How would you feel if the government could freely use sophisticated satellite technology and computers to secretly turn your property into an invisible surveillance device and track your every public move 24/7? The Obama administration recently told the Supreme Court that it’s perfectly constitutional for the government to do so, without so much as a warrant, probable cause or any basis for suspicion. The future of privacy in the digital age will turn on whether the Supreme Court agrees.

In the case before the Court, United States v. Jones, police and FBI agents in Washington, DC, surreptitiously attached a GPS device to the underside of a suspect’s car and used it to monitor everywhere the car went for a month. That information led to the discovery of cocaine and Antoine Jones’s conviction for drug trafficking. But the US Court of Appeals for the DC Circuit reversed the conviction, ruling that the police violated the Fourth Amendment when they monitored Jones’s car around the clock for a month without a warrant.

The government argues that this use of a GPS did not infringe privacy because the police tracked the car’s movements only in public. The argument relies on two Supreme Court decisions from the 1980s that ruled that using a beeper to assist in tailing a car does not violate privacy as long as it only facilitates the police in observing where the car went in public. As the Court explained in Karo v. United States, what a person “knowingly exposes to the public” is not private. But what we “expose to the public” when we go out changes dramatically when a GPS makes it possible, at vanishingly low cost and without human intervention, to follow us everywhere we go for an unlimited period. The Fourth Amendment protects “reasonable expectation of privacy,” and surely people reasonably expect that the pattern of their trips in public over an extended period will remain private.

So the Court of Appeals for the DC Circuit ruled, reasoning that the kind of information the police can glean from such monitoring is qualitatively different from what they learn from manually following a car on a single trip. They might learn, for example, what doctors or psychiatrists you visit, how religious you are, what political meetings you attend, your drinking habits and with whom you associate, whether a friend, lover or confidante.

Where, however, does the Constitution draw the line between a single trip and a series of trips? When does the collection of public information impermissibly intrude on personal secrets? These are extraordinarily difficult distinctions, and at oral argument the Court seemed troubled by how to draw clear and principled lines.

There may, however, be a simpler way to resolve this case, by focusing on the fact that the police attached the GPS to Jones’s car without his knowledge, thereby transforming his property into a secret surveillance device to be used against him. Justices Antonin Scalia and Anthony Kennedy expressed concern about that aspect of the police behavior, and their votes should be sufficient to establish that the Fourth Amendment applies.

But if the case is resolved on that ground, it will leave many difficult questions unanswered. What happens if the police simply employ a pre-existing GPS device that’s already in the car? Many new cars come equipped with the OnStar system, which automatically monitors the car’s location so that services can be directed to it swiftly in an emergency. And most people carry cellphones, which regularly emit signals to their phone provider as to their whereabouts. Should the police be able to get that information without obtaining a warrant or demonstrating that they have some objective basis for suspecting a person of wrongdoing? Lower courts are divided on the question. Some have reasoned, following Supreme Court precedents, that we have no “expectation of privacy” in information we share with others, so that if the cellphone company or OnStar has it, the government can get it without constitutional limitation.

Others have recognized that this rule, like the rule that what is “exposed to the public” is categorically not private, needs modification if we are to preserve privacy, and even democracy, in the twenty-first century. As Justice Stephen Breyer asked at the oral argument in Jones, “What would a democratic society look like if a large number of people did think that the government was tracking their every movement over long periods of time?”

Democracy depends on the freedom of citizens to engage with one another in public and private without broadcasting their every association to the government. If the government can hijack our cars, our cellphones, our GPS monitors, our computers and transform our property into invisible surveillance devices against us, without having to satisfy a judge that there is probable cause of criminal activity, what will be left of democratic freedom? As US District Court Judge Nicholas Garaufis recently wrote, in a decision requiring the police to get a warrant based on probable cause to obtain access to cellphone location information, “It is time that the courts begin to address whether revolutionary changes in technology require changes to existing Fourth Amendment doctrine.” If we do not adapt our laws to advancing technology, privacy may soon go the way of the typewriter.