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]

Re: Glass does not flow



Put a piece of glass in one arm of a Michelson interferometer. Assume the
glass in Jim Green's house thickens by, say, 0.1 mm in 130 years. Using
an index of refraction of 1.5 we obtain a two-way optical path difference
of 0.10 mm due to the thickening. For light of wavelength 0.0005 mm (500
nm) that's 200 wavelengths, or about one fringe every eight months. We
have a Michelson interferometer with a compensation plate that is thicker
than a normal windowpane. It is more than thirty years old, and shows no
sign of having lost its optical figure. If you consider much more
sensitive tests, like the one-meter diameter refractor at the Yerkes
Observatory which is more than a hundred years old and has kept its
youthful figure to within a fraction of a wavelength over that time you
will surely see that flow is inappreciable over such a period. Even so,
glass reflecting telescopes are prophylactically pointed to the zenith
while not in use to minimize the effects of creep*.

Such is the power of myth even among those best able to see it as such.

Leigh

*These telescopes are systematically tilted in the same direction
whenever they are in use, so one would expect them to show the effect
if creep were real. Perhaps the mirrors are rotated by one hundred
eighty degrees after each recoating to minimize this effect. It would
lead to some complications. Anyone know if that is done anywhere?