The real meat, for those interested in that question, comes in two concurring opinions, one from Justice Sotomayor (who also signed the majority's) and another authored by Justice Alito and signed by Justices Ginsburg, Breyer, and Kagan. The problem with Scalia's opinion, Alito writes, is that it overly relies on originalist reasoning -- that is, whether the case in question comports with the Fourth Amendment as the Framers meant it. But that interpretation of the Fourth Amendment -- one "based on 18th-century tort law" -- is insufficiently illuminating for what to do with today's technologies, technologies that allow for a level of tracking never imagined by James Madison and others.

As Alito writes, in one of the decision's most incisive passages, "In the pre-computer age, the greatest protections of privacy were neither constitutional nor statutory, but practical. ... The surveillance at issue in the case -- constant monitoring of the location of a vehicle for four weeks -- would have required a large team of agents, multiple vehicles, and perhaps aerial assistance. Only an investigation of unusual importance could have justified such an expenditure of law enforcement resources." Today, we need law to make up for the protections that technological difficulties once provided.

Alito goes on to say that the logical end of the Court's reasoning is a bit laughable. So it's possible that attaching something to a car is unconstitutional, but tracking their every move -- as long as you do not technically trespass while installing the device -- is just fine? This is why relying on 18th-century understandings of the Fourth Amendment fail to help us with modern questions.

The alternative standard for evaluating a Fourth Amendment violation, Alito writes, is a test introduced in a case called Katz v. United States, which was decided in 1967. In that case, which examined whether the right to privacy extends to public telephone booths, the Court said that the right extends to places where a reasonable person has an expectation of privacy. In Alito's view, long-term GPS tracking for "most offenses" falls beyond the scope of what a reasonable person would expect.

But this standard too has problems, "Dramatic technological change may lead to periods in which popular expectations are in flux and may ultimately produce significant changes in popular attitudes." How can courts evaluate whether an expectation is reasonable?

Perhaps they shouldn't have to, is Alito's response. Perhaps courts are the wrong savior for our privacy concerns. Legislation -- from Congress -- may be a better tool. This, Alito explains, is what happened in the case of wiretapping, which has been regulated by statute since 1986. So far, however, on the question of GPS tracking, Congress and most states have not taken any action.