Back to the Future
At common law, any trespass against a chattel (personal property) constituted a tort, but under modern law, one has to show actual damage as well. I'm guessing that you've gone your entire legal career without having to know that, but it's essential to understanding the Supreme Court's decision in US v. Jones, holding that the police had to secure a warrant before attaching a GPS device to a car and using it to monitor the car's movements over a period of time. The conclusion was unanimous, but the two major opinions offered radically different ideas of why that was and what the 4th Amendment was intended to do.
In Jones, the police had surreptitiously placed a GPS device on his vehicle and used it to track his movements over the next month. The 2,000 page log showing those movements was used to convict him and give him a life term for drug trafficking. There were actually two aspects of the police conduct which raised issues: the initial planting of the GPS device, and the use of it to track his movements.
The first one was problematic for the defendant. The current view of what constitutes a search or seizure under the 4th Amendment was articulated in 1968 in Katz v. United States, a wiretapping case. The Court had considered that issue 40 years earlier, and had concluded then that no 4th Amendment violation occurred because the wiretaps were attached to public phone lines, and the defendant had no property interest in those lines. A similar argument could have been made in Katz, where the police had attached a listening device to a public phone booth. Katz found that property interests weren't controlling, though: the 4th Amendment was intended to protect privacy, not property. Since then, the determination of whether an action constitutes a search or seizure under the 4th depends on whether it intrudes upon a defendant's "reasonable expectation of privacy." And there are loads of cases holding that a person doesn't have a reasonable expectation of privacy in the outside of his car. When Jones parked his van in a public lot, he knew that anyone could walk up to it. Even the police.
Scalia, writing for five of the justices, finds that a warrant was required, but his motives are suspect. He gets around the privacy analysis by discarding it. He is the master originalist: his opinions in Blakely, Heller, and Crawford are filled with references to the cases of Walter Raleigh and George Dingler and people you've never heard of, while sorting through every historical just short of what James Madison had for breakfast the morning he wrote the rough draft of the Bill of Rights. To him, and a bare majority of the court, it is a simple matter: "The Government physically occupied private property for the purpose of obtaining information," and "such a physical intrusion would have been considered a 'search' within the meaning of the Fourth Amendment when it was adopted."
Such an intrustion would have been considered a "trespass" under common law. Trespass, though, is a matter of property law, and here Scalia's purpose is clear: he wants to unmoor Fourth Amendment law from Katz and return it to a property-interest analysis. Not that he calls for overruling Katz: he simply sees it as adding to, rather than replacing, the property-interest analysis.
This is too much for Alito, who, along with Breyer, Kagan, and Ginsburg, concur only in judgment. For a conservative, Alito certainly doesn't do anything to burnish his own originalist credentials, noting that there are no 18-century analogues to what occurred here. After a debate on the fine points of trespass law, Alito argues that it the majority opinion overlooks the main problem: the use of the device to monitor Jones' movements over a month's period of time. To Alito, that certainly constituted a search, and a warrant was required.
Well, you say tomahto, I say tomayto, the bottom line is that everybody agrees a warrant is required to attach a GPS device, right? Ummm... no. The government had also claimed that even if the attachment was a search, they had probable cause to believe that Jones was the leader of a large drug ring, and thus the search was reasonable. The majority doesn't address this issue because they deem it forfeited, since it wasn't raised in the court below. Alito wholly ignores the placement of the device and concentrates on its use, but implies that short term use, or even longer-term use for certain crimes, might not be a 4th Amendment violation. The former, while injecting uncertainy -- how long is "short-term"? -- at least has some logical integrity: one might argue that a person could have a reasonable expectation that the police personlly might follow him around for a few days, but wouldn't have the resources to do that for an entire month. The latter argument is simply untenable: there is no legal or logical basis for concluding that one's reasonable expectation of privacy hinges upon the nature of the crime which one is being investigated for.
Perhaps the most interesting opinion is Sotomayor's. Although she concurs in Scalia's opinion, she also agrees with Alito that privacy expectations are critical here, and she, more than either of the other two, seems fully aware of the implications of modern technology for privacy interests. What if the government requires automakers to install GPS devices in all vehicles? Presumably for safety, but it's not hard to see how the executive, with no oversight, could abuse that power. In fact, Sotomayor suggests it may be necessary to rethink the whole "reasonable expectation of privacy" concept in light of technological advances. We voluntarily disclose a vast amount of information to third parties; my Internet provider has information on every website I've visited. I expect them to keep that information, but that doesn't mean I should have a reasonable expectation that they'll turn it over to the government.
So what's the upshot? Some commentators have suggested that the police will still use GPS devices, at least for short periods to get around the concurrence's objections, and to argue that the police have probable cause to place the device -- the argument they forfeited in Jones -- to get around Scalia's. The latter argument poses its own problems, though. Even assuming the police had probable cause to believe that Jones was a drug kingpin, what exception to the warrant requirement would cover placing a GPS device on the car? One might think the automobile exception would apply, but that exception is based on the mobility of the vehicle, and the resultant lack of time for the police to obtain the warrant. In the absence of any evidence showing that the car would imminently be moved to an undisclosed location, it's difficult to see how the police wouldn't have the time to obtain a warrant.
So Jones is not nearly as defining as the 9-0 vote would indicate. The details could be worked out in subsequent cases, but there's another outcome here that Alito suggests. He points out that in the wake of the Katz decision, Congress passed a detailed wiretapping law, and in fact statutory law, not case law, has governed that subject since. The same approach might be used to handle GPS monitoring and other similar technological encroachments on privacy.
In fact, that would probably be a superior to having the courts address it on a case-by-case basis. Perhaps the best way of deciding what our reasonable expectation of privacy should be is to have the people democratically decide, through their legislature, just what reasonable expectation of privacy they want.