Of the four resemblances this project keeps circling, this is the one that tempts the loosest writing, because the words line up almost too well. Decoherence: a delicate quantum state loses its special coherence, dissolving into the ordinary classical world. Impermanence: nothing conditioned holds its form, everything dissolves into what comes next. Say both sentences back to back and they sound like paraphrases of one insight. They aren't. One describes a specific, fast, physically caused process affecting a narrow class of quantum states. The other is among the oldest and most general claims in Buddhist philosophy, addressed to every phenomenon whatsoever, at every scale, for reasons that have nothing to do with quantum mechanics and were formulated roughly twenty-five centuries before it existed.
What decoherence actually explains
A quantum system in superposition can, in principle, show interference between its different possible states — the signature that makes quantum behavior genuinely strange. In practice, that interference vanishes almost immediately for anything larger than a handful of particles, and decoherence is the mechanism that explains why. Developed from the 1970s by H. Dieter Zeh and given its now-standard treatment by Wojciech Zurek in the following decade, decoherence theory shows that a quantum system's coherence "leaks" irreversibly into its surrounding environment through ordinary physical interaction — collisions with air molecules, scattering of ambient photons, thermal noise. A dust grain's quantum interference pattern, calculations suggest, can be destroyed in a fraction of a nanosecond simply through exposure to sunlight. This is why the everyday, macroscopic world looks solidly classical even though it's built from quantum constituents: not because quantum weirdness has some size cutoff, but because interaction with a complex environment scrambles the coherence needed to see that weirdness almost instantly.
Decoherence is also, importantly, not a complete account of quantum measurement. It explains why superpositions become practically unobservable — why we don't see interference — but it doesn't by itself explain why a measurement yields one specific, definite outcome rather than a very good simulation of "many things happening but hidden from view." That remaining question, the heart of the measurement problem, is still debated among competing interpretations of quantum mechanics. Decoherence solved one part of a hard problem, cleanly and rigorously, and left the rest exactly where it was.
What impermanence actually claims
Anicca appears throughout the Pali Canon as one of the three marks of existence, alongside dukkha (suffering, or unsatisfactoriness) and anatta (the absence of a fixed, permanent self). The compact formula recorded in the canon states it plainly: "sabbe sankhara anicca" — all conditioned things are impermanent. The claim is total in a way decoherence never attempts to be: it covers mountains and mental states, civilizations and single breaths, applying not because any physical mechanism forces things to dissolve on a particular timescale, but because nothing that arises through causes and conditions can, by the nature of arising that way, be a fixed, unchanging thing to begin with. It isn't a prediction awaiting confirmation by better instruments. It's offered as something to be verified directly, through sustained attention to one's own experience, and its purpose is explicitly practical: the Buddha's teaching connects the failure to recognize impermanence to the arising of craving and attachment, and attachment to suffering. Anicca isn't there to describe the world for its own sake. It's there to change how a person relates to it.
A dust grain's coherence can vanish in a fraction of a nanosecond. Anicca was never claiming a timescale at all.
Where the vocabulary of dissolving hides the real difference
Line the two up honestly and the resemblance is almost entirely lexical. Decoherence is caused — by specific, identifiable physical interactions, in principle reversible if you could perfectly isolate a system from its environment again, and it operates on a narrow class of delicate quantum states under specific conditions. Impermanence is not, in the Buddhist framework, something that could in principle be undone by better isolation; it's presented as intrinsic to anything conditioned, with no isolated case exempt and no special apparatus required to observe it, only sustained attention. Decoherence times are measured in fractions of seconds for microscopic systems and effectively don't apply to a mountain, which is already thoroughly decohered and stays looking solid for geological ages. Anicca applies to the mountain too, on its own vastly longer timescale, precisely because the doctrine was never about coherence loss through environmental coupling — it was about the impossibility, for anything built from causes and conditions, of being the kind of fixed thing our concepts want it to be.
What's left when the pun is retired is a much smaller and, I think, genuinely worthwhile observation: physics and contemplative philosophy each independently rejected, for their own internal reasons and by entirely different methods, the intuitive picture of stable objects persisting unchanged until something happens to them. Physics found that intuition false at the level of quantum coherence, through experiment and formal derivation. Buddhist philosophy found it false at the level of direct experience, through sustained first-person investigation carried on for millennia before either decoherence or the word "quantum" existed. Neither discovery explains or validates the other. Both, taken on their own terms, are worth taking seriously — which is the only kind of claim this project is interested in making.
For the physics of entangled systems next to the Buddhist teaching of interdependence, see Quantum Entanglement and Dependent Origination. For what "observation" means in each tradition, see The Observer Effect and Mindful Attention.