The Supreme Court heard arguments Wednesday in Carpenter v. United States. But even before the attorneys and spectators assembled that morning at 1 First Street, it was clear this would be a monumental year for digital privacy rights at the Court.

This case will not be decided in a vacuum, and it appears that the justices appreciate both the magnitude of the moment and the complex nature of the problem that they have taken on. To understand the significance of this case, it is helpful to consider the history leading up to Carpenter and the points raised at the Court on Wednesday.

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This case has captured the public interest more than most privacy cases at the court, and that is not surprising. The case is about cell phones, location tracking, and the amount of privacy protection that attaches to the digital breadcrumbs that we create every day. Cell phones are no longer seen as a niche gadget or an extravagant convenience. Cell phones, in the words of Justice Sotomayor, are an “appendage.”

Indeed, many individuals rely on mobile phones for the majority of their communications, and no longer use traditional landline phones at home. And the Supreme Court is keenly aware of this dynamic.

Questions Focused on the Court’s Earlier Cell Phone and Location Tracking Cases

In 2013, the Court unanimously held in Riley v. California that officers cannot search an individual’s cell phone without a warrant, even during an arrest. Chief Justice Roberts famously quipped that cell phones are so “pervasive” that “the proverbial visitor from Mars might conclude they were an important feature of human anatomy.” Given the important role that cell phones now play in our daily lives, and the sensitive nature of the data they hold, the Court found a simple answer to the question of what the police must do: “get a warrant.”

During the Carpenter argument, Chief Justice Roberts implied that a similarly simple standard could apply to location tracking. When the government’s attorney argued that cell phone records should not be protected because they are “voluntarily” conveyed to third parties, Chief Justice Roberts pushed back firmly: “that sounds inconsistent with our decision in Riley, though, which emphasized that you really don’t have a choice these days if you want to have a cell phone.” The chief justice was not alone in his skepticism of the Government’s position.

The views of the justices were previewed in United States v. Jones, a case about whether officers could attach a GPS device to an individual’s car and track their movements over several months without a warrant. The Court unanimously agreed that the Fourth Amendment protects against such searches, but could not agree on what test should apply.

Justice Scalia, writing for the majority, found that the attachment of the GPS device to the defendant’s car was a “trespass” that triggered the Fourth Amendment. Justice Alito, writing in concurrence, reasoned that tracking the defendant’s movements over an extended period violated a reasonable expectation of privacy. Justice Sotomayor wrote separately, agreeing both with Justice Scalia’s property-based majority and with Justice Alito’s reasonable expectation analysis, and predicted that “it may be necessary to reconsider the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties.”

Justices Considered How A Pro-privacy Decision Would Apply in Future Cases

During the Carpenter argument, Justice Alito distanced himself from his conclusion in Jones. He expressed concern over the impact on searches of other types of business records, even though Justice Alito agreed that location tracking “is raising very serious privacy concerns.” In particular, he questioned the ability to distinguish between the sensitivity of location records, telephone call records, and banking records.

Justice Kagan, in contrast, was skeptical of the government’s ability to distinguish the location tracking in this case from the GPS tracking that the Court had found unconstitutional in Jones. Justice Breyer also agreed that long-term location tracking was invasive, but questioned how to draw meaningful lines to guide future cases.

Given the apparent consensus on the sensitivity of these location records, you might wonder why a decision in this case would represent such a monumental shift for digital privacy rights. The answer lies in the questions posed by Justice Alito and Justice Breyer, and the decisions reached by lower courts across the country. Up until this point, courts have rejected the argument that cell phone location records are protected under the Fourth Amendment.

Even after GPS tracking was found unconstitutional in Jones and warrantless cell phone searches rejected in Riley, courts across the nation have dismissed location privacy arguments based on a pair of cases from the 1970s denying protection for banking and telephone call records. Those two cases — Smith v. Maryland and Miller v. United States — have been a roadblock for privacy litigators in cell phone privacy and other digital rights cases. But the justices on Wednesday seemed to acknowledge that is all about to change.

Both the Court and Congress Will Play A Role In Defining Future Privacy Protections

The justices seem to agree that the Fourth Amendment protects the privacy of location records. The question now is what rule will explain why such records are protected, and what factors should apply in future cases.

It is helpful to consider points previously raised by Justice Alito as well as the history and development of modern privacy law. Prior to the Federal Wiretap Act in 1968, the Supreme Court held that the interception of telephone calls violated the Fourth Amendment. The Court first established the broad legal principle, then Congress codified a set of specific and administrable rules to apply that principle. Justice Alito alluded to that same dynamic in his concurring opinions in Jones and Riley, noting that Congress is in a better position to establish specific rules governing these new technologies.

My organization, EPIC, made a similar point in the “friend of the court” brief that we filed in support of Carpenter. Congress can provide detailed rules in a privacy law concerning the collection and use of personal data. But in the first instance, the Supreme Court’s role is to set the Fourth Amendment baseline and identify government conduct that crosses the line.

As the justices met this week to discuss Carpenter, they likely considered all of these factors as they debated, and tentatively concluded, the outcome in the case. Now we wait to see what happens and how the Court defines the scope of the Fourth Amendment in the digital age.

Alan Butler is senior counsel at the Electronic Privacy Information Center (EPIC) in Washington, DC. EPIC filed a “friend of the court” brief in support of Carpenter.