Contemporary Physics Top 100 Dilemmas, Episode 95: the glass transition problem. Start with something you can picture immediately: hot syrup cooling on a spoon, or molten glass slowly losing its glow. At high temperature, it can still flow. The molecules are crowded and disorderly, like people in a packed station, but they can still step aside, trade places, and reorganize. Now cool it quickly. The material does not have time to line up into the neat marching grid of a crystal, yet it also stops behaving like an ordinary liquid. Structurally, it still looks messy, like a soup that never formed ranks. Mechanically, it becomes hard and solid-like. Its viscosity and relaxation time can shoot upward by many orders of magnitude, as if every main road in a city were being closed one after another. Inside one piece of glass, some regions can still rearrange a little, while others are like old blocks with every alley sealed. That is the puzzle. The material looks frozen, but you cannot point to a new crystal lattice. It acts like a solid, but its internal arrangement still looks like a trapped liquid. Mainstream physics struggles with this in-between character. If the glass transition is a true thermodynamic phase transition, where is the clean order parameter? When water freezes, a lattice appears. If glass formation is only kinetic arrest, then why do the time scales explode so violently? Why does a door that was still movable suddenly become almost impossible to open? Different theories each hold an important piece: energy landscapes give the valley maze, RFOT treats hidden rearrangement as a special transition, and facilitation explains how one mobile patch helps another move. These tools matter, but the full chain is still hard to make visible: why can a material gain rigidity without obvious long-range order, and why do the flow channels nearly all close before global crystallization finishes? EFT begins by rewriting the word “phase.” A phase is not just a label on a material ID card: gas, liquid, solid, crystal, glass. In EFT, a phase is a long-lived working mode of a node-and-connection network under a given sea condition, temperature, boundary, and disturbance pattern. From that view, crystal and glass are not simply “structure” versus “no structure.” Glass has structure. What it lacks is completed large-scale self-consistency. A crystal is like a newly planned city: road directions, block interfaces, and traffic rules all belong to one shared plan. Glass is like an old city hit by a sudden snowstorm. Each district closes gates, builds temporary detours, and settles just enough to survive locally, but the cross-city highways never finish opening. Then the glass transition is no longer a mysterious leap from disorder into solidity. It is the gradual shutdown of large-scale rearrangement corridors. As temperature falls, the beat slows. Structures have a harder time crossing local rearrangement thresholds. Small clusters can still find temporary stable poses, like each neighborhood parking its own cars wherever it can. But reorganizing the whole city takes too long. The system becomes trapped in a rough self-consistent network: locally settled, globally unfinished. Glass is not perfectly random, and it is not fully ordered. Local ledgers have closed; the global ledger remains unpaid. Why does viscosity explode? Because flow is no longer a few molecules casually changing seats. It requires a chain of locally closed regions to unlock, make room, pass the motion along, and lock again. The cost becomes like a growing chain of approvals. Why does relaxation become so slow? The system has not lost every route, but only a few narrow repair corridors remain. It must wait for the right defect, gap, vibration window, or soft spot before it can move forward by a tiny amount. Why does aging exist? Because the network was not finished when it froze. It was only pinned by its cooling history. Afterward, it keeps settling through the few slow channels that remain, sinking toward lower-cost local arrangements. History dependence also becomes natural. The path into the glass decides which gates close first, which defects survive, and which corridors get blocked. Dynamic heterogeneity follows from the same picture: some districts still have a side street open, while others are locked into a dead end. Fast and slow patches coexist because the network never reached one completed global plan. One guardrail matters. EFT is not saying energy landscapes, RFOT, facilitation, simulations, thermodynamics, or laboratory measurements are useless. They remain valuable lenses. What EFT rejects is the habit of first demanding one elegant label and then forcing the material to fit it. The better question is not only “Does glass deserve to be called a phase transition?” The better question is: which rearrangement corridors are still open, which local networks have temporarily closed their accounts, which defects have been frozen by history, and whether the system can still complete global self-consistency. In this language, glass is not a structureless solid, and it is not a failed liquid. It is a material city already occupied locally while the full city plan was never completed. It is hard because the roads are locked. It is slow because the repair corridors are too few. It ages because the total account is still being settled at an almost invisible pace. The mystery of the glass transition comes from this middle state: not a clean cut between disorder and order, but a step-by-step darkening of the large-scale corridors between free flow and rough self-consistency. Open the playlist for more. Next episode: the universality and intermittency of turbulence. 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