THE nine justices of the Supreme Court are used to applying 18th-century principles to an America that would bewilder the constitution’s framers. Yet sometimes this is really hard. On November 29th the court considered how a 226-year-old rule, the Fourth Amendment’s ban on unreasonable searches and seizures, bears on one arrow in the government’s investigative quiver: tracking people’s movements via their mobile-phone signals. At least six justices seemed keen to widen the Fourth Amendment umbrella for the digital age, but no single way to do so emerged. “This is an open box”, a forlorn Justice Stephen Breyer said. “We know not where we go.”

The matter dates to 2011, when Timothy Carpenter was arrested for masterminding a string of armed robberies in Michigan and Ohio. The FBI built their case against Mr Carpenter on 127 days of mobile-tower data placing him near the scenes of the crimes. Under the Stored Communications Act of 1986, investigators who have “reasonable grounds to believe” a suspect’s electronic data include “specific and articulable facts” that are “relevant and material” to their investigation can secure an order compelling providers to hand it over. That’s a far easier bar to reach than reasonable suspicion—the threshold for a search warrant. In Carpenter v United States, the justices are considering whether this higher standard, known as “probable cause”, should apply when the government seeks to track digital footprints.

According to Nathan Wessler, Mr Carpenter’s lawyer from the American Civil Liberties Union (ACLU), collecting location information without a warrant defies a “long-standing, practical expectation” that Americans’ “longer-term movements in public and private spaces will remain private”. Government collection of location data “is a categorically new power that is made possible by these perfect tracking devices that 95% of Americans carry in their pockets”. And with an explosion of newly built cell towers, providers can now estimate their users’ positions within “a broadcast radius as small as ten metres”, or “half the size of this courtroom”.

Justices Samuel Alito and Anthony Kennedy pushed Mr Wessler to explain why an individual would be more worried about keeping his tracks out of spooks’ file drawers than safeguarding his bank or landline phone records—data Supreme Court precedent says the government may access without a warrant. As sensitive as that information may be, Mr Wessler replied, it does not compare to “a minute-by-minute account of a person’s movements and associations” over weeks or months.

Justice Sonia Sotomayor agreed—and then some. Carpenter may concern only cell-tower data, but “a cell phone can be pinged in your bedroom”, she said. “It can ping you in the most intimate details of your life. Presumably at some point even in a dressing room as you’re undressing.” One day soon, she mused, “a provider could turn on my cell phone and listen to my conversations.” Given these imminent risks to Americans’ privacy, Justice Sotomayor gestured towards a more robust application of the Fourth Amendment than Mr Wessler was requesting.

Chief Justice John Roberts, a conservative, was an unlikely ally in this line of questioning. When Mr Wessler said that police should be able to see no more than 24 hours of cell-location data without a warrant, the chief suggested that warrantless access to even a smidgen of data may violate an individual’s privacy, outflanking the ACLU lawyer from the left. He asked Michael Dreeben, the government’s lawyer, how the claim that people voluntarily share location data when they wander about with their phones squares with Riley v California, a ruling of 2014 that police need a warrant to search a mobile phone. Riley, the chief said, “emphasised that you really don’t have a choice these days if you want to have a cell phone.”

With only Justices Alito and Kennedy seeming to buy the government’s argument, the newest justice, Neil Gorsuch, voiced his own take on why Mr Carpenter should win. Justice Gorsuch doubted that “the original understanding of the constitution” sanctioned easy access to individuals’ location information—especially if Americans have a “property interest” in their own data. Citing John Adams, he noted that one impetus for the revolutionary war was the government’s use of “snitches and snoops” to spy on Americans. Open-ended “writs of assistance” gave authorities licence to search anything they liked, infuriating the colonists and inspiring the Fourth Amendment.

For Jeffrey Rosen, president of the non-partisan National Constitution Centre in Philadelphia, Justice Gorsuch’s approach was somewhat surprising, if reassuring. “It suggests that he, like his predecessor Justice Antonin Scalia, may be a vigorous defender of the Fourth Amendment right to be free from unreasonable searches of our digital effects.” Ian Samuel of Harvard Law School agreed. The colonial-era reference caught the government’s lawyer “entirely off-guard”, he says. Now the justices must reckon with how to find for Mr Carpenter—no mean feat in light of the competing interests of privacy and policing. As he often does, Justice Breyer turned to the lawyers for help. “So where are we going? Is this the right line?” A solid majority of the justices know what decision they want, but “how do we, in fact, write it?” An answer should appear by the end of June.