You run a nightclub. You also drive a Jeep. And sometimes you use the Jeep to bring cocaine and cocaine base to your nightclub.
The police suspect you. They get a warrant to put on your Jeep a device that will allow the police, by using global positioning satellite technology, to track where your Jeep goes. But they fail to comply with the warrant, which issued from the District of Columbia, by attaching the GPS signal-emitter to your Jeep in Maryland. That doesn't keep them from watching you via satellite for about a month. And they use the info to help convict you on drug charges.
You appeal on the ground that the fourth amendment barred the police from tracking your Jeep's movements. And, today, the Supreme Court holds that the warrantless use of the GPS thing did amount to an unlawful "search" under the fourth amendment.
Justice Scalia's majority opinion, which Chief Justice Roberts and Justices Kennedy, Sotomayor, and Thomas joined, based the ruling on the fact that in Merry Olde England the means the police used to gather info on you would have counted as a tort — trespass to chattels. United States v. Jones, No. 10-1259 (U.S. Jan. 23, 2012).
Justice Sotomayor concurred to point out that in the digital age, which requires us to share info all the time, the fourth amendment ought not "treat secrecy as a prerequisite for privacy." Slip op. at 6.
Justice Alito concurred in the judgment, urging that the case should turn on your "reasonable expectations of privacy" and not whether the police committed a trespass in the 1791 sense. Slip op. at 2. Justices Breyer, Ginsburg, Kagan, and Sotomayor joined in the Alito concurrence.