The Search for Technosignatures: a Review of Possibilities
This paper reviews the diverse range of technosignatures that have been proposed in the literature. We organize the review by spatial scales from the Earth and the solar system to the edge of the Universe, not tied to a specific observatory or technical capability. This provides a more comprehensive view of technosignature science beyond the search for radio signals that has dominated SETI since its inception. We introduce the Kardashev and Barrow scales before exploring exoplanetary technosignatures, from surface, atmospheric to orbital sources. Next, we consider stellar technosignatures that may involve massive energy utilization, stellar modification or stellar pollution, and end with a section about compact objects. We then review attempts to detect interstellar communication, and discuss many dimensions of the search space from first principles. We also consider interstellar travel technosignatures, and end with galactic, extragalactic and universal signatures. The paper argues for stronger synergies between biosignature and technosignature searches, and the importance of multimodal and multimessenger strategies. Central to this is the importance of anomaly detection as a fundamental and agnostic search strategy. We highlight instruments for detecting technosignatures, as well as a science traceability matrix template.