As Popular Science explains, Wi-Fi radio signals are found in 61 percent of households nationwide. Researchers Karl Woodbridge and Kevin Chetty developed a suitcase-size device that can use these signals along with the Doppler effect to detect movement. The Doppler effect happens when a radio wave reflects off a moving object. When it does, the wave's frequency changes.
The device reads these frequency changes and calculates not only when an object is moving, but also its speed, location and the direction it's going. For example, if a person is moving toward the Wi-Fi source, the frequency of waves increases; if a person is moving away from the source, the frequency of the waves decreases.
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Despite all of this wave reading and frequency changing, the device itself gives off zero waves, making it impossible to detect. The most interesting potential use of the device would be for security purposes like scanning buildings during hostage situations or war torn cities where insurgents might be hiding.
Woodbridge and Chetty hope to fine tune the device to include the ability to pick up subtle movements like the rise and fall of one's chest when breathing, in order to sense someone even if they are standing still.
via Popular Science
Credit: Kevin Chetty / University College London