“I am not beyond the belief that someday a provider could turn on my cell phone and listen to my conversations,” Justice Sonia Sotomayor said on Wednesday, in the oral arguments in the case of Timothy Ivory Carpenter v. United States. She is correct; indeed, there have been indications that that day may have already arrived. And yet the government argued that when prosecutors—without a warrant—looked at some of the most intimate information that a cell-phone company can collect about its customers, they were not doing anything distinctly intrusive. The Carpenter case began with a string of robberies in which cell phones, of all things, were stolen from Radio Shacks and T-Mobile stores in Michigan and Ohio—not, in other words, a major terrorism or national-security case. One suspect turned over his phone, and on it prosecutors found that he had been in touch with Carpenter, who lived in Detroit. They then collected a hundred and twenty-seven days’ worth of Carpenter’s past cell-phone locations, revealing where and, by implication, with whom Carpenter had been at every moment of those four months. They were able to get that information with a simple order with a far lower standard than probable cause (which is what a warrant requires) and without anyone in law enforcement even having to swear out an affidavit. They used the information to convict Carpenter of six of the robberies; thanks to mandatory minimums, he was sentenced to a hundred and sixteen years in prison.

But, as Sotomayor and several other Justices pointed out—the oral arguments made it clear that both liberals and conservatives on the Court were troubled—prosecutors could, by the government’s standard, have asked for almost anything. Soon enough, they could ask for more than we can even imagine now. Nathan Wessler, an American Civil Liberties Union lawyer who was arguing Carpenter’s case, mentioned “information about the state of the body, like heart-rate data from a smart watch, or fertility-tracking data from a smartphone app; information about the interior of a home, for example, from a smart thermostat that knows when the homeowner is at home and perhaps what room they’re in.” Later, when Sotomayor was questioning Michael Dreeben, the government’s lawyer, she noted, “I don’t, but I know that most young people have the phones in the bed with them.”

Dreeben, nonetheless, persisted in his argument that there was nothing new for the Court to see here, even if there was so much new for the police to see. He said that smartphone records held by any third party (in this case, a cell-phone company, but it could also be an app maker) were no different than records of the numbers that a landline had called. In a 1979 decision, Smith v. Maryland, the Court found it permissible for a police officer to find out from the phone company what calls had been made from a single phone number that was associated with a suspect in a robbery. Now, Dreeben said, when the government asked for a reconstruction of someone’s movements over a period of multiple months, “it is doing the same thing that it did in Smith.” The technology had changed, he said, but the idea of getting records from a third-party communications provider had not, and “does not implicate the Fourth Amendment rights of the customer”—a stunningly broad standard.

It was Sotomayor’s questioning, in particular, that drew out the fundamental contradiction in the government’s case. It wanted the Court to accept that the technology didn’t matter—only the Smith precedent. But, in doing so, it was asking the Court to reject other precedents, in cases involving things other than phones, that related to the kind of very private information and person-to-person communications that the government can get from a smartphone. In many ways, the government was asking the Court to reject or at least severely curtail the Fourth Amendment itself, with its promise of “the right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures.” This is the basic question: Why should the government get to say that “phone” can mean something unrecognizably new, if citizens don’t get to say that “papers” do—that they might include, for example, what amounts to a digital diary of one’s doings. There can’t be a lopsided redefinition that only hurts privacy. For one example, Sotomayor mentioned United States v. Jones, a case restricting the government from placing G.P.S. trackers in cars; for another, she returned to the bedroom. “If it’s not O.K. to put a beeper into someone’s bedroom, why is it O.K. to use the signals that phone is using from that person’s bedroom, made accessible to law enforcement without probable cause?”

Dreeben answered, weakly, that the location information gathered about Carpenter wasn’t quite so precise. “That’s today, Mr. Dreeben, but we need to look at this with respect to how the technology is developing,” Sotomayor said. This was another example of the government trying to have it both ways: it was claiming that it could both apply a case involving obsolete technology in ways that ran past the imagination of the nineteen-seventies Court, and, at the same time, insist that the Justices curtail their imagination of how broadly it might be applied in the future. If Smith can apply to long-term location data today, what might a decision for the government in Carpenter be used to justify forty years from now? “Most Americans, I still think, want to avoid Big Brother,” Sotomayor said. And Americans had an expectation that the government should not be able to track and closely observe them anywhere, anytime—even in the past—without probable cause and a warrant. “That’s not a new standard,” Sotomayor said. “That’'s an old standard”—as old as the Constitution.

The liberals, again, were not alone in this. Chief Justice John Roberts mentioned the Court’s unanimous 2014 decision, which Roberts wrote, in Riley v. California, involving David Riley, of San Diego, who was stopped with an expired license and tags, and with two guns in his car. The police, relying on precedents saying that they could, after an arrest, examine the contents of a suspect’s pockets, also rifled through contacts and photos in a smartphone that they found in Riley’s pocket; they used that data to associate him with the Bloods gang. Roberts and the other Justices said that the police were wrong. In that instance, it mattered to the Court that a pocket, by containing a phone, could hold much more information than earlier judges had anticipated. In fact, the Roberts decision suggested that it was almost an accident of marketing that a phone was called a phone, when it is a mini-library.

Wessler, the A.C.L.U. lawyer, noted on Wednesday that, for the police, smartphone records can be used as “a time machine,” allowing them to re-create scenes from the past. He also questioned the distinction between routing information, or metadata, which can include locations and “content.” As he put it, “in the digital age, content as a category is both under-inclusive and unadministrable.” He described the issue in this case as one of “a categorically new power that is made possible by these perfect tracking devices that ninety-five per cent of Americans carry in their pockets.” He also emphasized that the government would not truly be hamstrung if it lost in Carpenter, because it could still rely on an old but very strong power: it could get a warrant.

“So where are we going?” Justice Stephen Breyer asked Wessler, wondering about where “the right line” was, and what rule the Court might adopt. “How do we, in fact, write it?” This seemed to be the issue for several Justices. (Neil Gorsuch, in a somewhat tedious series of questions, indicated that he wanted to write something based on “property rights.”) Wessler had earlier said that he did not believe that the Justices needed to explicitly overturn Smith. But did they? How broad should they be?