UCLA School of Law's Professor Eugene Volokh has a very interesting piece in Slate concerning surveillance technology and the Fourth Amendment (The Fourth Amendment Meets the War on Terror) [Note Prof Volokh's great collaborative blog, The Volokh Conspiracy]. Although I agree with much of Prof. Volokh's reasoning, I don't believe his example makes the case.
In this article. Prof. Volokh proposes the following hypothetical:
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."
The reason for this speculation is that, in an important case decided last year (Kyllo v. United States), the Supreme Court determined that infrared surveillance of a home was a "search" that required a warrant to be constitutional. Taking a broad reading of the decision (which is justified since Kyllo is a broad ruling), Prof. Volokh reasons that police sweeps of a neighborhood streets using geiger counters (such as this nifty device) could be held to be unconstitutional.
Nevertheless, Prof. Volokh rightly concludes that it is unlikely that any judge would prevent the police from trying to find a terrorist's cache of nuclear material. His reasoning is that potential terrorist actions change the balance of Fourth Amendemnt protections: "finding dirty bombs must simply be different from fighting normal crime." While I agree with his analysis that what is a "reasonable" search is dependent on the exigencies of the crime, I am skeptical of a call that we distinguish "terrorist" searches from normal crimes.
In the present scenario, for example, I believe that no change to Fourth Amendment doctrine needs to be made with regard to the use of geiger counters on the streets. As part of his logic, Prof. Volokh states that, "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." I believe this is an error. A geiger counter measures local levels of radiation. Thus, when using a geiger counter, one isn't detecting information regarding the interior of the home, but the local level of radiation in the street. Using a geiger counter to measure local levels of radiation is similar to using a very sensitive thermometer to measure the local temperature in the street, which is quite unlike even a crude thermal imaging device.
Now, perhaps, a directional geiger counter (rather unusual devices in the first place) directed toward particular homes would be unconstitutional. However, why would one want to use a directional geiger counter in the first place? Presumably, law enforcement only has a general idea of the location of the radioactive materials. Wouldn't they be better off using a regular, omnidirectional geiger counter to localize the materials in the first place? Using such devices, it should be possible to localize the radiation source enough to justify a probable cause search.
Furthermore, I am not sure that there is any expectation of privacy for the production of gamma radiation (the most likely type to be detected from the street). One has to ask, "what the hell are you doing if you are producing significant amounts of gamma radiation in your home?" The fact that one is capable of measuring above-background levels of gamma radiation from the street at all is probably direct evidence of the violation of numerous NRC, OSHA and EPA regulations. In such cases, I don't believe that society is prepared to recognize as reasonable an expectation that gamma radiation levels cannot be measured in the street.
Perhaps there is a need to make adjustments to the Fourth Amendment. However, until a more compelling case can be found, I remain skeptical.