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TheoryNet gravitation goes to Danbury

My friend is teaching physics at the Wooster School in Danbury, CT.  So
I spent two days there meeting with about 50 ninth grade students spread
across 4 classes.  I met with each class twice.  The first day we did
the Cavendish experiment and the second day I talked about gravitational
waves and showed the interferometer.  I think this is the furthest reach
so far of TheoryNet and the first visit in Connecticut.  We have now
gone to schools in every New England state but Maine.

I was able to do the Cavendish experiment four times with no particular
issues.  The essential things for this to work well are having a sturdy
table on a sturdy floor and going the day before to set things up and
leave time for damping of the pendulum motion.  If anyone ever wants to
do it, I have some detailed notes.

With two of the classes we computed the mass of the earth, getting a
result that was around half the correct value.  Later I realized that
this was because I hadn't accounted for the laser beam reflected by the
mirror on the pendulum turning through twice the angle that the mirror
turns.  This gave me the chance to come back the next day and explain
the mistake, so perhaps they learned something about how scientists make
mistakes and how we detect and correct them.  With this correction the
result was within about 20%.

The interferometer was a success, though it's always heard to tell what
students are getting out of something.  The comparison between a
tabletop interferometer and LIGO is quite straightforward.  With my
setup I could probably measure 0.01 of a fringe and detect gravitational
wave strain of 10^-8.  LIGO's beams are 3000 times longer, and the light
goes back and forth 300 times before interfering.  They also are
sensitive to about a 10^-10 of a fringe.  These multiply to an
improvement of 14 orders of magnitude and so a strain sensitivity around

Since this instrument cannot detect gravitational waves, I detected
sound waves instead.  I put a photosensor in the interference pattern
and connected it to the microphone input of my tablet.  Then when I
spoke next to one of the mirrors, the tablet could record and play back
my voice from the interference pattern changes.  But I'm not sure this
really added anything to student understanding.

Another thing I learned is that visiting in the next to the last week of
school may be less than ideal.  Some of the students had their minds on
other things.