Quantcast
  • Register
PhysicsOverflow is a next-generation academic platform for physicists and astronomers, including a community peer review system and a postgraduate-level discussion forum analogous to MathOverflow.

Welcome to PhysicsOverflow! PhysicsOverflow is an open platform for community peer review and graduate-level Physics discussion.

Please help promote PhysicsOverflow ads elsewhere if you like it.

News

New printer friendly PO pages!

Migration to Bielefeld University was successful!

Please vote for this year's PhysicsOverflow ads!

Please do help out in categorising submissions. Submit a paper to PhysicsOverflow!

... see more

Tools for paper authors

Submit paper
Claim Paper Authorship

Tools for SE users

Search User
Reclaim SE Account
Request Account Merger
Nativise imported posts
Claim post (deleted users)
Import SE post

Users whose questions have been imported from Physics Stack Exchange, Theoretical Physics Stack Exchange, or any other Stack Exchange site are kindly requested to reclaim their account and not to register as a new user.

Public \(\beta\) tools

Report a bug with a feature
Request a new functionality
404 page design
Send feedback

Attributions

(propose a free ad)

Site Statistics

146 submissions , 123 unreviewed
3,961 questions , 1,408 unanswered
4,890 answers , 20,766 comments
1,470 users with positive rep
506 active unimported users
More ...

Large and small gauge transformations?

+ 6 like - 0 dislike
1917 views

I've a questions about the difference between small and large gauge transformations (a small gauge transformation tends to the identity at spatial infinity, whereas the large transformations don't). Many sources state (without any explanation or reference) that configurations related by small gauge transformations are physically equivalent, whereas large gauge transformations relate physically distinct configurations. This seems odd to me (and some lecturers at my university even say that this is wrong), because all gauge transformations relate physically equivalent configurations.

Some of the literature that mentions the difference between small and large gauge transformations:

In Figueroa-O'Farrill's notes it is mentioned in section 3.1 (page 81-82) in http://www.maths.ed.ac.uk/~jmf/Teaching/EDC.html

In Harvey's notes, see equation (2.13) in http://arxiv.org/abs/hep-th/9603086

In Di Vecchia's notes, see the discussion above (and below) equation (5.7) http://arxiv.org/abs/hep-th/9803026

They all say that large gauge transformation relate physically distinct configurations, but they don't explain why this is true. Does anybody know why this is true?

Best regards,

Hunter

This post imported from StackExchange Physics at 2014-04-05 17:25 (UCT), posted by SE-user Hunter
asked Jul 31, 2013 in Theoretical Physics by Hunter (510 points) [ no revision ]
pg 23 of this reference talks a little bit about this lepp.cornell.edu/~pt267/files/documents/A_instanton.pdf

This post imported from StackExchange Physics at 2014-04-05 17:25 (UCT), posted by SE-user DJBunk
Tanks for your reply. I have read a bit about large gauge transformations in regards to instantons and understand that for instantons they can change the winding number due to quantum tunneling. However, I don't know whether or not this is the reason that this distinctions is made for solitons as well. Especially, because for (classical) solitons the winding number is gauge invariant. If anybody could give me a conclusive answer that this is indeed the reason for the distinction between small and large gauge transformations for solitons, then I will accept that.

This post imported from StackExchange Physics at 2014-04-05 17:25 (UCT), posted by SE-user Hunter

1 Answer

+ 4 like - 0 dislike

In the cases when the gauge group is disconnected, both choices of defining the physical space as a the quotient of the field space by the whole gauge group $\mathcal{A}_{physical} = \frac{\mathcal{A} }{ \mathcal{G}}$ or by its connected to the identity component $\mathcal{A}_{physical} = \frac{\mathcal{A} }{ \mathcal{G}_0}$ are mathematically sound. In the second case, the large gauge transformations are not included in the reduction, thus they transform between physically distinct configurations., and in quantum theory between physically distinct states.

However, as N.P. Landsman reasons, the first choice overlooks inequivalent quantizations that correspond to the same classical theory. In the case of the magnetic monopoles these distinct quantizations correspond to monopoles with fractional electric charge (Dyons). This phenomenon was discovered by Witten (the Witten effect). If the whole gauge group including the large gauge transformations is quotiened by, no such states would be present in the quantum theory.

In the monopole theory, the inequivalent quantizations can be obtained by adding a theta term to the Lagrangian (just as the case of instantons). Landsman offers an explanation of this term in the quantum Hamiltonian picture: Assuming $\pi_0(\mathcal{G})$ is Abelian, then when the gauge group is not connected, then a gauge invariant inner product can be defined as:

$\langle \psi| \phi \rangle_{phys} = \sum_{n \in \pi_0(\mathcal{G})} \int_{g\in \mathcal{G_0}} e^{i \pi \theta n} \langle \psi| U(g) |\phi \rangle$

Where the original states belong to the (big) gauge noninvariant Hilbert space. This inner product is $\mathcal{G}_0$ invariant for all values of $\theta$.

This post imported from StackExchange Physics at 2014-04-05 17:25 (UCT), posted by SE-user David Bar Moshe
answered Aug 4, 2013 by David Bar Moshe (3,875 points) [ no revision ]
Thanks for your reply. I've read your message and try to read the paper you provided a link for, but I don't really understand it to be honest. I understand that taking the quotient $\mathcal{A}/\mathcal{G}$ is mathematically correct. However, I am still not sure why large gauge transformations relate physically distinct configurations. It seems that this is contradictory to the whole idea of gauge transformations, which are by definition redundant degrees of freedom.

This post imported from StackExchange Physics at 2014-04-05 17:25 (UCT), posted by SE-user Hunter
If large gauge transformations do relate physically distinct configurations, then they are clearly not redundant. I don't know how to rhyme these things.

This post imported from StackExchange Physics at 2014-04-05 17:25 (UCT), posted by SE-user Hunter
@Hunter The identity component $\mathcal{G}_0$ that you gauge away is almost the whole gauge group, it is an infinite dimensional group. The group of large gauge transformation is only $\mathbb{Z}$, it is discrete. Thus I think that you may consider this as a fine tuning of the gauge principle.

This post imported from StackExchange Physics at 2014-04-05 17:25 (UCT), posted by SE-user David Bar Moshe
Ok, thank you for your answer.

This post imported from StackExchange Physics at 2014-04-05 17:25 (UCT), posted by SE-user Hunter
@Hunter Transformations that don't change the state are called gauge symmetries or redundancies. These transformations tend to the identity at spatial infinity. "Large gauge transformations" are by definition not continuously connected with the identity transformation (they don't live in the same "island" of the disconnected group as the identity). Therefore they don't tend to the identity anywhere. Therefore they do change the state (and in fact they take the state from one Hilbert space to another one).

This post imported from StackExchange Physics at 2014-04-05 17:25 (UCT), posted by SE-user drake
@Hunter Therefore a better name would be " large local (space-time dependent) transformations" instead of "gauge" (this should be reserved for transformations not changing the state). David, please, correct me if I am wrong.

This post imported from StackExchange Physics at 2014-04-05 17:25 (UCT), posted by SE-user drake
@drake Thank for your answer. Do you by any chance have any references where I can find more information? I couldn't find anything in Peskin and Schroeder or Lewis Ryder. I've tried to Google it, but most sources refer to changing the homotopy of instantons (including Ryder) and I don't think that is what you are talking about.

This post imported from StackExchange Physics at 2014-04-05 17:25 (UCT), posted by SE-user Hunter

Your answer

Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead.
To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL.
Please consult the FAQ for as to how to format your post.
This is the answer box; if you want to write a comment instead, please use the 'add comment' button.
Live preview (may slow down editor)   Preview
Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
Anti-spam verification:
If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:
p$\hbar$ysicsOverfl$\varnothing$w
Then drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds).
To avoid this verification in future, please log in or register.




user contributions licensed under cc by-sa 3.0 with attribution required

Your rights
...