YOU are old. You live alone. You suffer a heart attack. You cannot raise the alarm. You die. That, unfortunately, is the way that many people in the rich world shuffle off this mortal coil. One way around the problem is to wear an alarm bracelet that detects when something is wrong and calls an ambulance. That means remembering to put the bracelet on, however—and many people do not want to wear one in the first place.

PassivSystems, of Newbury, England, is therefore working on an alternative. This is a device that can detect remotely when something is wrong with somebody in the room it is monitoring. The technology to do this, which was developed by Helen Prance and her colleagues at the University of Sussex, is based on electrocardiography.

A conventional electrocardiogram (ECG) is made by attaching electrodes to a person's body and recording signals that originate in the heart. Dr Prance's version uses ultra-sensitive electrodes to record the effects of bodily electrical signals on the ambient electric fields that pervade a room, and deduce from that what is going on. As Dr Prance puts it, “a person is just like a big bag of water”: any movement which the bag makes will disrupt the fields in the room.

Apply enough computing power to the signal caused by this disturbance and it is possible to detect an individual's movement (and thus, for example, a fall) from several metres away. More subtle movements, such as those of the chest caused by breathing and the beating of the heart, can be registered from a distance of about half a metre, and PassivSystems is trying to increase that range.

If the firm's engineers succeed in doing so, the result will be like an electronic nurse that can keep an eye on someone and know when to call the doctor. Not so much Big Brother, then, as Big Sister.