News for hackers. Stuff that matters.

A comment on We Know Simple Fluids Can Flow. Turns Out, Some Can Fracture. Parent comment

Re:We Know Simple Fluids Can Flow. Turns Out, Some Can Fracture by vlovich123 on Sunday July 12, 2026 @01:12AM (#48878465)
Nope, lots of fluids that just flow over such a long period they appear solid.
[ Parent | on HN ]
Re:We Know Simple Fluids Can Flow. Turns Out, Some Can Fracture by vlovich123 on Sunday July 12, 2026 @01:31AM (#48878570)
Not sure why klustregrif got flagged - the answer was informative and correct.

What I said is true but not for glass. Pitch is a liquid even though it feels like a solid and shatters when smashed.

[ Parent | on HN ]
Re:We Know Simple Fluids Can Flow. Turns Out, Some Can Fracture by bonesss on Sunday July 12, 2026 @01:28AM (#48878552)
That ‘long period’ can be many billions of years, glass is an amorphous solid.
[ Parent | on HN ]
Re:We Know Simple Fluids Can Flow. Turns Out, Some Can Fracture by klustregrif on Sunday July 12, 2026 @01:26AM (#48878532)
That’s a misconception. Glass does not flow over time; it is a rigid, amorphous solid. The uneven thickness seen in old cathedral windows is a result of historical manufacturing. Glass was spun into discs that naturally became thicker at the edges, and builders installed the heavier side at the bottom for stability. Physicists calculate that it would take longer than the age of the universe for room-temperature window glass to visibly deform under gravity. Reference: Zanotto, E. D. (1998). "Do cathedral glasses flow?" American Journal of Physics, https://doi.org/10.1119/1.19026
[ Parent | on HN ]
Re:We Know Simple Fluids Can Flow. Turns Out, Some Can Fracture by autoexec on Sunday July 12, 2026 @02:19AM (#48878803)
> That’s a misconception. Glass does not flow over time;...Physicists calculate that it would take longer than the age of the universe for room-temperature window glass to visibly deform under gravity.

so then it does flow over time, just not on a scale where it matters to us.

[ Parent | on HN ]
Re:We Know Simple Fluids Can Flow. Turns Out, Some Can Fracture by rsfern on Sunday July 12, 2026 @08:00AM (#48880453)
But this is true for lots of solids, not just glass. Consider creep deformation [0] which is deformation mediated by diffusion of defects in solid materials. It’s a big problem in metal turbine blades, it limits the maximum usable temperature to well below the melting point.

The physical mechanism would be similar in glass flowing in this way, so I don’t think evidence of glass flowing like this should make us think of it as a liquid instead of an amorphous solid

0: https://en.wikipedia.org/wiki/Creep_(deformation)

[ Parent | on HN ]
Re:We Know Simple Fluids Can Flow. Turns Out, Some Can Fracture by klustregrif on Monday July 13, 2026 @01:24AM (#48888253)
How do you go from “calculate that it would take longer than the age of the universe for room-temperature window glass to visibly deform” to “so the. It does flow”?

Glass flows a higher temperatures where it does act like a liquid, you can study glass blowing for examples, however no, room temperature glass is not a liquid and does not flow, for any practical definition of what flowing is.

[ Parent | on HN ]