Today the ACLU argues Carpenter v. United States before the Supreme Court. At the heart of the argument is this question: Do we have a constitutionally protected privacy interest in our cell phone location data? More specifically, can the government access this information without a warrant, and then use it against us in court?

The ACLU says: Get a warrant. Over a dozen groups ranging from the Electronic Frontier Foundation to technology experts to First Amendment defenders filed friend of the court briefs supporting the ACLU’s petition.

On the other side are the Department of Justice, the National District Attorneys Association, the Attorneys General of nineteen states, and a law professor named Orin Kerr. They all argue that the Justices should rule against the ACLU, and allow prosecutors and cops to track our phones without warrants.

[UPDATE: Here’s a transcript of the oral argument.]

Here are three important things you need to know about the case. 

1. The third party doctrine is in the spotlight—and maybe in the crosshairs. The Carpenter case provides the Supreme Court with an opportunity to reject—or at minimum, narrow—the outdated “third party doctrine,” as applied to cell phone location privacy. This precedent, established in 1979, says that we give up our right to privacy in information we knowingly and voluntarily hand over to third parties like phone companies. But in the 21^st century, when nearly everything we do requires that we leave a digital trail of sensitive data behind us, in the hands of private companies like Google, Apple, Microsoft, and AT&T, that can’t be right. Indeed, even the lawyer who argued in favor of the case establishing that terrible precedent has come out against its modern day application. He wrote this week in the Washington Post, arguing that the 1979 precedent he helped establish shouldn’t apply in the Carpenter case. If the Court limits the third party doctrine, or does away with it altogether, it’ll be a huge privacy victory for everyone in the United States, with potentially far-reaching implications that go well beyond cell phone privacy. We have reason to hope the Court will rule the right way—especially if the Court follows Justice Sotomayor’s concurrence in the Jones case. “[I]t may be necessary to reconsider the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties,” Justice Sotomayor wrote. “This approach 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.”
2. The Massachusetts high court showed the way. My colleagues at the ACLU of Massachusetts convinced our state’s high court to carve out a location tracking exception to the third party doctrine. In that case, Commonwealth v. Augustine, prosecutors obtained our client’s historical cell phone location data without a warrant. We challenged that seizure, arguing that a warrant was required. The Court agreed, affirming that Mr. Augustine—and all Massachusetts residents and visitors—have a reasonable expectation of privacy in our historical cell phone location data (technically known as cell site location information, or CSLI). The third party doctrine simply doesn’t apply, the court ruled.
3. The sky won’t fall if law enforcement is required to get warrants to track our location using our historical cell phone data. In addition to the ruling in Massachusetts, courts and legislatures across the country have come to the conclusion that law enforcement must obtain warrants to conduct cell phone location tracking. In these states, the sky has not fallen. Law enforcement is still able to investigate serious crimes like murder, and there has been no demonstrable, negative impact on public safety.

Privacy matters, especially in the 21^st century. We hope the Supreme Court agrees.