How do you stop a suicide bomber on his way to a target? Until recently, that wasn't an urgent question for scholars in the West. But it is now, and scientists, military strategists and security experts are scrambling, in different directions, to find an answer.

Last year, Darpa, the Pentagon's research arm, convened a panel of experts through the National Research Council to study methods to detect suicide bombers from a distance, before they strike. The panel's report presented some provocative ideas, including "detection by detonation." Under this plan, soldiers at a military checkpoint would fire radiation at each approaching car. If there were no explosives on board, the car would pass through the beam safely. But if the car carried suicide attackers, the radiation would cause their bombs to explode, killing everyone on board (and anyone unlucky enough to be nearby), but leaving the checkpoint unharmed. Another intriguing notion from the report: "distributed biological sensors" -- bees, moths, butterflies or rats specially trained to pick up bomb vapors, buzzing or fluttering through a crowd, sniffing for fumes. (The rats, equipped with global-positioning-system chips, would work the sewers.)

Even if you manage to detect a suicide bomber, what do you do next? This question was taken up by Edward H. Kaplan, a professor of public health at Yale, in a paper he published in July, written with Moshe Kress of the Naval Postgraduate School in Monterey, Calif. Kaplan and Kress investigated the physics of a belt-bomb blast and reached some unexpected conclusions. It turns out that very few people are killed by the concussive force of a suicide explosion; the deadly weapon is in fact the shrapnel -- the ball bearings, nails or pieces of metal that the attacker attaches to the outside of his bomb. The explosions, though, are usually not powerful enough to send these projectiles all the way through a human body, which means that if your view of a suicide bomber is entirely obscured by other people at the moment of detonation, you are much more likely to escape serious injury. Because of the geometry of crowds, Kaplan found, a belt bomb set off in a heavily populated room will actually yield fewer casualties than one set off in a more sparsely populated area; the unlucky few nearest to the bomb will absorb all of its force.

The authors used these calculations to question some assumptions about what authorities should do if they detect a bomber. The International Association of Chiefs of Police issued guidelines this year suggesting that police officers who find a bomber in a crowd should fire shots into the air to cause people near the bomber to scatter or hit the deck. But Kaplan's calculations demonstrate that in many cases, this would make things worse -- as a packed crowd ran away from a bomber or dropped to the ground, the circle of potential victims around him would get wider and thus more populous, and more lives could be lost.