In a blockbuster 5-4 decision issued today, the U.S. Supreme Court ruled that warrantless government tracking of cellphone users via their cellphone location records violates the Fourth Amendment. "A person does not surrender all Fourth Amendment protection by venturing into the public sphere," declared the majority opinion of Chief Justice John Roberts. "We decline to grant the state unrestricted access to a wireless carrier's database of physical location information." The case is Carpenter v. United States. It arose after the after FBI obtained, without a search warrant, the cellphone...