Supreme Court justices on Wednesday wrestled with how to apply Fourth Amendment privacy protections to cell phone location records.

Cell phones produce a "minute-by-minute account of a person's locations and movements and associations over a long period regardless of what the person is doing at any given moment," the ACLU's Nathan Freed Wessler pointed out in an argument before the Supreme Court. The ACLU is urging the Supreme Court to rule that the government can't access these records without a warrant.

But the government pointed to a 1979 Supreme Court ruling called Smith v. Maryland. In that case, the Supreme Court ruled that the government doesn't need to get a warrant to obtain a customer's dialing history because they are merely the business records of the phone company. The government argues that the same principle, known as the third-party doctrine, applies here: data about which cell phone towers a customer's phone has talked to are merely the cell phone company's business records and should be available to the government without a warrant.

Several justices seemed uncomfortable with the potentially Orwellian implications of this position. Indeed, five years ago, the Supreme Court ruled that it was unconstitutional for the police to attach a GPS tracking device to a suspect's car for almost a month without a warrant. The justices wondered how it could be constitutionally permissible to accomplish the same goal by asking a phone company for cell phone records.

"It seems like your whole argument boils down to 'if we get it from a third party, we're OK,'" Justice Neil Gorsuch said. "John Adams said one of the reasons for the war was the use by the government of third parties to obtain information—forced them to help as their snitches and snoops. Why isn't this exactly what the framers were concerned about?"

The challenge facing the justices in Wednesday's argument, however, is that no one was offering them a great alternative to the third-party doctrine.

Cell phone location data nabbed a Radio Shack robber

The Supreme Court was hearing an appeal from a man accused of orchestrating a string of robberies of Radio Shack and T-Mobile stores in Michigan and Ohio in 2010 and 2011. Ultimately, Timothy Carpenter was found out after one of his co-conspirators was arrested at the scene of one of the robberies, and he eventually cooperated with the government. Authorities obtained Carpenter’s phone number, which they used to serve his phone company, Metro PCS, with a court order seeking 127 days of his call records, including his location.

The data confirmed that Carpenter was often at the scene of the crime during the robberies. However, such a vast quantity of data also revealed a lot of personal information that had nothing to do with his crime spree—things like where he went to church, and where he slept at night.

The government obtained Carpenter's phone records using Section 2703(d) of the Electronic Communications Privacy Act, a provision that allows the government to get a court order under a much more lenient legal standard than is imposed by the Fourth Amendment.

The American Civil Liberties Union has argued that if the government wants to get this level of personal detail about someone, it should get a warrant. That means the government has to convince a judge that there's probable cause to believe a crime has been committed, and it also means the warrant would have to be limited to information that's likely to be relevant to the crime. A federal court in Michigan and then the 6th US District Court of Appeals sided with prosecutors.

This week, the case reached the Supreme Court, and the government argued that the high court should simply apply the third-party doctrine it articulated 38 years ago: as long as information is held by a third party, the government can get it without a warrant no matter how sensitive it might be. "There's a distinction between acquiring GPS information from a phone and acquiring cell site information from a business," the government's lawyer, Michael Dreeben, argued. "This case involves acquiring cell site information from a business."

Dreeben argued that cell site information was similar to the telephone numbers at issue in the Smith case because both constituted "routing information"—information that allows the telephone company to deliver a cell phone call to the customer. In Dreeben's view, the fact that the data also provides a wealth of information about the customer's location and movements—even when he's not actively using his phone—was immaterial.

Lots of people feel that takes the third-party doctrine too far, however. This week, Stephen Sachs, who was the state attorney general of Maryland in 1979 and won the Smith case, wrote in a Washington Post op-ed that the government’s position in Carpenter is "taking the Smith precedent way too far, in a vastly different technological age."

"When the Supreme Court decided Smith, in the pre-dawn of the digital age, we didn’t know about the Internet, smartphones, cloud computing, Facebook or Twitter," he continued. "No one involved in the case could foresee the digital revolution that was to come."

The Supreme Court was searching for a new principle

None of the justices seemed satisfied with the government's position that law enforcement should have easy access cell phone customers' location data. They pointed back to the Supreme Court's 2012 GPS decision, United States v. Jones, which held that tracking a suspect's car for a month using a GPS device ran afoul of the Fourth Amendment.

"In both cases, you have a new technology that allows for 24/7 tracking," Justice Elena Kagan said. In the Jones case, she said, justices concluded that weeks of surveillance with a GPS device was a "new and different thing that did intrude on people's expectations of who would be watching them when." She suggested the same was true with cell phone tracking.

So the justices were on the lookout for a sensible way to rein in cell phone tracking. The big question is how.

In the 2012 Jones GPS tracking case, Justice Sonia Sotomayor wrote a famous concurrence arguing that the third-party doctrine is "ill-suited to the digital age." Third parties today—from MetroPCS to Google and Facebook—hold vastly more sensitive information about us than they did 38 years ago. So one option would be to overrule the third party doctrine—or hold that it doesn't apply to location data.

But even the ACLU wasn't pushing the court to go that far. "We're not asking the court to overturn" Smith or another 1970s case that established the third-party doctrine, the ACLU's Wessler said. Instead, he argued that location records were different from other kinds of data third parties collect. Location records are particularly sensitive, he said. Also, many customers don't even know their phones are generating location data their phone companies can collect—a contrast with phone numbers that customers explicitly punch into their telephones.

Wessler suggested that the court could limit warrantless access to location data to 24 hours. If the government wanted more than 24 hours of location data, Wessler suggested, then it would need a warrant.

Justices seemed skeptical about setting a bright-line rule like this, however. They wondered if it would make more sense to limit location data to the duration of the crime. In some cases, that would mean limiting location data to the hour when a crime actually occurred. In other cases—like the crime spree here—it might allow the government to obtain location data from several different days when criminal activity was occurring.

Gorsuch suggested the court could take a different approach—one championed by conservative Justice Antonin Scalia in the Jones case. Writing for the majority, Scalia focused on the fact that the government had trespassed on Antoine Jones' property when it attached the GPS tracking device to his car. The Jones opinion suggested the court could move away from the "reasonable expectation of privacy" principle that has guided Fourth Amendment law for the last 50 years.

Gorsuch suggested that the court could take a similar approach here: arguing that data about Carpenter's location actually belonged to Carpenter, and that's why the government needed a warrant to access it without his consent.

But the government's lawyer pushed back forcefully against this suggestion. He pointed out that the law has never treated data about someone as being that person's property. Creating this kind of property right could have wide-reaching and unpredictable effects, the government argued.

In some cases, the Supreme Court faces a binary choice between two different legal principles. But this case was different. All the justices seemed to agree that unfettered government access to location data was troubling. The big challenge was coming up with a principle that provided people with privacy protections without unduly upending other aspects of privacy law. They didn't seem much closer to developing such a principle at the end of the argument than they had at the beginning.