1920s and the early 1950s by merging the earlier theory of quantum mechanics with Einstein’s special theory of relativity. It provides the conceptual underpinnings of the Standard Model of particle physics, which describes the fundamental building blocks of matter and their interactions in one common framework. In terms of empirical precision, it has been described as the most successful theory in the history of science: scientists turn to it every day to calculate the aftermath of particle collisions, the synthesis of matter in the big bang, the extreme conditions inside atomic nuclei and much besides[2].
In conjuring up a mental impression of what constitutes a particle, we naturally come up with ideas of little billiard balls bouncing off one another. However, this notion is misleading. It implies something that exists in a certain location, but the particles of QFT do not have well defined locations. So, many physicists have now come to think that particles are not things, but excitations in a quantum field, the modern successor of classical fields such as the magnetic field. But fields too are paradoxical, we are told. A point in space does not take on a specific physical quantity, merely a spectrum of possible quantities. So if then neither particles nor fields are fundamental, what is?
QFT asserts that the world, at root, does not consist of material things, but the relations in which those things stand, and that relations between things is all there is. Alternatively, what we call a thing may be just a bundle of properties: colour, shape, consistency, mass, charge, spin and so on[3]. The conception of properties as particulars rather than universals is referred to by philosophers as ‘tropes’. In other words, a thing’s properties or tropes are the one and only fundamental category.
Particles cannot be pinned down to any particular location, and in quantum fields, a point in space does not take on a specific physical quantity, merely a spectrum of possible quantities. The value that is actually chosen depends on a separate mathematical construct known as the state vector, which represents the configuration of the system in question. It is not assigned to any specific location; it spans all of space[4]. So, what we call an electron is in fact a bundle of properties of tropes: three fixed essential properties (mass, charge, spin), as well as numerous changing, non-essential properties (position and velocity). The trope conception helps makes sense of QFT theory, which predicts that elementary particles can pop into and out of existence very quickly.
The behaviour of the vacuum in QFT is particularly mind-boggling: the average value of the number of particles is zero, yet the vacuum seethes with activity. In a particle ontology, this activity is paradoxical. If particles are fundamental, then how can they materialise, and what do they materialise out of? In the trope ontology, this situation is quite natural. The vacuum, though empty of particles, contains properties. A particle is what you get when those properties bundle themselves together in a particular way[5].
QFT combines both physics and philosophical metaphysical ways of thinking in order to provide a comprehensive picture of the world. The two disciplines are complementary. . Metaphysics supplies various competing frameworks for the ontology of the material world, but it cannot decide amongst them. Physics lacks a coherent account of fundamental issues, such as the definition of objects, the role of individuality, the status of properties, the relation of things and the significance of space and time.
Physicists are not even sure what the ontology or basic physical picture of this theory is. However, metaphysical thinking is influencing many of those who are trying to unite QFT with Einstein’s theory of gravitation, and the alternatives to the standard particle and field views may inspire physicists in their struggle to achieve the grand unification[6].
[0] Michael Box, "The Fundamental Nature of Light", WEA course, 26 August 2019, 4.3.
[1] http://en.wikipedia.org/wiki/Quantum_field_theory Quantum field theory is also a foundation stone of Feynman's sum over paths
[2] Meinhard Kuhlmann, “What is real”, Scientific American, August 2013, 32-39.
[3] On these ideas, see also Anthony Quinton, The Nature of Things, (London, 1973).
[4] A state vector is not a property of a physical system.. It does not evolve continuously between measurements, nor suddenly ‘‘collapse’’ into a new state vector whenever a measurement is performed. Instead it represents a procedure for preparing or testing one or more physical systems:
© 1984 American Association of Physics Teachers (edited).: http://scitation.aip.org/content/aapt/journal/ajp/52/7/10.1119/1.13586
[5] This is a summary drawn from Meinhard Kuhlmann, “What is real”, Scientific American, August 2013, 32-39.
[6] Ibid at 39.