In Carpenter v. United States, the Court had to grapple with modern technology and expectations of privacy in a world of shared data. Some called this decision “the most consequential privacy decision of the digital age”.
Mr. Carpenter was suspected of participating in a series of robberies. Having obtained his cell phone number, FBI applied for (under the Stored Communications Act) and got an order from a magistrate judge to obtain his cell phone transactional record. This information included date and time of calls, and the approximate location where calls began and ended, based on his connections to cell phone towers. Using this cell phone information, government charged Mr. Carpenter with six counts of robbery. Mr. Carptenter moved to suppress the cell phone evidence on Fourth Amendment grounds, arguing that FBI needed a warrant based on probable cause to obtain his cell phone records.
The Court had to decide whether the warrantless search and seizure of cell phone records, which include the location and movements of cell phone users, violated the Fourth Amendment. In a 5 to 4 opinion, the Court decided that Mr. Carpenter had a reasonable expectation of privacy in his physical movements, as chronicled by his cell-site location information. Even though Mr. Carpenter was aware that his cell phone provider keeps logs, and knowingly shared his cell-site data with his wireless carrier, this did not wholly eliminate his privacy interest in the data. Given the indispensability of cell phones in modern life, the Court decided that users do not volunteer to share this data. Tracking a person's movements and location through extensive cell-site records is far more intrusive than the precedents might have anticipated.
As a result, the government’s acquisition of Mr. Carpenter’s cell-site records was a Fourth Amendment search subject to warrant requirements. Government’s warrantless acquisition of data violated Mr. Carpenter’s Fourth Amendment right against unreasonable searches and seizures.
When all the various technologies that people use daily, collecting our data (Google, Facebook, Amazon’s Echo, fitness apps, etc), the potential for government’s easy access to personal data is unlimited. This ruling has potential broad implications for government access to all manner of data, as the Court rejected government’s argument that people lose their privacy rights merely by using today’s technologies.