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# [Phys-l] barrels of gauge invariance

• From: John Denker <jsd@av8n.com>
• Date: Tue, 19 Jan 2010 12:25:30 -0700

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.

Reference:
http://www.leoyan.com/century-dictionary.com/nph-chw.php?query=gage&type=dicts

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