Measurement Without Observers: A Layered Reading of Causal Links and Decoherence

## Abstract Textbook quantum mechanics defines measurement using ideas that are not themselves basic physics: "observer," "macroscopic apparatus," "irreversible record." We take this definition apart and find that it bundles two different things: the physical event of an interaction, and the statistical fact that the event's result becomes hard to erase. We separate them. A measurement, in its minimal form, is a causal link: when event A and event B interact, a fact is established between them — B's state relative to A. This happens at every interaction, at every scale, with no apparatus required. What laboratories call "a measurement" is the same kind of event, after its result has spread — fanned out — into so many further causal links that undoing it is no longer practical. This reading gives the quantum eraser a clean explanation: an outcome is erasable exactly as long as its causal fan-out stays narrow. It also clarifies what decoherence theory does and does not do: decoherence is the physics of fan-out, and it does that job well; the question it was criticized for not answering (why one outcome, with Born-rule probability) belongs to a different layer. Nothing in the existing formalism changes. What changes is the definition — and with it, the need for a separate "collapse mechanism" disappears. Because the basic unit here is a causal link, the whole picture can be built directly on discrete causal structures, which we identify as the natural next step.