The damaged remains of Building 4 of the World Trade Center shake and shimmy as passing trucks and nearby cranes rattle its very foundations. But thanks to a new laser-based motion detector, cleanup crews will have ample warning if the shattered building begins to collapse.
The device, known as a laser Doppler velocimeter, was originally created to detect land mines in third world countries. But on the night of September 11, the U.S. Army rushed the velocimeter and its creator, James Sabatier, to ground zero in lower Manhattan.
Under normal conditions, the building might sway 25 microns -- about one-quarter the width of a human hair. But with an armada of heavy machinery removing rubble all around, Sabatier has recorded oscillations in Building 4 of nearly 300 microns -- about one-third of a millimeter. Thankfully, that's still not enough to topple a structure, and none of the metal workers, garbage carters, and rescuers at the site have had to be evacuated so far.
"It takes displacements of a few centimeters for windows to start falling out," says Sabatier, a research scientist at the U.S. Army Night Vision Electronic Sensors Directorate in Fort Belvoir, Virginia.
Sabatier's device works by bouncing a laser beam off the building's facade. As the building sways, it alters the frequency of the reflected beam, much the way a train whistle appears to change pitch as it moves through a station. Conventional survey equipment, like theodolites that measure how far a building leans, are accurate to a few millimeters. But Sabatier's detector makes continuous real-time measurements down to a few millionths of an inch.
-- Trevor Thieme