Cell phones, the Indiana Supreme Court observed yesterday, are "double-edged swords, increasing convenience at the expense of privacy." That court then demonstrated just how expensive the costs to privacy can be.

Yesterday's ruling in Zanders v. Indiana centered on one of most pressing questions in modern Fourth Amendment law: Namely, when the police want to track somebody's movements via that person's cell phone location data, do the police need a search warrant in order to get that data from the person's cellular service provider? In this particular case, the police obtained Marcus Zander's cell site data without a warrant and used that information to trace his whereabouts during the time periods in which several armed robberies had been committed. Those cell phone records were later used against Zanders at trial.

"Zanders presumptively knew that his phone makes and receives calls by sending signals to towers," the Indiana Supreme Court said, "and that Sprint keeps records of these signals for business purposes like billing and tracking tower usage." Because Zanders had no "reasonable expectation of privacy" in such records, the Indiana Supreme Court ruled, the Fourth Amendment offers him no protection when the police obtain such records without a warrant.

The Indiana Supreme Court claimed that its hands were tied and that it had no choice but to rule in favor of the police on account of several U.S. Supreme Court precedents. In Katz v. United States (1967), for example, the U.S. Supreme Court held that "what a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection." Even more significantly, in Smith v. Maryland (1979), the Court ruled that "a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties."

Fourth Amendment advocates are increasingly urging the U.S. Supreme Court to reconsider those precedents in the digital age. As I noted in a column earlier this week, five different petitions are pending before SCOTUS right now that ask the justices to step in and review lower court rulings that have allowed warrantless police searches of cell site data. Those five cases, as I explained, all turn on "the conflict between the Fourth Amendment right to be free from unreasonable search and seizure and the third-party doctrine's vast grant of warrantless search and seizure powers to law enforcement agencies." We may found out as early as next week if the Court agrees to take one or more of those cases.

It all boils down to this: How many Fourth Amendment protections do we forfeit when we use a cell phone? That's the question that the U.S. Supreme Court is eventually going to have to face.