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Hi, TheoryNet people.  I'd like to continue a discussion that we started
at Saturday's meeting.  For those that weren't there, I'm trying to
acquire an interferometer to take to schools to show how LIGO works.  I
said at the meeting that Marco Cavaglia at LIGO had developed such a
thing and we could try to follow his lead.  This is true, but it is more
an idea or a range of ideas than a specific design.  As far as I know,
no one has actually written up what they did as a demonstration and how
it worked.

People at the meeting were asking how what Marco developed is different
from an interferometer kit that we could buy.  We don't have one
already, do we?  Mike Hirsh, were you saying that you had one?

In any case, Marco said that I could buy a bunch of parts from Thor
Labs, or even buy their interferometer kit, which is
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=10107 So I
think the answer is that it is not very different.  This Thor kit seems
to me better than the PASCO kit,
because of being more flexible.  It is built on an optical breadboard,
so you can do whatever you want with the components, whereas PASCO's
seems to have a fixed spot for everything, so you can't change as much.

Another interesting idea is this design from the University of Glasgow:
https://twitter.com/UofGravity/status/1032643461411946496 I like it
because it locks on a fringe, i.e., there is a computer which adjusts
one of the mirrors to keep the relative lengths of the beam paths always
the same.  The readout is then how much motion was necessary to keep
things aligned, rather than motion of the fringes.  This is the same way
that LIGO works.  However, it's possible that it's worse for
demonstrations, because it requires some additional understanding to
make sense of this idea.

In any case, I'll try to find out whether the Glasgow people have
written up their design.