Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

[Phys-l] barrels of gauge invariance

In the context of shotgun barrels,

On 01/19/2010 08:23 AM, chuck britton wrote:

That word 'gauge' has TOO MANY meanings ! ! ! ! !
Why can't it be INVARIANT ! ! ! ! ;-)

I guess you've got us over a barrel ... or should
I say over a gagga?

On 01/19/2010 10:47 AM, Bernard Cleyet wrote:

not field theory

Actually it is the same as field theory.

All uses of the word "gauge" have the same root and
the same core meaning. This includes shotgun barrels,
wine barrels, railways, field theory, et cetera.

The common meaning is a standard of measure, a reference
against which things are measured ... from gagga, an old
standard for the volume of wine casks.


As we discussed back in 2001, I would say a gauge is:
1) A system of measurement based on an artifact or an arbitrary reference
2) An artifact used to define such a system, or to propagate it
3) By extension, any measuring device.


Pedagogical remark: If an introductory-level student asks
what gauge invariance is, point out that a voltmeter has
two leads. It doesn't work if you only hook up one of the
leads. Why? Because there's a deep principle of physics
that says voltages are gauge-invariant.

Funny story: Once upon a time, a hiring candidate started
to tell me about his thesis work, about how gauge invariance
placed strict limits on how much CMRR (common mode rejection
ratio) and line-noise rejection ratio certain types of
circuit could have. I said I understood the idea. He
kept on explaining. I insisted he didn't need to explain
it. He was shocked, and said I was only the second person
he had met who knew what he was talking about. He said
he had nearly flunked his PhD exam, because the examiners
didn't think his methods were valid. I said, "well, you
don't need to deal with them anymore. Whatever you did
to come up with the insight about the gauge invariance
of circuits, keep doing that. Working out sophisticated
solutions to real-world problems is a big part of what
we do here."