Indeed, the court developed the "reasonable expectation of privacy" standard precisely because technologies unknown to the Framers -- telephones and eavesdropping equipment -- made it possible to secretly collect sensitive information without trespassing on the target's property. Until the 1967 case Katz v. United States, the court held that surveillance of telephone calls did not constitute a search unless it involved a physical intrusion.
But the Katz test is notoriously fuzzy. While Alito thought a month of GPS tracking was clearly a search, for instance, he said "relatively short-term monitoring of a person's movements on public streets accords with expectations of privacy that our society has recognized as reasonable." He added that even long-term monitoring might be acceptable without a warrant "in the context of investigations involving extraordinary offenses" -- a loophole big enough to drive many GPS-tracked vehicles through.
More fundamentally, the very technologies that threaten privacy also change people's expectations. "Even if the public does not welcome the diminution of privacy that new technology entails," Alito wrote, "they may eventually reconcile themselves to this development as inevitable."
But as Alito noted, there is another possible outcome: A public alarmed by the erosion of privacy can demand statutory limits on government surveillance, which then provide clear evidence of expectations "our society has recognized as reasonable." That is ultimately what happened with wiretapping, although only after the court decided that Fourth Amendment rights were at stake.
There is a chicken-and-egg problem here that reflects the circularity of the Katz test: Privacy is expected when it's protected, and it's protected when it's expected. We need to expect more, or we will end up with less.
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