Editor's note: Peter Rez is a professor of physics at Arizona State University.
(CNN) -- Most passengers want to get to their destination. A very small number, by my estimates about one in a few billion, desire immortality and believe they will achieve some greater objective by blowing themselves up and taking hundreds of their fellow passengers with them. The problem for the authorities is how to intercept this small number of individuals and frustrate their plans.
There are two different philosophies. The Israeli view is that one looks for the perpetrator, not the tool or weapon. All passengers about to board a flight are interviewed. Many in this country erroneously describe the process as "profiling," but I think a better description would be "triage." The Israeli view is that one does not waste time and resources on passengers who do not pose any threat to safety and security.
Those doing the interviews are generally college graduates who have completed their military service and have had significant additional training. The key point is that they are trusted to use their discretion on how much to investigate any given passenger. This system clearly catches those dressed in a T-shirt, with no checked baggage who have bought a one-way ticket for travel to Detroit, Michigan, in midwinter!
The Transportation Security Administration system in the United States relies on searching for the tool or weapon. Those doing the screening have no discretion and follow a ritual ordained from above. We have now reached the point where this method is unworkable.
In the period since 9/11, the authorities have responded to a series of incidents by adding more to the screening process. Shoes are removed and X-rayed in response to the would-be "shoe bomber," liquids and toothpaste are restricted to miniscule amounts in response to the plot to use liquid explosives, and now body scans and enhanced pat-downs are deemed necessary as a response to the alleged underwear bomber.
It is interesting to note that in all the recent terrorist attempts on airplanes nothing actually happened, no one was killed or injured. The aim of terrorism is societal disruption. With these new security measures, we have handed the terrorists a victory, even when they failed to detonate a bomb that caused casualties.
I believe that these measures are based on an erroneous view of what explosives actually do. Explosives damage structures from the pressure of the expanding shock wave. This pressure depends on the amount of explosive and its yield, and falls off rapidly the farther you get from the explosion. As a consequence, small quantities of explosives will generally not damage an airplane so extensively that it falls apart.
Calculating exactly what would happen in the event of an explosion is difficult, but reasonable estimates can be made from a simple analysis.
This would show that even if the shoe bomber's shoe had exploded, the airplane would not have been destroyed and fallen out of the sky. He would likely have blown his foot off, and a small hole would have been made in the aircraft outer skin. The aircraft would have lost cabin pressure, something that occasionally happens for other reasons. Oxygen masks would drop, the pilots would descend to a lower altitude, and due to the increased fuel burn might have to land somewhere other than the intended destination.
At the time of the failed underwear bomb plot, the passengers in nearby rows reported seeing a flash and hearing a bang. These are signs of a detonation. Press reports indicated that there were 80 grams of the explosive PETN, although not all of that material appears to have exploded.
No damage was done to the aircraft structure -- something that is not surprising since this happened at low altitude. That is because the airframe is already designed to withstand the pressure differential between a low altitude and maximum cruising altitudes.
Even if the device had detonated at cruising altitude it is unlikely to have resulted in structural failure. Again it is probable that a hole would have been punched in the outer skin, followed by a loss of cabin pressure. There could have been injuries and even fatalities among passengers who were sitting next to him.
So where do we draw the line? With the way the TSA is structured, it's not possible to change to the Israeli system. Clearly everything should be done to prevent explosives getting on board an aircraft in quantities sufficient to cause structural failure and bring the plane down. But is it worth chasing lesser quantities that would result in zero or minimal damage? The enhanced pat-down that some find so offensive is designed to search for these small amounts. It often ends with a swab being taken to test for explosive residues.
Technology does have a role to play, but imaging is not the solution. Operator fatigue sets in after short periods of time staring at computer images. That's why there are reports that contraband items have been smuggled through X-ray units used to scan carry-on bags.
The aim should be to detect high explosive in quantities that are sufficient to cause significant damage. We don't need a machine that takes pictures of the human body. It makes more sense to develop a detector that clearly discriminates between high explosives and human tissue or water. In principle, millimeter-wave scattering, used in some of the body scanners, could do that.
If you direct these high frequency radio waves at a human being and at high explosives, the intensity of the signal coming back is different. A simple system that combines millimeter-wave scattering and the conventional metal detector would adequately screen for guns, knives and high explosives. Anomalies could be resolved by a light pat-down.
Of course, some individuals will want the extra reassurance that the more thorough screening can give. The solution is to let the markets decide. Make some concourses at airports, and the flights that leave from them, high security flights. Other concourses would just have the normal metal detectors and X-ray scanning of carry-on baggage. Extra layers of security cost money, and this will mean higher fares for those flights. I expect most passengers will opt for the speed, convenience and lower cost of the lower security option.
The optimal solution would be to accept that small amounts of explosive do not bring airplanes down, and to concentrate screening procedures on detecting those quantities of explosive that, when detonated, will result in mass casualties. Technology designed to detect, rather than image, high explosives will achieve this objective as well as being less costly and intrusive.
The opinions expressed in this commentary are solely those of Peter Rez.