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Fundamental theories of physics

Fermions

Gerard ’t Hooft

Published Jan 1, 2016• 0 citations
Publisher

Abstract

Classical theories starting with continuous degrees of freedom will typically generate bosonic quantum objects; fermionic quantum objects in turn, arise when a classical system contains discrete degrees of freedom—typically: flip-flops. Here, we explain how anti-commuting operators enter the scene, and illustrate the situation using an elegant model: the idealized notion of a ‘neutrino’. What do neutrinos have in common with infinite, at sheets, and where does spin \(\frac{1}{2}\) come from? The associated mathematical tool kit is quite delicate. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

DOI
10.1007/978-3-319-41285-6_15

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Classical theories starting with continuous degrees of freedom will typically generate bosonic quantum objects; fermionic quantum objects in turn, arise when a classical system contains discrete degrees of freedom—typically: flip-flops. Here, we explain how anti-commuting operators enter the scene, and illustrate the situation using an elegant model: the idealized notion of a ‘neutrino’. What do neutrinos have in common with infinite, at sheets, and where does spin (\frac{1}{2}) come from? The associated mathematical tool kit is quite delicate. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
This chapter gives an introduction to a chain of results attempting to exclude deeper layers underneath quantum mechanics that restore some form of classical physics: ‘[Such results] more or less illustrate the ways along which some opponents might hope to escape Bohr’s reasonings and von Neumann’s proof and the places where they are dangerously near breaking their necks.’ (Groenewold, 1946, p. 454). In so far as they are mathematically precise, such no-go results have their roots in von Neumann’s 1932 book, which gave rise to two traditions that were often in polemical opposition to each other. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
The degrees of freedom studied so far have been either real or complex variables. These variables commute under multiplication, in the sense that two numbers a, b satisfy ab = ba ; commuting variables are generically called bosonic variables, or bosonic degrees of freedom. Typical of the bosonic case are the degrees of freedom for a collection of quantum mechanical particles. Interactions of fundamental particles are generally mediated by bosonic fields such as the Maxwell electromagnetic field, whereas mass is usually carried by particles that are fermions, the most familiar being the electron. Two key features distinguish fermions from bosons: Fermions obey the Pauli exclusion principle, which states that no two fermions can occupy the same quantum state. This is the reason the concept of intensity does not apply to a fermion. A high intensity electric field is a reflection of the presence of a large number of photons, which are bosons, in the same quantum state; for photons, any number of photons can be in the same quantum state. In contrast, an electron is either in a quantum state or it is not; in particular, ignoring spin, an electron exists at a point or there is no electron there. The state function of a multi-bosonic system is totally symmetric in that the exchange of any two bosonic degrees of freedom yields the same state function. In contrast, a multi-fermion system is totally anti-symmetric: the exchange of any two fermion degrees of freedom gives the same state – but with a negative sign .
Many physical problems can be described as a single (or a few) degrees of freedom interacting with a free field, regarded as a bath or reservoir. It is commonly assumed that a nontrivial coupling to the field serves as a mechanism for the small system to dissipate its energy. This somewhat vague formulation should be made precise by mathematical physics and we take here the point of view that it is reflected in the spectrum of the coupled Hamiltonian. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Usually one uses completely different mathematical descriptions to formulate classical and quantum mechanics. Classical theory may be nicely formulated in terms of symplectic geometry, and the quantum one in terms of algebraic objects related to a complex Hilbert space. However, it turns out that standard, nonrelativistic quantum mechanics possesses natural geometric structure that is even richer than that found in classical mechanics. This section reveals the beauty of the geometric approach to quantum theory and stands as a basis for the elegant geometrical ideas of Pancharatnam and, later on, of Aharonov and Anandan. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.