For decades, the search for extraterrestrial intelligence has waited for quiet, coherent signals from space — a kind of 'whistle' in the radio silence. However, our own technosphere has created a continuous radio background — the hum of a vast metropolis, emitted by all communications, radars, and broadcasting stations. If neighboring stars have sensitive antennas, they already hear this noise. SETI is now learning to detect such 'noisy cities' on other planets — not deliberate messages, but the unintentional technological hum of entire civilizations.

The method is two-stage: first, a quick sky survey for bright radio spots, then a detailed examination with a network of antennas — radio interferometers that, working in sync, provide high angular resolution. A technological source is identified by several signs. First, anomalous radio brightness — the object 'shines' in the radio range like an overheated body, but without corresponding thermal emission in the infrared or optical ranges. Second, the absence of circular polarization, characteristic of many natural sources such as pulsars or active galactic nuclei. Third, scintillation — rapid flux variations due to scattering by inhomogeneities in the interstellar plasma, which for compact technological sources manifests differently than for extended natural ones. The final test is co-motion with a star: if the radio source maintains a constant position relative to its parent star over many observations, background objects are ruled out. These observations will complement data on exoplanets from the Kepler mission William Borucki, which searched for planets using the transit method, detecting tiny dips in stellar brightness.