In a blockbuster 5-4 decision issued today, the U.S. Supreme Court ruled that warrantless government tracking of cellphone users via their cellphone location records violates the Fourth Amendment. "A person does not surrender all Fourth Amendment protection by venturing into the public sphere," declared the majority opinion of Chief Justice John Roberts. "We decline to grant the state unrestricted access to a wireless carrier's database of physical location information."

The case is Carpenter v. United States. It arose after the after FBI obtained, without a search warrant, the cellphone records of a suspected armed robber named Timothy Carpenter. With those records, law enforcement officials identified the cell towers that handled his calls and then proceeded to trace back his whereabouts during the time periods in which his alleged crimes were committed. That information was used against Carpenter in court.

The central issue in the case was whether Carpenter had a "reasonable expectation of privacy" in the information contained in those records, or whether he had forfeited such privacy protections by voluntarily sharing the information with his cellular service provider. As the Supreme Court put it in United States v. Miller (1976) and Smith v. Maryland (1979), "a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties."

In his ruling today, Chief Justice Roberts "decline[d] to extend Smith and Miller to cover these novel circumstances. Given the unique nature of cell phone location records, the fact that the information is held by a third party does not by itself overcome the user's claim to Fourth Amendment protection." He continued: "Whether the Government employs its own surveillance technology…or leverages the technology of a wireless carrier, we hold that an individual maintains a legitimate expectation of privacy in the record of his physical movements as captured through [cell site location information]."

Roberts' opinion was joined by Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor, and Elena Kagan. Justice Anthony Kennedy filed a dissent, joined by Justices Clarence Thomas and Samuel Alito. Alito also filed a dissent, which Thomas joined. Thomas also filed a dissent of his own. Justice Neil Gorsuch dissented alone too.

Kennedy, joined by Thomas and Alito, complained that "the Court's stark departure from relevant Fourth Amendment precedents and principles…places undue restrictions on the lawful and necessary enforcement powers exercised not only by the Federal Government, but also by law enforcement in every State and locality throughout the Nation." In their view, the Court should have followed its precedents in Miller and Smith and held that "individuals have no Fourth Amendment interests in business records which are possessed, owned, and controlled by a third party." Cellphone records, they maintain, "are no different from the many other kinds of business records the Government has a lawful right to obtain by compulsory process."

Justice Neil Gorsuch struck an entirely different note in his lone dissent. Indeed, his dissent reads much more like a concurrence. It seems clear that while Gorsuch agreed with the majority that Carpenter deserved to win, he strongly disagreed with them about how the win should have happened.

"I would look to a more traditional Fourth Amendment approach," Gorsuch wrote. "The Fourth Amendment protects 'the right of the people to be secure in their persons, houses, papers and effects, against unreasonable searches and seizures.' True to those words and their original understanding, the traditional approach asked if a house, paper or effect was yours under law. No more was needed to trigger the Fourth Amendment." Furthermore, Gorsuch wrote, "it seems to me entirely possible a person's cell-site data could qualify as his papers or effects under existing law."

"I cannot fault" the majority "for its implicit but unmistakable conclusion that the rationale of Smith and Miller is wrong; indeed, I agree with that," Gorsuch explained. "At the same time, I do not agree with the Court's decision today to keep Smith and Miller on life support." In other words, Gorsuch would scrap these third-party precedents and have the Court start adhering to an originalist, property rights-based theory of the Fourth Amendment. That's how Gorsuch wanted Carpenter to win.

The importance of today's ruling in Carpenter v. U.S. should not be underestimated. Both the majority opinion and Gorsuch's dissent raise questions about the future viability of two key Fourth Amendment precedents. What is more, the decision itself represents a massive win for Fourth Amendment advocates. Carpenter may well be remembered as the most significant decision issued this term.