Not until I walked with Bruce Schneier toward the mass of people unloading their laptops did it occur to me that it might not be possible for us to hang around unnoticed near Reagan National Airport’s security line. Much as upscale restaurants hang mug shots of local food writers in their kitchens, I realized, the Transportation Security Administration might post photographs of Schneier, a 48-year-old cryptographer and security technologist who is probably its most relentless critic. In addition to writing books and articles, Schneier has a popular blog; a recent search for “TSA” in its archives elicited about 2,000 results, the vast majority of which refer to some aspect of the agency that he finds to be ineffective, invasive, incompetent, inexcusably costly, or all four.

As we came by the checkpoint line, Schneier described one of these aspects: the ease with which people can pass through airport security with fake boarding passes. First, scan an old boarding pass, he said—more loudly than necessary, it seemed to me. Alter it with Photoshop, then print the result with a laser printer. In his hand was an example, complete with the little squiggle the T.S.A. agent had drawn on it to indicate that it had been checked. “Feeling safer?” he asked.

Ten years ago, 19 men armed with utility knives hijacked four airplanes and within a few hours killed nearly 3,000 people. At a stroke, Americans were thrust into a menacing new world. “They are coming after us,” C.I.A. director George Tenet said of al-Qaeda. “They intend to strike this homeland again, and we better get about the business of putting the right structure in place as fast as we can.”

The United States tried to do just that. Federal and state governments embarked on a nationwide safety upgrade. Checkpoints proliferated in airports, train stations, and office buildings. A digital panopticon of radiation scanners, chemical sensors, and closed-circuit television cameras audited the movements of shipping containers, airborne chemicals, and ordinary Americans. None of this was or will be cheap. Since 9/11, the U.S. has spent more than $1.1 trillion on homeland security.

To a large number of security analysts, this expenditure makes no sense. The vast cost is not worth the infinitesimal benefit. Not only has the actual threat from terror been exaggerated, they say, but the great bulk of the post-9/11 measures to contain it are little more than what Schneier mocks as “security theater”: actions that accomplish nothing but are designed to make the government look like it is on the job. In fact, the continuing expenditure on security may actually have made the United States less safe.

The first time I met Schneier, a few months after 9/11, he wanted to bet me a very expensive dinner that the United States would not be hit by a major terrorist attack in the next 10 years. We were in Washington, D.C., visiting one of the offices of Counterpane Internet Security, the company he had co-founded in 1999. (BT, the former British Telecom, bought Counterpane seven years later; officially, Schneier is now BT’s chief security technology officer.) The bet seemed foolhardy to me. Defense Secretary Donald Rumsfeld had just told The Washington Times that al-Qaeda was dispersing its killers all over the world.

From an airplane-hijacking point of view, Schneier said, al-Qaeda had used up its luck. Passengers on the first three 9/11 flights didn’t resist their captors, because in the past the typical consequence of a plane seizure had been “a week in Havana.” When the people on the fourth hijacked plane learned by cell phone that the previous flights had been turned into airborne bombs, they attacked their attackers. The hijackers were forced to crash Flight 93 into a field. “No big plane will ever be taken that way again, because the passengers will fight back,” Schneier said. Events have borne him out. The instigators of the two most serious post-9/11 incidents involving airplanes— the “shoe bomber” in 2001 and the “underwear bomber” in 2009, both of whom managed to get onto an airplane with explosives—were subdued by angry passengers.