Healey, Richard (2007) Gauge Theory and the Theta Vacuum. In:  EPSA07: 1st Conference of the European Philosophy of Science Association (Madrid, 15-17 November, 2007).
This is the latest version of this item.
Download (129Kb) | Preview
According to conventional wisdom, local gauge symmetry is not a symmetry of nature, but an artifact of how our theories represent nature. But a study of the so-called theta-vacuum appears to refute this view. The ground state of a quantized non-Abelian Yang-Mills gauge theory is characterized by a real-valued, dimensionless parameter theta—a fundamental new constant of nature. The structure of this vacuum state is often said to arise from a degeneracy of the vacuum of the corresponding classical theory, which degeneracy allegedly arises from the fact that “large” (but not “small”) local gauge transformations connect physically distinct states of zero field energy. If that is right, then some local gauge transformations do generate empirical symmetries. In defending conventional wisdom against this challenge I hope to clarify the meaning of empirical symmetry while deepening our understanding of gauge transformations. I distinguish empirical from theoretical symmetries. Using Galileo’s ship and Faraday’s cube as illustrations, I say when an empirical symmetry is implied by a theoretical symmetry. I explain how the theta-vacuum arises, and how “large” gauge transformations differ from “small” ones. I then present two analogies from elementary quantum mechanics. By applying my analysis of the relation between empirical and theoretical symmetries, I show which analogy faithfully portrays the character of the vacuum state of a classical non-Abelian Yang-Mills gauge theory. The upshot is that “large” as well as “small” gauge transformations are purely formal symmetries of non-Abelian Yang-Mills gauge theories, whether classical or quantized. It is still worth distinguishing between these kinds of symmetries. An analysis of gauge within the constrained-Hamiltonian formalism yields the result that “large” gauge transformations should not be classified as gauge transformations; indeed, nor should “global” gauge transformations. In a theory in which boundary conditions are modeled dynamically, “global” gauge transformations may be associated with physical symmetries, corresponding to translations of these extra dynamical variables. Such translations are symmetries if and only if charge is conserved. But it is hard to argue that these symmetries are empirical, and in any case they do not correspond to any constant phase change in a quantum state.
|Export/Citation:||EndNote | BibTeX | Dublin Core | ASCII/Text Citation (Chicago) | HTML Citation | OpenURL|
|Social Networking:|| |
|Item Type:||Conference or Workshop Item (UNSPECIFIED)|
|Additional Information:||To appear in the EPSA conference proceedings.|
|Keywords:||gauge, symmetry, theta vacuum, Yang-Mills|
|Subjects:||Specific Sciences > Physics > Symmetries/Invariances|
Specific Sciences > Physics > Fields and Particles
Specific Sciences > Physics
Specific Sciences > Physics > Quantum Field Theory
|Conferences and Volumes:|| EPSA07: 1st Conference of the European Philosophy of Science Association (Madrid, 15-17 November, 2007)|
|Depositing User:||Richard Andrew Healey|
|Date Deposited:||15 Jan 2009|
|Last Modified:||07 Oct 2010 11:17|
Available Versions of this Item
- Gauge Symmetry and the Theta Vacuum. (deposited 09 Jul 2007)
- Gauge Theory and the Theta Vacuum. (deposited 15 Jan 2009)[Currently Displayed]
Actions (login required)