MIT Technology Review

Thursday, January 24, 2013

Physicists Prove "Dowsing" Bomb Detectors Useless in Double Blind Trial

A widely used detector that relies on dowsing to find explosives and drugs has finally been proven ineffectual by physicists in Mexico

The detection and identification of drugs and explosives is an important goal in the fight against crime. Indeed there are numerous promising methods for spotting this stuff that depend on technologies such as artificial noses, x-ray imaging, terahertz scanners and so on.

But there are also devices based on the controversial process of “dowsing” that claim to do a similar job. These devices consist of a pair of swivelling rods, are “powered” by the user’s electrostatic electricity and are supposedly capable of tracking illegal substances in tiny quantities over vast distances.

The GT200 is one of these devices, manufactured and sold for a cool $20,000 each by Global Technical Ltd, a company based in the UK.  It is widely used by the Mexican armed forces to detect contraband but with a growing sense of suspicion by those who have been targeted. In various court cases, the evidence provided by the device has been called in to question.

Two of the most outspoken expert witnesses are Wolf Luis Mochan at the National University of Mexico and Alejandro Ramirez-Solis at the Universidad Autonoma del Estado de Morelos, who are both physicists. These scientists say the device is useless and in one trial, carried out double blind experiments to test its efficacy.

The results of these tests were not originally published because they were part of an ongoing trial. But with the court proceedings now over, Mochan and Ramirez-Solis say they are happy to release the paper (although a Mexican newspaper somehow got hold of the results and published them last year). Today, their work appears on the arXiv.

The test is simple. They placed a quantity of amphetamines–over 1600 pills-or some bullets, in one of eight boxes chosen at random in a large room.  A GT200 operator, in this case a soldier, then entered the room and used the device to locate the stash. 

They repeated this four times, each time allowing the solider to see where the contraband was place and then a further twenty times without the soldier being aware of the box in which it was placed.

The results are unsurprising. When the solider was aware of the location, the GT200 worked perfectly, identifying the correct box on all four occasions.

But when the soldier was unaware of the location, the GT200 located the contraband on only three occasions out of 20, a result that is entirely compatible with chance. 

“We conclude that the GT200 is worthless as a substance detector,” say Mochan and Ramirez-Solis.

That’s hardly unexpected. What is extraordinary is that the Mexican armed forces have bought 940 of these devices and continue to use them, although the Mexican Supreme court is currently reviewing the use of the GT200 to provide evidence.

There is of course a bigger story here about the many people who have been convicted using evidence obtained with these devices and the lives that have been threatened by the use of bomb detecting equipment that clearly does not work. Indeed, the controversy rages in various countries around the world including the UK where the government has banned the export of these devices to Iraq and Afghanistan.  

But thanks to the work of people like Mochan and Ramirez-Solis this episode may finally be drawing to a close.

Ref: arxiv.org/abs/1301.3971: Effectiveness of the GT200 Molecular Detector: A Double-Blind Test