I ran a practice inside Brooklyn’s public defense office focused on the police department’s use of science and technology, so I am well accustomed to the department’s collection of personal information on people being investigated for crimes. But more than a decade of observing traditional police work did not prepare me for what the department is doing today: building vast, hidden repositories of data it collects on everyone in the city, with no clear boundaries on how it can be used…. The city’s police force has spent more than $3 billion amassing information that reveals where you have been, whom you have interacted with and what you have said, thought and believed. Unlike previous surveillance methods, new digital tools allow law enforcement agencies to conduct surveillance persistently, universally, at an unimaginable scale. They can do so with no special permission, no oversight and no advance planning. The results amount to a digital time machine that not only makes our past constantly available to law enforcement officers but also can provide them with predictions about our futures.