Wherever you take your phone, your wireless provider knows. And these days, you take your phone everywhere.
So if they want to, law enforcement investigators can go back in time and trace your every movement by getting your location records from your wireless provider.
But do they need a warrant to do that? That's the question at stake in a case before the US Supreme Court, Carpenter v. USA, and justices heard both sides of the argument in court on Wednesday.
The government wants to keep the current understanding of the law, which lets investigators request location records from wireless providers without a warrant. Opponents say that's a violation of Fourth Amendment protections against unreasonable searches.
The court appeared open Wednesday to creating a new standard for obtaining location information -- but also a little lost as to what exactly that standard should look like.
"This is an open box," Associate Justice Stephen Breyer said during the arguments. "We know not where we go."
Why is it so hard? Technology has changed dramatically since the rules about government access to location data first emerged.
Right now, investigators consider your location information to be something you freely handed over to your wireless carrier, so they don't need to get a warrant to look at it. (They still need to get a court order, but that doesn't require law enforcement officers to clear as high a bar to get permission from a judge for the search.)
This approach is called the "third-party doctrine," and the American Civil Liberties Union argues it shouldn't apply so broadly now that location data can provide such a precise and complete picture of your movements. Tech and telecommunications companies including Apple, Google, Facebook and Verizon havefor the ACLU's argument.
The rules for accessing location information emerged from the Stored Communications Act, enacted in 1986 and last amended in 1994. But things have changed since then, said ACLU attorney Nathan Wessler, who argued in favor of a new standard.
Location records, he argued, provide police with the equivalent of a time machine that can let investigators go back and trace your movements for long stretches.
"That is a categorically new power that is made possible by these perfect tracking devices that 95 percent of Americans carry in their pockets," Wessler said.
But US Deputy Solicitor General Michael Dreeben argued the existing standard should still apply. Phone users know they're continually handing over their location information to cell service providers, he said, and that means they've willingly given up that information. They shouldn't have an expectation of privacy as a result.
Requesting those records, then, doesn't require a warrant, Dreeben said. "It is asking a business to provide information about the business's own transactions with a customer."
The justices are expected to rule on whether that's the case by the end of June 2018.
Wessler said in an interview that he's confident the justices appreciate what's at stake in the case.
"The court clearly has been thinking very hard about how to keep the Fourth Amendment meaningful in the digital age," Wessler said.
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