Wi-Fi can pass through walls.



This fact is easy to take for granted, yet it's the reason we can surf the web using a wireless router located in another room.



But not all of that microwave radiation makes it to (or from) our phones, tablets, and laptops. Routers scatter and bounce their signal off objects, illuminating our homes and offices like invisible light bulbs.



Now, German scientists have found a way to exploit this property to take holograms, or 3D photographs, of objects inside a room — from outside it.



"It can basically scan a room with someone's Wi-Fi transmission," Philipp Holl, a 23-year-old undergraduate physics student at the Technical University of Munich, told Business Insider.



Holl initially built the device as part of his bachelor thesis with the help of his academic supervisor, Friedemann Reinhard. The two later submitted a study about their technique to the journal Physical Review Letters, which published their paper in early May.



Holl says the technology is only in its prototype stage and has limited resolution, but he is excited about its promise.



"If there's a cup of coffee on a table, you may see something is there, but you couldn't see the shape," Holl says. "But you could make out the shape of a person, or a dog on a couch. Really any object that's more than 4 centimeters in size."

How to see through walls with Wi-Fi

The ability to see through walls using Wi-Fi has been around for years.



Some setups can detect home intruders or track moving objects with one or two Wi-Fi antennas. Others use an array of antennas to build 2D images. But Holl says no one has used Wi-Fi to make a 3D hologram of an entire room and the stuff inside it.



"Our method gives you much better images, since we record much more signal. We scan the whole plane of a room," he says.



Holl's method differs from the others in few significant ways.



