Imagine this scenario: The government learns there might be a dirty bomb hidden in your town, so the police start driving around with Geiger counters, looking for houses that have more radioactivity than normal. (Assume this is practically feasible, though there may be various potential difficulties with it.) Enter the courts: "No, no, no," they say, "that's an unconstitutional search, because it is an effort to determine what's in people's homes without probable cause and a warrant."
"Insanity!" some might say; and I'm pretty sure that judges would not, in fact, find a constitutional violation in such a situation. But existing Fourth Amendment precedents suggest that such police searches would indeed be illegal. And this, I think, helps show that the war on terror requires some special rules that differ from those used in ordinary criminal investigations.
The law's logic is often counterintuitive, even when it's ultimately sound. Communists, Nazis, and Islamo-fascists, for instance, are currently entitled to the same First Amendment protection as Democrats and Republicans. It may seem odd to let people claim rights under the very Constitution that they are trying to subvert. But bitter experience has shown us that the power to punish such bad speakers often ends up being used to punish good ones, too. Likewise, it might seem reasonable to provide less Fourth Amendment protection when the government is investigating really serious crimes, like murder. Why not give the police carte blanche, for instance, to search murder scenes?
But the Supreme Court unanimously rejected such a rule in a 1978 case, Mincey v. Arizona; and even Justice William Rehnquist—no fan of broad readings of the Fourth Amendment—agreed on this point. First, more serious crimes may make the police even more zealous and may thus make it even more necessary to guard against police overreaching. And beyond that, the court was understandably reluctant to draw constitutional lines based on subjective estimates of "seriousness." As Justice Potter Stewart (hardly a doctrinaire liberal) wrote in Mincey:
[T]he public interest in the investigation of other serious crimes is comparable [to the interest in investigating murder]. If the warrantless search of a homicide scene is reasonable, why not the warrantless search of the scene of a rape, a robbery, or a burglary? No consideration relevant to the Fourth Amendment suggests any point of rational limitation of such a doctrine.
So is a police car driving a Geiger counter through my neighborhood unconstitutional? (John Elwood, writing in a legal journal in the summer of 2001, was, to my knowledge, the first to raise this question.) Here's some purely precedential logic about that:
1. Pointing infrared thermal imagers at a home (for instance, to find evidence of heat lamps used to cultivate marijuana) constitutes a "search" under the Fourth Amendment. That's what the court held—in an opinion by Justice Antonin Scalia!—in last year's Kyllo v. United States. The court found that the government's "obtaining by sense-enhancing technology any information regarding the interior of the home … constitutes a search." And such searches of homes are allowed only if the police get a warrant based on probable cause to think that this particular home contains evidence of a crime.
2. A Geiger counter is sense-enhancing technology that detects information regarding the interior of the home: whether there's an unusual amount of radiation (rather than heat) present in the home.
3. As I noted above, the Fourth Amendment generally applies equally to investigations of all serious crimes.
4. Therefore, based on the most analogous precedent, citywide sweeps using a Geiger counter to detect radiation coming from homes—sweeps not based on any individualized probable cause as to any particular house—are unconstitutional.