Physics/Elementary explanation for advanced concepts
I have a background in physics, but I have difficulty in explaining some concepts to lay colleagues. Wikipedia and other on-line sources are not much help - they usually assume some background, and do not start from first principles. So I'd like you to explain (in everyday language / non-mathematical) two such concepts. The first is "Gauge Symmetry" or "Gauge Theory". The second is "Yang-Mills Theory". Please try and limit the explanation to one or two sentences - the sort of thing one would say to a friend who has no special background in physics. Thanks for your help.
OK, but let me preface this by saying that you probably won't like my answer. It's kind of like asking me to describe in two layman's sentences how a computer works without any background in semiconductors, electronics, or even learning how to count in binary. Or perhaps for an aerospace engineer to describe the dynamics of an F35 Joint Strike Fighter with no knowledge of physics or engineering.
In short, a couple of sentences to describe something that is a fully-developed system, worked on for decades by a whole lot of very smart people (and without speaking their jargon), will almost always be unsatisfying. Words like "abelian" and "group" have very specific mathematical meanings, and that's just the basis of conversation for gauge symmetry and Yang-Mills theory. My first exposure to abelian groups, rings, and other such constructs was in abstract algebra in college. I was in there with a bunch of junior year math majors, all of whom were struggling with the concepts. Simple explanation will be just that, simple... and neither detailed not satisfying to those who want to truly understand but are unwilling to follow a very (VERY) lengthy explanation of the details. But let me give it a swing:
In short, a gauge theory is a theory about particle interactions which uses equations to describe the forces between them. "Gauge" just tells the theorist (in math jargon) the types of mathematical rules that the force equations happen to obey, which makes it possible to work with them, and which lets them know the types of shortcuts they can and can't take when they're wading through all that math.
Yang-Mills theory is a gauge theory that is specific to fundamental particles, and applies for the Standard Model of particle physics. It's basically even more specialized jargon to tell the theorist the specific sub-types of mathematical rules which their equations obey.
I told you that you probably wouldn't like it. Unless you're willing to dig into the specific meaning of the math terms underneath, there's no way to break it down and say "it's like how a potato bounces, but loses some energy doing so" or something...because the theories have so many pieces and parts that have been piled on for many decades and by many very smart people.