On Wednesday, the Supreme Court will hear opening arguments in a case that could help define a new digital era. Justices will need to apply a centuries-old framework, the Fourth Amendment, to a world in which people’s movements are now continuously recorded by mobile devices.

The Fourth Amendment protects against unreasonable searches. The case, Carpenter v. United States, will test whether prosecutors violated that amendment by collecting vast amounts of data from cellphone providers showing the movements of a man prosecutors say organized a spree of robberies in 2010.

“A lot of what we’re getting at in the Carpenter case,” said Woodrow Hartzog, professor of law and computer science at Northeastern, “is a growing sense of discontent from the judges over the seemingly simplistic rules we crafted years ago about when and how the government can surveil and collect information about us in light of all these powerful information technologies.

“These technologies significantly changed the way in which we interact with each other, but also our risk calculus as to what we consider safe activities and what we consider dangerous activities,” Hartzog said.

In the Carpenter case, police used cellphone data—including the time and location of calls relative to the nearest cell tower—to reconstruct Timothy Carpenter’s whereabouts during a string of Radio Shack robberies. Carpenter contends the investigators violated the Fourth Amendment by not getting a warrant first. The state contends that since the information was freely given to a third party, the cell phone company, it’s not protected by the Fourth Amendment.

It’s difficult to imagine the drafters of the Bill of Rights envisioning a future in which people are constantly monitored by palm-sized computers. “This is way beyond the imagination of the founders,” said Daniel Urman, a Supreme Court scholar and director of the undergraduate minor in law and public policy at Northeastern. “The founders had very specific concerns they were addressing with the Fourth Amendment. At the time it was drafted, the British government was searching and seizing people’s homes, papers, and effects using general warrants.

“But,” Urman said, “the Supreme Court has never let the historical context behind the Fourth Amendment hold them back from a modern application.”

Jessica Silbey, a Northeastern law professor and expert on constitutional law, said the Constitution was indeed designed to be flexible. “The Bill of Rights itself has experienced a ton of evolution since 1789,” Silbey said. “Part of what the Constitution was meant to do was adapt. It was never meant to be static. The founders anticipated that there would be flexibility in the document.”

Evolving understanding of privacy

The heart of interpreting the Constitution in this case, Silbey and Hartzog said, is a question of privacy. “Do we have a reasonable expectation of privacy in our geolocation as tracked through cellphone behavior?” she asked.

It gets more complex, though. Even that phrase, “reasonable expectation of privacy,” is problematic, Hartzog said. “From that seemingly simple concept comes a host of problems,” he said, “including how, exactly, we define privacy. If we’re going to have a more realistic sense of privacy, the law has to become more nuanced.”

Silbey noted the third-party doctrine, invoked in the case. Information that’s voluntarily given to third parties falls under the third-party doctrine, and is exempt from protection under the Fourth Amendment. That is, as cellphone users, we give up certain information—including geolocation when making calls—to cellphone companies. In that sense, such location information would be exempt from Fourth Amendment protection.

But the extent to which we give up information in order to use cellphones is perhaps beyond the third-party doctrine’s binary “on/off privacy switch,” Silbey said.

“We sign these boilerplate contracts in order to get a cellphone, but that doesn’t mean that we think what we’re doing on the phone isn’t private,” she said. “Just because we give information to our phone company doesn’t mean we expect it to be public. That’s true of how we live our lives. Creating the types of private spaces the Bill of Rights imagines is more and more challenging if everything is digital and we’re always having to give information to third parties in order to function in the digital world.”

Hartzog echoed her sense of our increasingly digital world. “Try interacting with society in any meaningful way, or doing anything relevant to human flourishing without using some sort of modern digital technology. It’s almost impossible,” he said.

The ghost of cases past

Urman noted that the late Justice Antonin Scalia was an originalist who authored “some of the most important Fourth Amendment cases that are setting the stage for Carpenter,” including Kyllo v. United States in 2001, and United States v. Jones in 2012, both of which upheld the Fourth Amendment’s protection against warrantless searches.

“He understood that cellphones and cars were not things the founders were worried about, but there were principles they were concerned about, and those included people going through your personal stuff,” he said.

Two other cases are likely to come into play in the justices’ decisions.

In the first, Smith v. Maryland, police requested without a warrant that a telephone company use a device that would record a suspected robber’s outgoing calls, though only the numbers he dialed. They ultimately used the information to charge him with robbery. The Supreme Court decided in 1979 that the action did not violate the Fourth Amendment because phone numbers themselves are voluntarily given to third parties (the phone companies, in this case).

In the second, Riley v. California in 2014, police arrested David Leon Riley then searched through his cellphone—which they did without a warrant per an exception following an arrest—to connect him to a local gang and ultimately to an attempted murder of a rival gang member. Riley argued that the evidence regarding his gang affiliation shouldn’t have been admissible as it was obtained in a way that violated the Fourth Amendment. The Supreme Court unanimously agreed.

Hartzog and 41 other legal scholars joined an amicus brief urging the Court to “resist extending the reasoning of Smith v. Maryland—a 38-year-old case built on a faulty privacy premise—to the modern, hyper-connected, technology-dependent world,” the brief reads. “Instead, the Court should recognize that the new realities of this world require new legal doctrines to fit the privacy expectations shared by most Americans.”

Justices may likely try to decide to which of these two cases Carpenter is most similar, Urman said. “Choosing one or the other means ruling two different ways,” he said.