The Supreme Court tells us that a Fourth Amendment “search” is a matter of “reasonable expectations of privacy.” Scholars meanwhile debate “search” on the axes of value, doctrine, institutionalism, interpretation, and judicial politics. Yet neither prevailing judicial doctrine nor normative academic discourse has had much impact on the Court’s actual “search” interpretations. This article suggests that this static between “paper” rules and “real” rules (and, more generally, normative prescriptions and judicial decisionmaking) is a function of a deep constraint on the judiciary’s capacity to form “search” doctrine in free accordance with evolving juridical and policy norms. This constraint is one that I call the “atomic code.”

The atomic code has three properties. First, justices evaluate each “search” issue without regard for coherence across the spectrum of “search” issues. In effect, the “search doctrine” is simply an “aggregation of search ‘atoms.’” A second property of the atomic code is that justices attribute content to new search atoms through analogies to old search atoms. While initially every search atom is dormant, over time a justice, either informally or through adjudication, will gradually attribute content—“search” or “non-search”—to her atoms by drawing from the content of analogous search atoms. By this account, the holding in Olmstead v. United States reflects nothing grander than the endorsement by five justices of an analogy between wiretapping and visual surveillance from a public vantage point. The third property of the atomic code is that a justice will not reassess an atom’s content once she has made her initial attribution. One might think of this as stare decisis writ small: a justice (but not necessarily the Court) will decide like cases alike. And so the overturning of “search” precedent is not the result of a majority’s new attribution of content to their search atoms, but, more accurately, a new majority of initial attributions.

I should emphasize at the outset that the atomic code is not simply the result of a vague doctrinal test, suboptimal or inaccessible empirical data on “privacy expectations,” or judicial politics. Rather, the code’s roots run deeper. It is in large part attributable to two factors. One is the concreteness of the term “search.” The other is the justices’ preference for a calibrated retroactivity of criminal procedure rules. These two factors, more than any others, have caused justices to treat the “search” doctrine as atomistic, to attribute content through analogical reasoning, and to fix content upon attribution.