We are continuously surrounded by radio waves, thanks to the millions of WiFi routers around us. While you can’t see, hear or feel them, you’re living in a field of 2.4- and 5-gigahertz radio signals. Anytime you move—take a sip of water, or look out your window, wash your hair—you are distorting the waves.

The WiFi transmitter in your home or office is continually sending and receiving information, which it converts into radio waves.

The signals aren’t very strong, only filling up the space around you (and possibly spilling just outside to the street). It turns out that, with the right device, it’s possible to watch us moving through the signals as they bounce off us and onto other objects.

What this means in practice: WiFi signals can be harnessed to recognize us and our movements through our walls.

Researchers at the University of California-Santa Barbara used ambient wifi signals and a smartphone to look for revealing pattern changes in signal strength. MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL) and Massachusetts General Hospital developed a device that uses an advanced A.I. algorithm to analyze the radio signals around someone when they’re sleeping. The system then translates all of their body movements into the stages of sleep: light, deep or REM (rapid eye movement).

Imagine a future in which your WiFi router collects your physical movements, then calculates your health metrics, and automatically adjusts the devices and appliances in your house to help you live a better life—if you’re snoring, for example, your pillow could automatically inflate or deflate to adjust the angle of your head and neck.

Another CSAIL team built a WiFi device that could read human emotion using a wireless router. Called EQ-Radio, it successfully detected emotions without disturbing the person being monitored. In 2018, they were able to generate images of a person’s skeleton in motion, showing posture and movements in real time using WiFi.

Practical applications of the technology range from motion capture for video gaming to giving law enforcement and military new ways to see through walls.