Just curious, how was the presentation on Phys-L received?
Well, I'm not the best one to judge how my presentation was received... but
there were a number of positive indications.
-- Tom Greytak asked some penetrating questions about how the
electrostatics spreadsheet worked, starting with how to arrange for the
boundary conditions to be gauge-invariant (which is indeed the part that
most people get wrong the first time). We also discussed how a
spatially-discrete model could exhibit _exact_ local charge conservation
and _exact_ gauge invariance, even though the model was only an
approximation to Maxwell's equations for the continuum. Then he said,
"This is wild. They should just give that spreadsheet to every freshman."
-- I mentioned that a lot of people start out with the misconception that
there is a dead short to phi=0 "at the edge of the universe". I said "I
don't think so... Every voltmeter I've ever seen has two leads, not one,
and I think this has something to do with gauge invariance." Everybody
laughed at this point. I take it as a very good sign when people laugh at
my jokes. Sometimes when I give the identical talk to two different
audiences, one will laugh and the other won't.
-- David Mermin and Bob Richardson asked deep questions about how to
infer cause-and-effect relationships from the data.
-- In order to make the point that you cannot infer _causation_ from
typical data (even when you can infer _implication_ from the data), my talk
included the riddle: "Suppose you have a certain ZIP Code where lots of
people have horrible weird diseases. Policy question: should you move
them out?"
....... I let the audience think about that a while.......
Then I pointed out that if the ZIP Code was Love Canal, NY, then yes, the
people would be better off almost anywhere else. However, if the ZIP Code
was the Mayo Clinic, then the people would be worse off almost anywhere
else! Again, people laughed quite heartily. Again, that's a sign that I
was on my game.
-- After the talk, I was mobbed by people who wanted to play with my
masking-tape model of geodesics in curved spacetime, and wanted to know how
to make the "darts". (See next message.)
-- Over the next day or so, a number of people asked me how to sign up
for Phys-L. We'll see how many actually do.