Thursday, August 23, 2018

[unsqsuuh] Measuring glass flow with an interferometer

Glass is a liquid, and the myth is that centuries-old glass panes in old cathedrals show evidence -- noticeable deformation -- due to liquid flow.  This myth can be tested with an interferometer.

Assume "noticeable deformation" is 1 mm, and conservatively assume that the flow of glass takes 1000 years to cause noticeable deformation.  That is equivalent to 1 micrometer per year, or 100 nm every 36.5 days, which is a reasonable amount of time to run an experiment.

100 nm is 1/4 of the wavelength of 400 nm blue laser light.  sin(2pi/4) = 1.  This means that if you had calibrated your interferometer to have perfect destructive interference, minimum intensity, at the beginning of the experiment, the glass will have deformed enough to cause perfect constructive interference, maximum intensity, at the end of 36.5 days.  If the glass flows faster than our conservative assumption of 1 mm per millennium, one will see maximum intensity sooner.

The myth of cathedral glass flowing has already been busted, but it doesn't hurt to repeat the experiment yourself because replicability is the heart of science.

If glass really did flow that fast, astronomical telescope lenses and mirrors would have serious problems in a short period of time.  These instruments need to maintain precision to within a small fraction of wavelength of the light they observe.  Every high precision optical experiment would be extremely difficult as its glass mirrors, lenses, prisms, and windows change shape during the course of the experiment.

This source says glass flow at room temperature requires the age of the universe or longer to observe, not 1000 years.  Can that very slow flow be measured with sophisticated interferometry techniques, for example those used by LIGO?

No comments :