# Don't quite Understand the terminology -- Local gauge

1. Jun 17, 2015

### say_cheese

I hope someone with a deep conceptual understanding of terminologies would help me out here. I am having starting problems in understanding the approach of gauge theories.

I have read suggested threads and I am still at a loss. I am an experimental physicist and know basics of electrodynamics and QM. I am reading the Primer for Gauge Theory by Moriyasu. Though the book is reviewed well and is suggested for relative beginner physicists, the terminologies are not explained fully. In particular, while broaching the topic,

It gives the introduction to Weyl's scale or gauge invariance. The book states "Weyl proposed that the absolute magnitude or the norm of a physical vector should not be an absolute quantity, but should depend on the location in space time. A new connection would then be necessary in order to relate the lengths of vectors at different positions. This idea became known as scale or gauge invariance. It is important to note here that the true significance of Weyl's proposal lies in the local property of gauge symmetry ..."

I understand the connection is similar to the GTR connection. But that aside, why the terminology "local" and why the word symmetry here? There is actually a variation. Perhaps I need an analogy from classical physics. My present understanding is that certain transformations like the one for the EM vector potential leave the fields unchanged, but I dont understand that how is this or the one of Weyl is local. I do understand that in the EM case, the fact that the fields do not change with the transformation, is a form of symmetry, but Weyl's??

I also dont understand why the relationship between the phase of a wavefunction and the vector potential (Aharanov-Bohm) is a gauge symmetry and local.

I broadly know that an objects properties can be known by the way it changes or does not change under a transformation, while revealing the symmetry. But I am not seeing the context of the terminologies.

Thanks
Jay

2. Jun 18, 2015

### DEvens

https://en.wikipedia.org/wiki/Gauge_theory#Classical_electromagnetism

If a theory has a global symmetry, it means you can apply the symmetry transformation to the entire universe and get back something that is effectively the same. And example might be rotational symmetry. You could rotate your coordinates by some arbitrary amount, say around the z-axis, and get back a theory that that looks exactly the same.

If a theory has a local symmetry, it means you can apply the symmetry to parts of the universe. But notice that this means there is now some difference between the parts you applied it to, and the parts you did not. And this difference is important.

In electromagnetism we have gauge symmetry. Look down in the wiki article. You start out with V -> V+C. But eventually you elaborate this into the vector potential $A_i$, and then the 4-vector potential $A_\mu$. The first form, V+C, is a global symmetry. But turning it into the form given in the article means it can have different values at different locations. A local symmetry. But this local symmetry is carried by the A field, the photon. So we experience the gauge symmetry locally as photons carrying the transformation from one spot to another.

So too with Weyl gravity, for example. Local variations in the symmetry are proposed to look like an interaction moving from one part of space-time to another, applying the symmetry change as it goes.