We still can't explain why glass is a solid

The problem

When many liquids are cooled fast enough to avoid crystallizing, they thicken smoothly until they stop flowing and behave mechanically like solids — yet under a microscope their atoms remain as disordered as a liquid. No accepted theory explains what, if anything, fundamentally changes at this glass transition. The 1948 Kauzmann paradox shows that extrapolating a supercooled liquid's falling entropy would eventually drop it below the crystal's entropy, which is impossible, hinting at an unreached 'ideal glass' state. Philip Anderson called it, in 1995, the deepest unsolved problem in the theory of solids, and physicists still disagree about the answer.

Why it matters

Glasses are everywhere, from windows and optical fibers to pharmaceuticals, polymers, and metallic glasses, and a real theory would let chemists design materials with tailored stability and aging behavior. It also sits at the heart of understanding disordered matter more broadly.

Progress so far

• 1948Kauzmann articulates the entropy paradox that any theory of the glass transition must resolve
• 1995Philip W. Anderson names the nature of glass the deepest unsolved problem in condensed-matter theory
• 2026a Nature Reviews Physics perspective lays out three still-open problems: diverging length scales, a possible ideal-glass transition, and universal relaxation

References

1. Nature Reviews Physics (2026) — 2026 perspective on three unsolved problems in glass science
2. Wikipedia — why the transition is unresolved and whether an underlying phase transition exists
3. Wikipedia — the 1948 entropy paradox and the ideal-glass idea