Who could ever pass up the chance to try on a pair of real X-ray specs? You may not always like what comes into view, but endowed with Superman-like vision, you would be able to see through solid walls, peer in at your neighbours or keep a watchful eye on your kids upstairs. Unfortunately, they still don't exist, despite what those ads at the back of children's comics suggest. Yet there are some technological tricks that can give you the next best thing.
Unlike visible light, radio waves can pass through solid materials. In 2006, engineers at Cambridge Consultants in the UK announced they had built a briefcase-sized system called the Prism 200 which can detect people through a brick wall by firing off pulses of ultrawide-band radar and listening for returning echoes.
According to the company, these pulses can pass through building materials over 40 centimetres thick, and spot activity over a range of up to 15 metres. The device could be used to track people in hostage situations, the company suggests, but it has a crucial weakness. To avoid being blinded by walls and other fixed structures, it is designed to only register objects that generate rapidly varying echoes. In other words, it can only detect people when they move.
Even human statues would be hard pressed to avoid detection by Erwin Biebl's radar sensor, however. Biebl's team at the Technical University of Munich in Germany has built a device that can pick up tiny motions like breathing, or even a beating heart, through a closed door.
His team found that radio waves at between 433 megahertz and 24 gigahertz can pass through skin and bone but are partly reflected by the fatty layer surrounding muscles such as the heart. The team has exploited this by using the Doppler effect to pick up sub-centimetre changes in movement caused by a beating heart or the motion of the lungs.
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