Yesterday the Supreme Court handed down the most important privacy case of the Roberts era, U.S. v. Jones. The unanimous decision is an occasion for dancing in the chat rooms. In holding that the government needs a warrant before attaching a GPS device to a suspect’s car to track his movements 24/7 for a month, all the justices rejected the Obama administration’s extreme and unnecessary position that we have no expectations of privacy when it comes to the virtual surveillance of our movements in public places. Although the majority decision is narrow and many hard cases involving virtual surveillance lie ahead, all of the justices acknowledged that round the clock surveillance is far more invasive than tracking someone’s movements for a day. Now it’s up to Congress to fill in the legal gaps for GPS tracking and for the Court to build on this landmark decision in future cases.

Writing for five of his colleagues, Justice Antonin Scalia’s majority opinion focused on the fact that the police had committed a physical trespass when they put a GPS device on the bottom of a suspect’s car without a valid warrant. “The government physically occupied private property for the purpose of obtaining information,” Scalia wrote, explaining that such a physical intrusion doubtlessly qualifies as a “search.”

Scalia’s decision is welcome because it insists that even the smallest physical intrusions on private property require a warrant. But with his focus on physical trespass, Scalia does little to resolve the virtual surveillance questions that are on the horizon. As Justice Samuel Alito pointed out in his insightful concurrence—joined by Justices Ruth Bader Ginsburg, Stephen Breyer, and Elena Kagan—according to Scalia’s reasoning the government would need a warrant to put a GPS device under a suspect’s car but not to active a GPS tracking system already embedded in the car. And, as Alito also noted, smartphones allow Apple (or the police) to track a user’s movements 24/7 without any sort of physical trespass.

With his concurring opinion, Alito has cemented his reputation as the Supreme Court’s top privacy cop—one of the justices most sensitive to the impact of new technologies on expectations of privacy. As a Princeton student in 1971, Alito chaired a student conference on surveillance technologies, which concluded, “We sense a great threat to privacy in modern America.” In the Jones case, Alito proposed a welcome framework for meeting those threats, distinguishing between short-term monitoring and the longer-term GPS monitoring that impinges on our actual expectations of privacy. Refusing to identify the precise point between a day and a month when the surveillance become unconstitutional, Alito said that when the police are unsure, they can always get a warrant.

Justice Sonia Sotomayor, in a separate concurrence, indicated that she might in the future go further still. She asks if we should reconsider what’s known as the “third party doctrine,” which suggests that if I surrender my search terms to Google or my geolocation information to Apple, I have to assume the risk that Google and Apple will turn over that information to the government. Now that most of our private papers are stored not in locked desk drawers but on distributed servers in the digital cloud owned by Google and Yahoo, the Court does indeed need to revisit the third party doctrine if citizens are to have as much privacy in the twenty-first century as they did in the eighteenth. Given Sotomayor’s willingness to go beyond Scalia in translating the Constitution in light of new technologies, it’s surprising that she provided a fifth vote for his narrow opinion, rather than joining Alito and her liberal colleagues in making clear that the Constitution imposes limits on even long-term surveillance that doesn’t involve a physical trespass.