At any time, the government could dig through the past communications of anyone it wanted to victimize in search of a crime (and everybody’s communications contain evidence of something). At any point, for all perpetuity, any new administration—any future rogue head of the NSA—could just show up to work and, as easily as flicking a switch, instantly track everybody with a phone or a computer, know who they were, where they were, what they were doing with whom, and what they had ever done in the past.
