A woman uses her cell phones in Manhattan, New York, U.S., January 30, 2018.

© 2018 Reuters

In today’s five-to-four majority opinion in Carpenter v. United States, the US Supreme Court found that police need to get a warrant from a judge before demanding location data generated by  your cell phone. In this case, Chief Justice John Roberts did something crucial but rare in US court decisions about digital privacy: he recognized reality.

Cell phones, as Roberts had previously acknowledged, are an integral part of modern life. And in order to function, your phone continuously produces data about your location – data your cell phone company retains, sometimes for years. As the court observed, you have no meaningful choice about this, and this information about your movements can be highly revealing of your intimate relationships, professional activities, religious observance, protest attendance, and many other aspects of your life. Therefore, the court concluded, authorities need to obtain a warrant before obtaining historical cell phone location data.

In the 18th century, people in the American colonies objected to the British authorities intruding on homes and private papers to glean information about personal life and beliefs with little constraint, believing this created a risk of tyranny and abuse. Fear of this dangerous imbalance of power led to the Fourth Amendment to the US Constitution, which broadly requires a warrant for searches and seizures by the government. As the majority opinion in Carpenter recognizes, the ubiquity of digital technology and the resulting intrusiveness of the government’s surveillance powers have created similar risks today, making the need for court approval just as pressing now as then.

While focused only on historical cell-phone location information, today’s decision has potential implications for other technologies, especially since the court explicitly acknowledged that people do not automatically “surrender all Fourth Amendment protection by venturing into the public sphere.” That should be just as true for facial-recognition systems, automated license plate readers, and other surveillance methods the authorities use to track people whom they have no prior reason to suspect of wrongdoing.

We should celebrate this victory and carry the fight for privacy forward into other technological realms.