Quantum mechanics as usually taught bolts on an unnecessary extra rule

Carroll argues that the standard, textbook version of quantum mechanics — often called the Copenhagen interpretation — actually requires two separate rules: one smooth, deterministic equation describing how quantum states evolve (the Schrödinger equation), and a second, entirely different rule that says measurement causes the state to instantaneously and randomly 'collapse' into one definite outcome. These two rules coexist uneasily and are never derived from each other.

His complaint is that this collapse rule is never explained by the theory itself — it's simply asserted as something that happens whenever a measurement occurs, without a mechanism, without a precise definition of what counts as a 'measurement,' and without deriving randomness from anything more fundamental. Physics generally prefers unified, minimal frameworks, and this bolted-on second rule violates that preference.

Carroll treats this awkwardness not as a minor technicality but as a sign the standard interpretation is incomplete, motivating his search for an interpretation that only needs the one smooth equation and nothing extra. A theory that needs an unexplained extra rule just to handle measurement is a theory hiding something it hasn't figured out yet.