The Stardroppers — John Brunner

A stardropper got its name from the belief that the user was eavesdropping on the stars. But that was only a guess . . . nobody really knew what the instrument did.

The instrument itself made no sense scientifically. A conventional earpiece, an amplifier, a power source - all attached to a small vacuum box, an alnico magnet, and a calibrated 'tuner'. What you got from all this was some very extraordinary noises and the conviction that you were listening to beings from space and could almost understand what you were hearing.

What brought Special Agent Dan Cross into the stardropper problem was the carefully censored news that users of the instrument had begun to disappear. They popped out of existence suddenly - and the world's leaders began to suspect that somehow the fad had lit the fuse on a bomb that would either destroy the world or change it forever.

(First published 1972)