But the warrant was good for ten days. Joe Friday & Co. waited 11 days to attach the GPS; and they also did it in Maryland, where the DC judge had no jurisdiction. The prosecution sought admission of the GPS evidence without a warrant. The trial court allowed it, but the D.C. Circuit reversed the conviction, holding that four weeks of warrantless GPS surveillance is too much.

Hard cases famously make bad law; but an easy case like Jones's can distort the resulting rule as well. The government is asking the Supreme Court to hold that putting a GPS on a car isn't really a "search" at all, since it's "no more" than 28 days of Joe Friday on stakeout. If so, of course, the government can use a GPS on any of us, with or without any suspicion that we've done anything wrong. A "low cost, real time GPS tracker with advanced features" will run you just $59.99 on amazon.com. If you don't need a warrant, you can stick trackers on a lot of cars, and raw computer power will give you a picture of many people's lives -- where they shop, where they worship, who they're sleeping with.

That, argue Jones's lawyers in their brief to the Court, "poses harrowing threats to personal privacy and security-- threats that have until now existed only in dystopian novels."

But in Fourth Amendment law, something either is a search or it isn't. In order to be a "search," we must have a "reasonable expectation of privacy." A phone booth (here's a picture for young readers who may never have seen one) is such a place (Clark Kent even used to change clothes there); a public street is not.

The nine Justices of the Supreme Court, for all their manifold excellences, are not futurists. (The Court still gives lawyers who argue before it a quill pen as a souvenir, and each Justice still has a spittoon at his or her feet during argument.) In the argument today, they had to guess at the direction in which technology is taking us, and at how people feel about it.

Early in the session, Chief Justice Roberts asked Deputy Solicitor General Michael Dreeben the question on all nine of their minds: "You think there would ... not be a search if you put a GPS device on all of our cars, monitored our movements for a month?"

"The Justices of this Court?" Dreeben asked cautiously.

"Yes," Roberts said.

Dreeben said "the Justices of this Court when driving on public roadways" have no reasonable expectation of privacy.

"So," Roberts said, "your answer is yes, you could tomorrow decide that you put a GPS device on every one of our cars, follow us for a month; no problem under the Constitution?"

Dreeben said yes.

Justice Breyer noted that under the government's rule, "there is nothing to prevent the police or the government from monitoring 24 hours a day the public movement of every citizen of the United States. ... So if you win, you suddenly produce what sounds like 1984 from their brief."