A man uses a smartphone in New York on Nov. 6. (Mark Kauzlarich/Bloomberg)

Stephen H. Sachs was Maryland attorney general from 1979 to 1987.

Stephen H. Sachs was Maryland attorney general from 1979 to 1987.

The Supreme Court will hear arguments this Wednesday in Carpenter v. United States, a criminal case testing the scope of the Fourth Amendment's right to privacy in the digital age. The government seeks to uphold Timothy Carpenter's conviction and will rely, as did the lower court, on the court's 1979 decision in Smith v. Maryland, a case I know well.

I argued and won Smith v. Maryland when I was Maryland's attorney general. I believe it was correctly decided. But I also believe it has long since outlived its suitability as precedent.

Smith began as a run-of-the-mill purse snatching. Unsatisfied with the just-purloined purse, the defendant stalked the victim with threatening and obscene telephone calls. The telephone company — at police request, but without a judicial warrant based on probable cause — attached a "pen register" at its central office that recorded numbers dialed from Smith's home. Within three days, the device revealed a call to the victim that led to additional incriminating evidence and Michael Smith's conviction.

The court, dividing 5 to 3, ruled that the warrantless use of the pen register didn’t constitute a search within the meaning of the Fourth Amendment. It held that Smith had no “legitimate expectation of privacy” in the dialed numbers he “voluntarily conveyed” to a third party, the phone company. The court reasoned that the defendant knew or should have known that the phone company maintained such records in the normal course of its business. He therefore assumed the risk that the phone company could do with the records what it wished.

The facts in Carpenter are markedly different — and so is the technology. The defendant was convicted of leading a gang of robbers. The prosecution produced cellphone-tower data that tracked the whereabouts of Carpenter's cellphone for more than four months and placed him at or near the sites of a string of armed robberies. The police acquired the data from Carpenter's wireless carriers without a warrant showing probable cause.

A majority of the U.S. Court of Appeals for the 6th Circuit held that the phone-based logic of Smith was also applicable to cell-tower communications and affirmed Carpenter's convictions. Carpenter had voluntarily conveyed data revealing his location to wireless carriers — the third-party operators of the cell towers. He knew, or should have known, that the wireless carriers were free to track his locations and furnish the information to the police.

This 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. No one involved in the case could foresee the digital revolution that was to come.

That new world is defined by the rapid increase in sophisticated — and invasive — technology. It is also defined by a relentless and pervasive assault on privacy. As journalist Julia Angwin has shown in her book "Dragnet Nation," the new digital world can track our movements, seize our secrets, manipulate our finances and much more.

In such a world, the very notion of a “legitimate expectation of privacy” seems antique.

There is evidence that the courts are catching on. Most predictive, perhaps, are the words of Justice Sonia Sotomayor, concurring in a 2012 case holding that the clandestine and warrantless attachment of a GPS tracking device to a defendant's car was an unconstitutional search. Sotomayor suggested that "the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties" is "ill suited to the digital age, in which people reveal a great deal of information about themselves to third parties in the course of carrying out mundane tasks." As Sotomayor noted, "People disclose the phone numbers that they dial or text to their cellular providers; the URLs that they visit and the email addresses with which they correspond to their internet service providers; and the books, groceries and medications they purchase to online retailers."

Sotomayor is right. The Supreme Court should develop a modern Fourth Amendment doctrine. Such a test would recognize the legitimate claims of law enforcement but set objective boundaries — such as the duration of an intrusion or the nature of the data seized — that constrain those claims. The Carpenter case is the court's opportunity to do so.

The world has changed profoundly since I argued Smith v. Maryland. And as Oliver Wendell Holmes Jr. taught us long ago: "The life of the law has not been logic; it has been experience."