5 ms·

It's kind of funny that even in glassblowing circles this myth gets propagated...where the old methods of making panes are generally well known and understood (

It's kind of funny that even in glassblowing circles this myth gets propagated...where the old methods of making panes are generally well known and understood (see the picture of the crown glass in the article).

Florin_Andrei 12y ago
If glass did flow at room temperature, telescopes would be a mess. 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. --- 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. --- (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".)
- abecedarius 12y ago
  Great way to connect this to other things we know about -- thanks. a few minutes: I'd guess more like a few days: 1 mm/100 years = 10 micron/year = .1 micron / 3.65 days.

Florin_Andrei 12y ago

If glass did flow at room temperature, telescopes would be a mess. 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. --- 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. --- (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".)

abecedarius 12y ago
Great way to connect this to other things we know about -- thanks. a few minutes: I'd guess more like a few days: 1 mm/100 years = 10 micron/year = .1 micron / 3.65 days.