New technologies are transforming the capabilities of law enforcement. Police agencies now have devices to track our cellphones and software to hack our networks. They have tools to sift the vast quantities of digital silt we leave behind on the Internet. They can deploy “big data” algorithms meant to predict where crimes will occur and who will commit them. They have even transformed the humble closed-circuit video camera—and its more recent companion, the body camera—into biometric tracking devices equipped with artificial intelligence meant to pick faces out of a crowd and, eventually, to mine gigabytes of stored footage to automatically reconstruct the movements of their targets. These kinds of novel police technologies test the constit...