Re: Sharon Begley's "Science Journal"
Posted: Tue Oct 29, 2019 2:24 am
Why Just Detecting Hidden Explosives May Not Cut Deaths
by Sharon Begley
July 8, 2005; Page B1
The explosions that ripped through three subways and a bus in central London yesterday underlined the urgency of determining what measures can reduce casualties from terrorist attacks. Although researchers have made strides in developing technologies to detect explosives at a distance, there is grave concern that such progress still falls short of protecting the public.
New research suggests that even perfect detection may not substantially lower the death toll from bombs set off in urban areas. And in some cases, terrorism experts now recognize a counterintuitive possibility: Warnings may lead to more fatalities.
Americans have an abiding belief in technological fixes, and that faith is now finding expression in the war on terror. The Pentagon's Defense Advanced Research Projects Agency recently commissioned the National Research Council to examine technologies able to detect suicide bombers before they reach their target. In its 2004 report, the NRC concluded that a number of sophisticated sensors, from millimeter-wave imagers to vapor-plume detectors, can detect hidden explosives more than 90 feet away, although none is perfectly specific (no false hits) or perfectly sensitive (no misses).
If, for example, a suicide bomber walked into a crowded plaza, "standoff" bomb detectors might well pick up an unambiguous signal. A terahertz imaging system could spy the telltale wires and explosives in 30 milliseconds, and ultraviolet-visible spectroscopy now in development could sniff out the trace vapors emitted by the ethylene glycol dinitrate in the plastique. Let's say the sensors alerted a security guard, who spotted the terrorist and yelled to the crowd, "Run, it's a bomb!"
In this scenario, the explosives-detection technology worked perfectly. An alarm sounded before a detonation. People were able to run or throw themselves to the ground. But when the bomber exploded, the casualty toll might not have been any less than if the sensors weren't deployed. Even worse, in some situations the intervention -- "Run!" or "Get down!" -- could lead to more casualties, conclude Edward Kaplan of the Yale University School of Management, New Haven, Conn., and Moshe Kress of the Naval Postgraduate School, Monterey, Calif., in a new study.
Even under the best-case assumption of sensors that are perfect, covert and cheap enough to deploy at every city intersection or throughout plazas, early detection unambiguously lowers the casualty count only if the bomber fails to detonate. Ensuring that outcome probably requires ubiquitous deployment of perfect sharpshooters, says Prof. Kress.
Early detection can backfire because of the grisly fact that human beings act as human shields. "There is a trade-off between crowd size and crowd blocking," says Prof. Kaplan. A large, dense crowd puts more people in harm's way, but "the probability of being exposed to a bomb fragment declines exponentially with the size of the crowd." As a crowd flees, there are fewer people near the bomber to absorb the fragments (as when a soldier falls on a grenade) and more people, unshielded, farther away. Simple geometry shows that you can hit more people at a radius 20 feet from a bomber than you can five feet from him.
"If the first ring of unshielded people is at a greater radius, there are more of them, and more will be hit," says Prof. Kaplan.
The same effect occurs if people throw themselves to the ground. That minimizes each person's exposed area, but also at the expense of decreasing human shielding. For bombs with 500 or more fragments (in Israel, 1,000 is typical), "hit the deck" can raise rather than cut casualties. If scores of people fall from an average height of five feet eight inches to 1.5 feet, the scientists calculate, casualties could rise as high as 50 from 37.
"We are not suggesting that standoff detection has no use, but having the ability to detect explosives doesn't automatically make you safe," says Prof. Kaplan. Since the conclusions reflect a best-case scenario -- perfect sensors do not yet exist -- casualty reduction with real-world devices would be even less than the researchers calculate in their study, published online this week in Proceedings of the National Academy of Sciences.
The limits of technology are becoming apparent to those leading the war on terrorism. "Response [to detection of a suicide bomber] is a very difficult problem," says Todd Brethauer, science adviser to the U.S. Interagency Technical Support Working Group. "While there are tremendous efforts under way, don't expect a miracle near term."
Instead, pre-empting bombers before they reach their target and destroying explosives labs is likely to bring a greater payoff. Israel suffered 26 suicide attacks in 2003 and 15 in 2004, a decline it attributes in part to earlier interdictions.
Fathoming who suicide bombers are, what motivates them, and what can stop them has become particularly urgent now that such attacks in Iraq have reached unprecedented levels, with more than 200 this year. That has prompted concern that a generation of terrorists is learning skills it can bring to the U.S. and Europe. Science and experience show that last-minute defense is the wrong way to play this lethal game.
by Sharon Begley
July 8, 2005; Page B1
The explosions that ripped through three subways and a bus in central London yesterday underlined the urgency of determining what measures can reduce casualties from terrorist attacks. Although researchers have made strides in developing technologies to detect explosives at a distance, there is grave concern that such progress still falls short of protecting the public.
New research suggests that even perfect detection may not substantially lower the death toll from bombs set off in urban areas. And in some cases, terrorism experts now recognize a counterintuitive possibility: Warnings may lead to more fatalities.
Americans have an abiding belief in technological fixes, and that faith is now finding expression in the war on terror. The Pentagon's Defense Advanced Research Projects Agency recently commissioned the National Research Council to examine technologies able to detect suicide bombers before they reach their target. In its 2004 report, the NRC concluded that a number of sophisticated sensors, from millimeter-wave imagers to vapor-plume detectors, can detect hidden explosives more than 90 feet away, although none is perfectly specific (no false hits) or perfectly sensitive (no misses).
If, for example, a suicide bomber walked into a crowded plaza, "standoff" bomb detectors might well pick up an unambiguous signal. A terahertz imaging system could spy the telltale wires and explosives in 30 milliseconds, and ultraviolet-visible spectroscopy now in development could sniff out the trace vapors emitted by the ethylene glycol dinitrate in the plastique. Let's say the sensors alerted a security guard, who spotted the terrorist and yelled to the crowd, "Run, it's a bomb!"
In this scenario, the explosives-detection technology worked perfectly. An alarm sounded before a detonation. People were able to run or throw themselves to the ground. But when the bomber exploded, the casualty toll might not have been any less than if the sensors weren't deployed. Even worse, in some situations the intervention -- "Run!" or "Get down!" -- could lead to more casualties, conclude Edward Kaplan of the Yale University School of Management, New Haven, Conn., and Moshe Kress of the Naval Postgraduate School, Monterey, Calif., in a new study.
Even under the best-case assumption of sensors that are perfect, covert and cheap enough to deploy at every city intersection or throughout plazas, early detection unambiguously lowers the casualty count only if the bomber fails to detonate. Ensuring that outcome probably requires ubiquitous deployment of perfect sharpshooters, says Prof. Kress.
Early detection can backfire because of the grisly fact that human beings act as human shields. "There is a trade-off between crowd size and crowd blocking," says Prof. Kaplan. A large, dense crowd puts more people in harm's way, but "the probability of being exposed to a bomb fragment declines exponentially with the size of the crowd." As a crowd flees, there are fewer people near the bomber to absorb the fragments (as when a soldier falls on a grenade) and more people, unshielded, farther away. Simple geometry shows that you can hit more people at a radius 20 feet from a bomber than you can five feet from him.
"If the first ring of unshielded people is at a greater radius, there are more of them, and more will be hit," says Prof. Kaplan.
The same effect occurs if people throw themselves to the ground. That minimizes each person's exposed area, but also at the expense of decreasing human shielding. For bombs with 500 or more fragments (in Israel, 1,000 is typical), "hit the deck" can raise rather than cut casualties. If scores of people fall from an average height of five feet eight inches to 1.5 feet, the scientists calculate, casualties could rise as high as 50 from 37.
"We are not suggesting that standoff detection has no use, but having the ability to detect explosives doesn't automatically make you safe," says Prof. Kaplan. Since the conclusions reflect a best-case scenario -- perfect sensors do not yet exist -- casualty reduction with real-world devices would be even less than the researchers calculate in their study, published online this week in Proceedings of the National Academy of Sciences.
The limits of technology are becoming apparent to those leading the war on terrorism. "Response [to detection of a suicide bomber] is a very difficult problem," says Todd Brethauer, science adviser to the U.S. Interagency Technical Support Working Group. "While there are tremendous efforts under way, don't expect a miracle near term."
Instead, pre-empting bombers before they reach their target and destroying explosives labs is likely to bring a greater payoff. Israel suffered 26 suicide attacks in 2003 and 15 in 2004, a decline it attributes in part to earlier interdictions.
Fathoming who suicide bombers are, what motivates them, and what can stop them has become particularly urgent now that such attacks in Iraq have reached unprecedented levels, with more than 200 this year. That has prompted concern that a generation of terrorists is learning skills it can bring to the U.S. and Europe. Science and experience show that last-minute defense is the wrong way to play this lethal game.