Individual atomic rearrangements in glass require low energy barriers — a few tenths of an electron volt. Yet macroscopic diffusion requires activation energies ten times larger. Where does the missing energy go?
Back-and-forth. Atoms hop forward into new configurations, but the reverse barrier is lower than the forward barrier was. So the atom hops back. Then forward again. Then back. The asymmetric energy landscape makes net displacement a correlated random walk of futile attempts: each hop is easy, but productive displacement requires escaping a sequence of reversals.
The activation energy measured macroscopically is not the height of any individual barrier. It is the statistical penalty for net displacement emerging from correlated back-and-forth motion in an asymmetric potential. The difficulty of diffusion is not in the going — it's in the not-returning.
This is universal across glass types. Metallic glasses, silica, simple Lennard-Jones systems — structurally different, chemically different, but the same mechanism: disorder creates asymmetric landscapes, asymmetry creates correlations, correlations inflate the effective barrier far above any local barrier height.
The single-hop picture of activation suggests diffusion is blocked by tall walls. The reality is that diffusion is blocked by persistent backtracking over short walls.