Studying distant exoplanets, scientists look for signs of life — for instance, water and carbon — and signs of technology: radio signals, megastructures. But between these clues there is a yawning gap. If intelligence hasn't created machines or they've vanished, we won't notice it. A new approach suggests searching for noosignatures — any ordered imprints of a thinking being.
Noosemiotics, the science of such traces, gives clear criteria to distinguish them from natural chaos. Its first examples are terrestrial: stone tools, cave paintings. Applying these methods to spectroscopy of distant planets' atmospheres, we may notice what we previously missed. The most surprising twist: the most reliable noosignatures are not messages, but waste. A rubbish heap is visible from distant stars and needs no decoding.
The most noticeable noosignatures are not signals, but garbage: our civilization has already left a layer of plastic that will persist in geological deposits for millions of years.
In the Strugatsky brothers' novel 'Roadside Picnic', the Zone left by aliens is full of artifacts whose meaning is completely incomprehensible — these are typical noosignatures.