Glass Doesn't Flow (2011)
cmog.org
cmog.org
I hoped would article would touch these special properties of glass.
http://en.wikipedia.org/wiki/Glass#Formation_from_a_supercoo...
I was furious, but my dogged insistence that I was right was starting to draw glares from both of the women, so reluctantly, in the name of peace, I dropped it. Then the ass had the gall to say "Haha, don't worry, I'm sure there's a lot of things about webpages that you could correct me on!"
The worst part is, if I sent him this article, and any number of others proving the same point, he'd just say "what? why are we talking about glass?" That conversation was nothing to him, and I'm sure he's already forgotten it.
This is precisely why authority has no role in science, why evidence outranks eminence.
"Science is the organized skepticism in the reliability of expert opinion." — Richard Feynman
All the error you're allowed when making the mirror of a reflecting telescope for visible light is 0.1 microns, or 100 nm. That's the top limit; a good telescope needs to push that down at least 2x or 3x, if not more. The mirror must remain within that limit for the entire lifetime of the instrument.
Now picture this big hunk of glass, resting on 3, 9, or 27 support points (or much more than that for the large professional scopes). It's not sitting on a flat table; it's sitting only on a few narrow points on the back, with weight distributed equally among all points. It is so thick so as to prevent it from sagging under its own weight and distorting the active surface via elastic deformation.
If glass was a "liquid" to the extent that cathedral windows would thicken noticeably at the bottom over centuries, how long would a telescope mirror remain within 100nm? A few minutes before it got irreversibly warped like icecream in the summer? Yet there are many telescopes still in use after more than 100 years - and the optics are just as good now as they were in the beginning.
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Before anyone tries to respond with a counter-argument: first try and realize what 100 nm really is. If you take one strand of human hair and cut it in two, and look with a microscope at the fresh section - if the surface of that fresh cut was the size of the rug in your living room, 100 nm would be the size of a large coin on that rug.
Think about that.
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(I make telescope mirrors and I've lost track how many times I've told this argument whenever there's a discussion about "glass is a liquid".)
a few minutes: I'd guess more like a few days: 1 mm/100 years = 10 micron/year = .1 micron / 3.65 days.
Does lead flow?
The article dismisses it but only in a casual way.
http://en.wikipedia.org/wiki/Pitch_drop_experiment
More seriously, there's possibly a viscosity at which gravity is no longer enough acceleration to overcome whatever mechanisms are holding the atoms together. If we assume that metallic lead does not exhibit any flow on Earth, then we can estimate this as being between 10^10 and 10^11P at g.
A lot of things once thought impossible have been proven to be possible. Offering an explanation of why it's impossible adds nothing to the table.
[EDIT] Missed the bit in the middle. That said, I still find the use of jokes and second hand experiences a bit weak in evidentiary terms. Would have been much more convincing argument if there was a count or documentation detailing how many panels were truly installed one way or the other.
"Half of the pieces in a window are thicker at the bottom, he said, but, he added quickly, the other half are thicker at the top. My own experience has been that for earlier windows especially, there is sometimes a pronounced variation in thickness over a distance of an inch or two on individual fragments. That squares with the experience of conservators and curators who have handled hundreds of panels. Although the individual pieces of glass in a window may be uneven in thickness, and noticeably wavy, these effects result simply from the way the glasses were made."
With today's production methods and quality control, we really don't have this issue. Glass panes are pretty consistent, but from the point of view of some 18th century carpenter, you probably had to deal with a lot of inconsistent glass panes.
"Orowan had quipped that there might, indeed, be some truth to the story about glass flowing. Half of the pieces in a window are thicker at the bottom, he said, but, he added quickly, the other half are thicker at the top."
Which reads to me as a joke. I note this simply because it seems sub-optimal to introduce myths about the thickness of glass in old cathedrals when correcting a myth about glass in old cathedrals :)
Rather, they would sag and warp, eventually becoming non-flat if there was any small deviation from verticality. They would remain thicker for longer around the edges (assuming the pane is embedded solidly in a frame) and any support elements that hold it firmly. The top edge would sag more quickly in the middle. Heck, prevailing winds, if any, would slowly push the pane inwards, like a soap bubble.
The neat shallow wedge shape simply cannot be obtained via regular viscous flow in a near-vertical pane.
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Also, if mere gravity could change the shape that much, any load-bearing glass element supporting a heavy item would sag visibly in a much shorter time. I'm thinking of the glass table supporting my very large, very heavy plasma TV.