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Police officers cannot retrace the location of your cell phone without a warrant, the Supreme Court ruled today — a narrow but critically important victory for privacy advocates as well as giant tech companies.

Why it matters: The court’s precedents have allowed a relatively broad range of warrantless searches, but Silicon Valley warned that if those “analog” rules were applied to modern smartphones, hardly anything we do would ever be private again. And they won.

The big picture: When the police go back and retrace a suspect's steps through the location of the suspect's cell phone, that constitutes a search under the Fourth Amendment, the court said in a 5-4 ruling. And so police will "generally need a warrant" to access that information, Chief Justice John Roberts wrote, though there will be exceptions.

Key quote: "Because location information is logged for all of the 400 million devices in the United States — not just those belonging to persons who might happen to come under investigation — this newfound tracking capacity runs against everyone," Roberts wrote. "Police need not even know in advance whether they want to follow a particular individual, or when."

The issue: The Supreme Court ruled in the 1970s that police didn’t need a warrant to track the phone numbers suspects dialed from their landlines. It said people know they’re relying on the phone company to connect calls, and have no right to believe the numbers they’ve called are private once they’ve sent that information to a third party.

  • That precedent has governed a lot of privacy-related cases, especially when it comes to telephones.
  • But by using a smartphone, you’re transmitting just about every detail of your life — including your physical movements — to third parties. That's just too much information for police to access without a warrant, the court said.

The case the court decided today dealt with Timothy Carpenter, who was convicted of participating in a string of robberies in 2010 and 2011 — a conviction based partly on data from Carpenter's cell phone providers, which showed that his phone pinged cell towers near the sites of the robberies at the times they occurred.

  • Police obtained nearly 13,000 individual records about the location of his cell phone, spanning four months.
  • The ACLU says obtaining all those cell-tower records without a warrant violated Carpenter's Fourth Amendment rights against unreasonable search and seizure.

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