A person outside of the Supreme Court in 2014, when it heard arguments in the cellphone privacy case Riley v. California. Jose Luis Magana/AP

The U.S. Supreme Court will hear a closely watched case on Wednesday that will determine whether authorities can search your cellphone location data without a warrant, in a dispute that could upend decades of privacy precedent.

The case, Carpenter v. U.S., will look at whether the Fourth Amendment — which guarantees your right against unreasonable searches and seizures — protects you against warrantless searches of cellphone records that track your movements.

For decades, the high court has said that when you share your information with a third party, like a bank or telephone company, you lose the expectation that it will remain private. But the Supreme Court may rule that cellphones have ushered in a new era of privacy expectations.

“I think Carpenter could significantly increase protection for the electronic data,” Erwin Chemerinsky, the dean of Berkeley Law and a Supreme Court expert, told Yahoo Finance in an email message.

He added: “The court may well overrule its earlier decisions that there is no privacy interest if third parties have the information.”

A cellphone privacy case that began with … the theft of smartphones

Fittingly, the Carpenter case began with a series of armed robberies of smartphones at Radio Shack and T-Mobile (TMUS) locations in Michigan and Ohio in 2010 and 2011. A man named Timothy Carpenter and his half-brother, Timothy Sanders, were convicted of nine armed robberies tied to those smartphone thefts, according to an appeals court opinion in the case. At trial, part of the government’s evidence consisted of records showing that each Timothy had used his cellphone within the vicinity of several of the robberies when they occurred.

In fighting their conviction, the Timothys contended the government had violated the Fourth Amendment when it collected their cellphone location records without a warrant. However, a federal appeals court in Cincinnati, Ohio, ruled against the half-brothers, finding the Fourth Amendment doesn’t “yet” extend to cellphone location data.

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The majority opinion of that court distinguished between the “content” of a communication and the “information necessary to send it.” The government can’t read your letters, or your emails for that matter, nor can it listen to what you’re actually saying on the phone, the court ruled. However, the court ruled that anything on the outside of an envelope is fair game — as is so-called metadata that’s used to route internet communications.

“The business records here fall on the unprotected side of this line. Those records say nothing about the content of any calls,” the court ruled. “Instead the records include routing information, which the wireless providers gathered in the ordinary course of business.”

The case for abandoning a long-held doctrine

Now Carpenter, and his lawyers at the American Civil Liberties Union, are asking the nation’s highest court to reconsider that ruling. At issue is the so-called third-party doctrine, which says that if you hand over information to third parties you shouldn’t expect it to remain a secret.

This doctrine may need to be updated for the digital age, some experts argue. One argument for reconsidering this doctrine is that, in reality, people actually have no clue to what extent their phone companies know where they go and how long they stay there.

“That makes the case a good opportunity for the court to reconsider whether location information that is shared with a cell-phone provider might still be kept private from the government,” Lior Jacob Strahilevitz, a professor at the University of Chicago Law School, told Yahoo Finance.

Added Strahilevitz, co-author of an amicus brief in support of Carpenter, “And the justices might use the opportunity to abandon a third-party doctrine that gives the government a lot of leeway to scoop up facts about individuals without judicial oversight.”

Some of America’s biggest tech giants — including Apple (AAPL), Facebook (FB), Google (GOOG, GOOGL), and Yahoo Finance parent Verizon (VZ) — also filed an amicus brief in the case. While that brief technically supported neither side in the case, the tech giants hinted that the court should reconsider whether the government should easily be able to get its hands on that data. At the same time, they seemed to want to ensure that companies can access that data for their own purposes.

“Because the data that is transmitted can reveal a wealth of detail about people’s personal lives … users of digital technologies reasonably expect to retain significant privacy in that data, notwithstanding that technology companies may use or share the data in various ways to provide and improve their services for their customers,” the brief said. “Fourth Amendment doctrine must adapt to this new reality.”

Do cellphone users ‘voluntarily expose’ themselves to observation?

Of course, not everybody agrees that the Fourth Amendment should adapt to shield your cellphone location data from law-enforcement scrutiny. The prominent legal expert Orin Kerr submitted a brief on behalf of the government arguing that the Fourth Amendment doesn’t protect that data because it’s the equivalent of being observed in a public space.

And the Fourth Amendment, he noted, provides zero protection against being watched in public.

“Just as a person voluntarily exposes himself to observation by traveling in public to deliver a communication, so does a person voluntarily expose himself to observation by hiring an agent [i.e., a cellphone network provider] to deliver his communications remotely,” Kerr’s brief stated. “The Fourth Amendment is not implicated by compelling testimony from an eyewitness or by observation in public … The same rule should apply in the analogous context of obtaining historical cell-site records.”

‘Not a knee-jerk vote for the government’

While Kerr may be siding with the government, the U.S. may have an uphill battle in convincing the Supreme Court of its argument. The high court has a history of protecting privacy, and in 2014 unanimously ruled that searching the digital contents of a cellphone without a warrant is unconstitutional. Of course, that case involved searching the actual contents of a smartphone without a warrant and not just the location data held by cellphone service providers.

Moreover, the makeup of the Supreme Court has changed since 2014. Still, Kerr himself told The New York Times earlier this year that the court’s newest judge — Donald Trump appointee, Neil Gorsuch — is not a “knee-jerk vote for the government.”

If the Supreme Court does end up taking a step further on digital privacy, the case, as University of Chicago Law professor Lior Jacob Strahilevitz noted, could turn out to be “a major landmark.”

Erin Fuchs is deputy managing editor of Yahoo Finance.

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