Question

Can anyone explain gauge theory? and the notation of Su(#)?

Answer 1

A field theory in which the fields and potentials are described by a symmetry group (the gauge group). A gauge theory arises when a particular symmetry is imposed on a theory even when the parameter labeling the symmetry is allowed to vary over spacetime.

The earliest physical theory to have a gauge symmetry was classical electrodynamics – the theory of electricity and magnetism originated by James Clerk Maxwell. It turns out that electrodynamic theory is invariant under redefinition of the electrostatic potential, which corresponds eventually to the law of conservation of electric charge. The mathematics of gauge theories was not fully developed, however, until the early 20th century by the German mathematician Hermann Weyl. In quantum theory, for example, the phase of the wave functions describing a system can be changed by an arbitrary amount without altering the physical content or structure of the theory, provided that all the wave functions are changed in the same way, everywhere in space. To gauge this symmetry the phase change is allowed to vary over spacetime. In order to maintain this as a symmetry of the theory it turns out that the electromagnetic force must be introduced. Thus quantum electrodynamics (QED) is an example of a gauge theory. The different wave functions can also be allowed to have different phase changes and this corresponds to their electric charge.

In a gauge theory there is a group of transformations of the field variables (gauge transformations) that leaves the basic physics of the quantum field unchanged. This condition, called gauge invariance, gives the theory a certain symmetry, which governs its equations. In short, the structure of the group of gauge transformations in a particular gauge theory entails general restrictions on the way in which the field described by that theory can interact with other fields and elementary particles. The classical theory of the electromagnetic field, proposed by Maxwell in 1864, is the prototype of gauge theories, though the concept of gauge transformation was not fully developed until the early 20th century by Weyl. In Maxwell's theory the basic field variables are the strengths of the electric and magnetic fields, which may be described in terms of auxiliary variables (e.g., the scalar and vector potentials). The gauge transformations in this theory consist of certain alterations in the values of those potentials that do not result in a change of the electric and magnetic fields. This gauge invariance is preserved in the modern theory of electromagnetism called quantum electrodynamics, or QED. Modern work on gauge theories began with the attempt of the American physicists Chen Ning Yang and Robert L. Mills (1954) to formulate a gauge theory of the strong interaction. The group of gauge transformations in this theory dealt with the isospin of strongly interacting particles. In the late 1960s Steven Weinberg, Sheldon Glashow, and Abdus Salam developed a gauge theory that treats electromagnetic and weak interactions in a unified manner. This theory, now commonly called the electroweak theory, has had notable success and is widely accepted. During the mid-1970s much work was done toward developing quantum chromodynamics (QCD), a gauge theory of the interactions between quarks (see quark). For various theoretical reasons, the concept of gauge invariance seems fundamental, and many physicists believe that the final unification of the fundamental interactions (i.e., gravitational, electromagnetic, strong, and weak) will be achieved by a gauge theory. The Standard Model is also built out of a gauge theory.

Answer 2

Don't you think that when you copy and paste such a large amount of text you should provide a citation to the source?

Answer 3

most likely wikipedia lols.

Answer 4

okay, thanks for the history, but can I get some physics please? Sorry to be a bit demanding, but I can find the history in my textbook. but the explination is not all that clear.

Answer 5

I posted on this here that you might find some useful stuff in http://openstudy.com/groups/physics#/groups/physics/updates/4e2439b00b8b3d38d3b7083b. I will copy and paste the more relavent part here it here though to save you the journey.

A gauge theory is a theory which obeys gauge symmetry (more strange terms, but bear with me). Gauge symmetry is a property of a field, where in the equations describing that field remain the same after one applies an operation to all particles everywhere in space. the term gauge just means "measure", and a field with gauge symmetry can thus be remeasured (or re-gauged) from different baselines without changing their properties. For example, if we had a ball on a step on a stair case, it would have a specific gravitational potential energy. If it then dropped down a step, it would loose a specific amount of gravitational energy due to the change of height with the earth's gravitational field. We can thus calculate the difference in energy between the two steps, but crucially, it doesn't matter where we measure the baseline to be (either from the lower step, higher step, top of the stairs, bottom of the stairs, centre of the earth, or geosynchronous orbit, or another galaxy entirely), the difference in energy will always come out the same, regardless of how you re-gauge the baseline. Recall that a gauge theory, such as Yang-Mills theory, will obey gauge symmetry. We can imagine what gauge theory is by picturing a sheet of paper infinite in extent, all painted a specific shade of grey. Now no matter how you rotate the page, or whatever angle you look at it, it will appear just like any other angle. It is called globally invariant. This is true regardless of what shade of paint we use, re-gauging the colour makes no difference as per gauge symmetry. Now imagine though we take a similar sheet of paper, but painted in different shades of grey. The symmetry would then be broken, since we rotation or viewing angle, will change how it looks, since we will be able to distinguish different parts of the paper from others. We can restore the global invariance if we overlay the multi-shaded paper with a a clear plastic sheet that has been painted in shades that exactly balance the pattern in the paper (dark where the paper is light, and vice versa). The combined effect is to produce a uniform shade of grey with a global invariance.

I am not too strong on the notation which comes down to group theory. It was ten years ago I last looked at this stuff, and I had trouble doing it then (which is one advantage of marrying a mathematician). If you have an \(N\times N\) matrix, then in group theory \(N\) is the dimension of the of the group. Thus Su\((N)\) is the Special Unitary group of dimension \(N\), and will have certain symmetry properties (please dont ask me what they are).