The Many Revolutions of Carpenter

Privacy and Security, Networks, the Internet, and Cloud Computing and Search and Advertising

Article Snapshot


Paul Ohm


Harvard Journal of Law & Technology, Vol. 32, No. 2, pp. 357-416 (Spring 2019)


The Supreme Court’s opinion in Carpenter v. United States is revolutionary. The Supreme Court recognizes that Fourth Amendment privacy rights were intended to preserve a society free of constant surveillance.

Policy Relevance

For the first time, police will often need to obtain a warrant to access commercial databases.

Main Points

  • In Carpenter v. United States, decided in 2018, the Supreme Court ruled that a customer’s cell phone location data is protected by the Fourth Amendment; for the first time, the Court held that police need a warrant to require a business to reveal information about its customers.
  • Carpenter uses three factors to determine whether privacy rights apply when police seek information from a database about individuals:
    • The deeply revealing nature of the information.
    • The depth and breadth of the information, and the number of people covered.
    • Whether collection is inescapable and automatic.
  • This revolutionary case means that the police probably need a warrant to access data from fitness trackers and connected cars, web browsing habits, or telephone and banking records.
    • Warrants might be needed to access medical records or genetic information.
    • Carpenter also should apply to surveillance by drones or license plate readers.
  • Under the “third party doctrine,” information a person voluntarily discloses to a third party is not protected by a reasonable expectation of privacy; Carpenter turns this idea inside out.
  • Formerly, cases were unclear as to whether the “reasonable expectation” inquiry was descriptive (what do ordinary Americans expect?) or normative (what kind of society does the Constitution seek to promote?); after Carpenter, the inquiry will be normative.
  • Carpenter ties the “reasonable expectations” inquiry to constitutional history; the case suggests that the Constitution requires the police to be more inefficient than technology allows, as the Framers of the Constitution sought to place obstacles in the way of permeating police surveillance.
  • Several Justices support a rule of “technological equivalence,” that is, if police use a new technology to gather information equivalent to traditionally protected information, use of the new technology also requires constitutional protection.
    • Police must have a warrant for thermal imaging to “see” inside the home.
    • Police might need a warrant to read email, which resembles private letters.
  • Some Justices embrace “tech exceptionalism,” the idea that recent advances in information technology are fundamentally different from what has come before.

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