A man types on a computer keyboard in this Feb. 28, 2013 illustration file picture. REUTERS/Kacper Pempel/Files  

In the wake of The Guardian’s remarkable revelation Wednesday that the National Security Agency is collecting phone records from millions of Americans, defenders of this dragnet surveillance program are insisting that the intelligence agency isn’t eavesdropping on the calls – it’s just scooping up “metadata.” The implication is that civil liberties complaints about Orwellian surveillance tactics are overblown.

But any suggestion that Americans have nothing to worry about from this dragnet collection of communications metadata is wrong. Even without intercepting the content of communications, the government can use metadata to learn our most intimate secrets – anything from whether we have a drinking problem to whether we’re gay or straight. The suggestion that metadata is “no big deal” – a view that, regrettably, is still reflected in the law – is entirely out of step with the reality of modern communications.

So what exactly is metadata? Simply, if the “data” of a communication is the content of an email or phone call, this is data about the data – the identities of the sender and recipient, and the time, date, duration and location of a communication. This information can be extraordinarily sensitive. A Massachusetts Institute of Technology study a few years back found that reviewing people’s social networking contacts alone was sufficient to determine their sexual orientation. Consider, metadata from email communications was sufficient to identify the mistress of then-CIA Director David Petraeus and then  drive him out of office.

The “who,” “when” and “how frequently” of communications are often more revealing than what is said or written. Calls between a reporter and a government whistleblower, for example, may reveal a relationship that can be incriminating all on its own.