The following Q&A is an excerpt from Law.com's What's Next briefing, a weekly newsletter on the future of law. Click here to learn more.

It's been nearly a year since the U.S. Supreme Court ruled in Carpenter v. U.S, establishing murky rules to govern law enforcement access to the cell-site records that map the movements of anyone who carries a cellphone.

To Orin Kerr, a professor at USC Gould School of Law, the decision signals a new era in Fourth Amendment jurisprudence. We caught up with Kerr by email to ask about the pressures that digital technology places on the Fourth Amendment and where things go next.

➤ You write that the Supreme Court's decision in Carpenter v U.S. signals a “major break from the traditional understanding” of the Fourth Amendment. Can you explain why it's so impactful? After all, it's not the first case to deal with transformational technology and policing.

Carpenter introduces a new way to think about the reasonable expectation of privacy test. Before Carpenter, Fourth Amendment rights were tied to places or things. The legal question was whether a person had a reasonable expectation of privacy in a place like a home, or in a thing like a car, that was invaded by the government's access of information from inside it. Under Carpenter, people have a reasonable expectation of privacy against certain levels of government power. If technology expands government power, the law adds constitutional protection to restore the level of privacy people had before the technological change. Now you can have Fourth Amendment rights in records that you didn't know existed, held by companies thousands of miles away, that might have information about you.

➤ Does Carpenter provide a clear framework for lower courts to apply in evaluating other forms of digital records? What other kinds of digital age data do you expect will come up in future cases?

No, it doesn't. The law professor cliché is true: Carpenter raises more questions than it answers. First, we don't know what other records about us are protected. For example, what if the government collects IP addresses? Or tracks website visits? Or engages in public camera surveillance? Or obtains numbers dialed? Carpenter says that the Fourth Amendment now protects some kind of non-content third party records, but we don't know which ones. Second, we don't know when collection of those protected records is a search. Is only long-term surveillance covered? What if the government obtains only a single record at a single time? We just don't yet know if any surveillance is protected or if some kind of long-term surveillance is needed.

➤ Where does this leave the companies that are collecting user data? If they receive a subpoena, should they comply?

It leaves everyone with a lot of uncertainty. Companies with user data often can't know if the records they have collected are protected under the Fourth Amendment. Many of the records will also be protected by a federal statute called the Stored Communications Act, which has an immunity provision. As a result, companies shouldn't face legal liability for complying with that statute as long as the Fourth Amendment law remains uncertain. But some companies will want to err on the side of caution and demand warrants. At that point law enforcement has three choices. First, if they have probable cause, they can obtain warrants. Second, they can bring the company to court and try to enforce the lesser legal process. Third, they can give up trying to get that information.

➤ We recently reported on “reverse location search warrants” where a company like Google is asked to identify all mobile devices in the area of a crime scene at a particular time. What Fourth Amendment considerations are raised by this type of investigation tactic and does Carpenter provide any guidance? 

Reverse location warrants raise a lot of legal issues. Carpenter doesn't so much provide guidance as add yet another hard question: Are the location coordinates protected by the Fourth Amendment at all? Before Carpenter, it was likely the case that the location records were not protected. They would have been considered metadata voluntarily communicated to the provider, and therefore outside Fourth Amendment protection. Carpenter leaves unclear whether these sorts of location records are still unprotected. It depends on how you read the Carpenter decision, I think, and in particular what you make of the Court's analysis of the voluntariness of generating records. If you take voluntary steps to generate records, then ordinarily that is on you and the records are not protected. Carpenter says that cell phone users don't make cell-site records voluntarily, however, because they have to use a cell phone to participate in modern life—and all cell phone use generates cell-site records. Creating such records is unavoidable unless you want to be a hermit, the Court reasons, so doing so is not voluntary in a “meaningful” sense. But is that also true for precise location coordinates stored in a Google account?

➤ What is the next digital Fourth Amendment issue that you expect to confront the Supreme Court? 

There are two deep circuit splits ready for the Court's attention.

The first issue is how to apply the Fourth Amendment to searching computers that cross the border. The traditional rule is that the government does not need any cause to search and read paper documents crossing the border. The Eleventh Circuit has held that same rule applies to computer searches. But other lower courts have disagreed. The Ninth Circuit has required reasonable suspicion to conduct a forensic search of a computer crossing the border, and the Fourth Circuit has also said that some cause is needed.

The second circuit split considers how the Fourth Amendment applies when a private person brings someone else's computer to the government and claims to have seen evidence of crime in a particular file on the machine. The traditional rule is that the government can reenact the private search without a warrant but cannot exceed the private search. Lower courts are divided on how that rule applies. Can the police only look at that one file? Can they look at the entire computer? The question is, what is the unit courts should use to determine when a government search exceeded a private search?

Orin S. Kerr joined the faculty at USC Gould School of Law in 2018 as the Frances R. and John J. Duggan Distinguished Professor of Law. He specializes in criminal procedure and computer crime law. Stay tuned for Kerr's forthcoming book “A Digital Fourth Amendment.”