But the ruling only dealt with the physical act of placing the GPS device on the vehicle and tracking Mr. Jones. Justice Antonin Scalia wrote the majority opinion. Justice Scalia said the case did not require the court to decide if electronic monitoring without trespassing onto someone's property is also a violation of privacy.

Law professor Renee Hutchins at the University of Maryland says that is a big question that remains to be answered. We spoke with her on Skype.

RENEE HUTCHINS: "Most people have have smartphones. A lot of people have cars that have GPS pre-installed. So the government doesn't have to do the installation. The installation, which was the hook for Justice Scalia, is already accomplished. We do it voluntarily."

Justice Sonya Sotomayor suggested that modern technology may soon force us to reconsider expectations of privacy. Professor Hutchins explains.

RENEE HUTCHINS: "Justice Sotomayor, actually in talking about the modern society that we live in, said, you know, we really have to perhaps rethink what it means for things to be private in a world where we voluntarily give up so much information. In a world where there's Facebook and GPS on your cell phone and GPS in your car, how should the court be thinking about constitutional protections in a world like that?"

Four other justices, led by Samuel Alito, questioned the wisdom of limiting the ruling only to a trespass of private property. They said the more important issue is the use of GPS for the purpose of long-term tracking.