What is biometrics? It is the science of identifying individuals by their unique biological characteristics. The best known and earliest example is fingerprints, used by ancient Babylonians as a signature and by police since the turn of the 20th century to identify criminals.

But in the last decade there has been a boom in more advanced biometric technology, allowing people to be identified, and sometimes remotely tracked, by their voices, the irises of their eyes, the geometry of their faces, and the way they walk. The FBI is currently consolidating existing fingerprint records, mug shots, and other biometric data on more than 100 million Americans into a single $1.2 billion database.

When it is completed, in 2014, police across the country will theoretically be able to instantly check a suspect against that vast and growing array of data. Law-enforcement officials are enthusiastic about this growing power, while civil libertarians are aghast. "A society in which everyone's actions are tracked is not, in principle, free," said William Abernathy and Lee Tien of the Electronic Frontier Foundation. "It may be a livable society, but would not be our society."

Every person has unique patterns within the colored part of his or her eye. A device scans your iris and compares it with photos of irises on record, identifying people with accuracy rates of 90 to 99 percent, depending on the conditions and system used. Iris scanners are now widely used on military bases, in federal agencies, and at border crossings and airports. An improved version can remotely assess up to 50 people per minute, making it possible to scan crowds for known criminals or terrorists whose iris patterns are on file.