Where do we go? Who do we talk to? What do we read about? Our mobile phones are troves of personal, private information, and the US Supreme Court weighed Wednesday how easily police should be able to get it.
In a case seen as a landmark for privacy protection in the digital age, the court heard arguments over whether, police have the right to obtain the location data of a person's phone from providers without a search warrant.
During the hearing, most of the high court's nine justices appeared deeply concerned about how phone companies can track a person's movements via their device and hand that information, sometimes going back years, to police when asked.
Civil libertarians say that information is protected by the US Constitution.
But law enforcement officials say the location data transmitted from a phone to a cell tower has been essentially made public and handed over to a third party, giving up any claim the owner might have to privacy.
The specific case involves Timothy Carpenter, who was tracked down and convicted of theft in 2011 after the police obtained some 12,898 cell tower location points for Carpenter's device over four months from phone companies.
Justice Sonia Sotomayor appeared to agree with the pro-privacy advocates.
The cell phone "is an appendage now for some people," she noted.
"Right now we're only talking about the cell site records, but as I understand it, a cell phone can be pinged in your bedroom. It can be pinged at your doctor's office. It can ping you in the most intimate details of your life -- presumably at some point even in a dressing room as you're undressing."
Constitutional Test Case
The US Constitution's Fourth Amendment guarantees the privacy of citizens from "unreasonable searches and seizures," and says police must obtain warrants based on "probable cause" if they want to search a suspect's "persons, houses, papers, and effects."
Parties on both sides of the case agree that the law did not anticipate an era in which everyone relies on a cell phone and technology providers can amass data on a person via those phones.
Nathan Wessler, an attorney with the American Civil Liberties Union representing Carpenter, said the police collection of the phone location data constituted a "search" that required a warrant.
"The concern here is with the privacy invasion, which is quite severe over the long term, over these more than four months of data," he told the court.
But the government argues that the location data is not like tapping a phone conversation, which is illegal without a warrant.
"We're dealing here with routing information. We're not dealing with the contents of communications," argued Michael Dreeben, deputy solicitor general for the Department of Justice.
He argued that giving up the information is a voluntary act by the cell phone user, and so it is not protected.
"There is an element here of voluntariness in deciding to contract with a cell company, just like there's an element of voluntariness in getting a landline phone and making a call," Dreeben told the court.
Implications for Private Data
The case has much broader implications than cell phone location data, experts say.
Today, a huge amount of information from people's lives is held by "third parties": personal files stored in the internet cloud, information from home electronics collected by the makers of those appliances, and communications sent via cell phones and the internet.
"The advance of technology means that information you used to store in your desk drawer is now stored somewhere with third parties," said Greg Nojeim of the Center for Democracy & Technology.
To get information in a drawer, he noted, police would have to ask the court for a warrant.
The Supreme Court will likely make a decision on the case before the end of its current term in June 2018.