About a century ago, to call a friend on a noisy street, you'd whistle loudly. Today everyone sends messages — almost no sound. The search for aliens is similar: astronomers point radio antennas at the sky hoping to hear a 'whistle' from a distant planet. But if the locals there have already switched to silent digital communication, we'll never catch them. Technological progress itself sweeps away the traces of intelligence.

The faster the development, the shorter the moment of audibility. Under the guidance of artificial intelligence, the visibility window might last only a century, and if intelligence has become purely digital — less than twenty years. Over the billions of years of a galaxy's existence, civilizations can easily miss each other. Earth stopped loudly 'whistling' back in the 1990s — sooner than its signal would have reached the nearest star. And the interstellar medium further dampens the faint whisper, making the search akin to trying to hear a mosquito behind a waterfall.

Now scientists search not only for radio waves but any oddities: excess heat from giant structures or unusual flickering of stars. This work is masterfully done by machine learning, sifting through spectral analysis and brightness measurements in hunt for anomalies. The very first unexplained spike might be the answer.