THE aether these days is abuzz with all manner of signals. Radio and television broadcasts which used to dominate the airwaves now vie with mobile-phone transmissions, Bluetooth tethers and, in many cities at least, massive deployments of personal and public Wi-Fi. So dense has this electromagnetic mesh become that, earlier this year, researchers showed how they can now track airplanes by detecting the ripples they leave in it as they fly. Now a group at the University of Washington, in Seattle, under the direction of Shwetak Patel, is trying to pull off a similar trick indoors. Dr Patel's earlier effort, Humantenna, measured changes caused by human bodies in the electromagnetic fields passing across electrical wiring in walls. Unfortunately, despite some early promise, the wiring was not dense enough and the signals too flimsy to recognise gestures consistently. His lab's latest project, called Wisee and led by Shyamnath Gollakota, aims to detect movements through the way they impede Wi-Fi signals which blanket homes and offices.

It does this by taking advantage of multiple-input, multiple-output (MIMO) antenna arrays, featured in most Wi-Fi devices since 2006. Wireless systems before MIMO would get confused when parts of the same signal arrived at different times after being reflected of walls and objects. MIMO, by contrast, cleverly uses reflection to improve network bandwidth. A chunk of data is sent out simultaneously from two or more antennas, each oriented slightly differently. Each of the chunks therefore takes a different path and is then picked up by several antennas on the receiver.

By measuring the differences in the arrival time at each antenna and using a variety of synchronisation cues embedded in the signal, the receiver can disentangle the signals and reconstruct the message. In fact, the process is so robust that a single base station or computer with MIMO can send and receive from two to four unique data streams using the same antennas; slight variations in signal strength, timing, reflection and radio encoding allow a receiver to pick apart each stream, even though the same radio frequencies are being used.

Researchers found that the high level of discrimination in timing and signal discrimination could be used not just to reassemble data, but to detect moving obstacles in the data streams' way. This detection and tracking is accomplished by measuring the tiny shift in frequency between the signals received by the Wi-Fi router and any mobile devices, computers or other gizmos wirelessly transmitting to it. Such shifts are caused by the Doppler effect, an everyday example of which is the changing pitch of an ambulance siren as it passes by. In the 5GHz band, one of two available for Wi-Fi in much of the world, a gesture that moves half a metre per second produces a 17Hz Doppler shift, which is readily distinguishable in the lab's monitoring equipment using off-the-shelf MIMO radios.

Sidhant Gupta, a graduate student at Dr Patel's Ubiquitous Computing Lab (ubicomplab), offers your correspondent an illustration of how the system works. It involves a lot of hand-waving. For once, this is justified. Mr Gupta's gestures—push, pull, dodge, circle or kick—all generate distinct readings on the Doppler display. Switching on the lab's light requires nothing more than moving a hand up and down in a chopping motion.

For the moment the system could be used to play video games, switch between television channels or regulate room temperature. This, though, can already be done using optical systems such as Microsoft's Kinect (or simply using a remote control or smartphone app). The researchers think their technology will truly come into its own in less frivolous areas such health monitoring. They hope that one day it will enable keeping track of whether a flat's residents have taken a fall, perhaps even measuring multiple individuals' breathing patterns and heart rates.

This, too, could in principle be done with Kinect-like cameras. But installing them in every room would be complicated and expensive, not to mention intrusive. Faddish wearable devices could do the trick, too, but people would need to remember to put them on, recharge them, keep them out of the wash, etc. Relying on pre-existing Wi-Fi solves all these problems, as well as dipensing with the need to be in a camera's line of sight. Mr Gupta's fellow student, Ruth Ravichandran, will spend the summer working out how to extract from the system's detections not just the type of gesture, but its extent and precise shape.