Researchers at the Massachusetts Institute of Technology say they’ve developed the first known system able to read people’s emotions by bouncing wireless signals off a person’s body.

Potential applications include more adaptive user interfaces as discussed in Co.Design. And while the team from MIT’s Computer Science and Artificial Intelligence Lab is taking measures to make it difficult to scan people’s emotions without their consent, the experiment still raises questions about privacy that some experts say current legal frameworks may be ill-equipped to handle.

“The whole thing started by trying to understand how we can extract information about people’s emotions and health in general using something that’s completely passive—does not require people to wear anything on their body or have to express things themselves actively,” says Prof. Dina Katabi, who conducted the research along with graduate students Mingmin Zhao and Fadel Adib.

The system, called EQ-Radio, works by generating a low-power wireless signal and measuring the time it takes the signal to reflect from various signals in its vicinity. Since the reflection time from people’s bodies vary as they inhale and exhale, and as their hearts beat, it can distinguish humans from other objects that generate static reflections, according to a paper the team plans to present next month at the Association for Computing Machinery’s International Conference on Mobile Computing and Networking.

Then, the system learns to distinguish heartbeats, which cause faster but smaller changes in reflections, from breathing, which leads to slower but larger differences. It’s roughly as accurate at measuring heartbeat time as a traditional electrocardiogram, say the MIT scientists, who are also working with researchers at Massachusetts General Hospital to study potential medical applications.

“We are able to extract breathing and heart rate in a very passive way without asking the user to do anything except for what he does naturally,” says Katabi, who in 2013 was awarded a MacArthur Foundation “genius grant” for her work on wireless networks.