Ars Tecnica reports how researchers have discovered a method to “see” humans move behind a wall using wifi mesh gear.
Wifi is a free wireless band with a frequency very close to the one used to heat food in microwave ovens. (Which is why it is free: everybody assumed the noise and interference of microwave ovens would prevent any commercial use of this frequency band: a historical mistake).
The microwave oven heats the food because bipolar molecules (aka H2O or water) absorb the energy in radio waves with this particular frequency very well.
A human body full of water will absorb some of the (extremely low level of) energy a wifi access point emits, which you can detect. The researchers measure the changes in the radio field generated by the absorption in a body and calculate the position of the body.
Pretty neat but not new.
The Serbians (in the recent civil war in former Yugoslavia) had discovered they could track the path of low flying Stealth planes by measuring the changes in the radio field emitted by cell phone towers. The mass of the plane had a noticeable effect on the radio fields.
So much for stealth…
