Technosignature

A technosignature or technomarker is something that provides scientific evidence of past or present technology. Technosignatures are analogous to the biosignatures that signal the presence of life. A large part of SETI is hence the search for technosignatures. These are generally megastructures that are observable from vast distances using extremely powerful hyper-telescopes, or advanced technology that emits identifiable wave patterns.

List of technosignatures

 * A Dyson sphere, constructed by life forms dwelling in proximity to a Sun-like star, would cause an increase in the amount of infrared radiation in the star system's emitted spectrum. Freeman Dyson wrote about this in his paper "Search for Artificial Stellar Sources of Infrared Radiation". SETI has adopted these assumptions in its search, looking for such "infrared heavy" spectra from solar analogs.
 * A Type III civilization, having "domesticated" its galaxy by building a large number of Dyson spheres, would be detectable over intergalactic distances of several million light-years. They could also live inside a supermassive black hole, in a stable periodic orbit, which would make them completely undetectable.
 * Shkadov thrusters with the ability to change the orbital paths of stars in order to avoid various dangers to life such as cold molecular clouds or cometary impacts, would also be detectable. Evidence of targeted extrasolar asteroid mining may also reveal extraterrestrial intelligence (ETI).
 * Interstellar spacecraft may be detectable thousands of light-years away through various forms of radiation, such as the photons emitted by an antimatter rocket or cyclotron radiation from the interaction of a magnetic sail with the interstellar medium. Self-replicating spacecraft or their communications networks could potentially be detectable within our Solar system or in nearby star-based systems.
 * A less advanced technology such as the Clarke Belt would be formed by all the artificial satellites occupying geostationary/geosynchronous orbits around an exoplanet.
 * The light from an extraterrestrial ecumenopolis could be easily visible as it surrounds the planet.
 * In Star Trek, a Vulcan ship detected the warp signature from the Earth's first warp-capable vessel. The Vulcans landed and made First Contact with the vessel's pilot.

Trivia
A Dyson–Harrop satellite is used for power generation using the solar wind. It is inspired by the Dyson sphere, but much harder to detect from another star system, therefore making it the opposite of a technosignature. This could be used by a civilization attempting to hide itself, and one of the reasons for the Fermi Paradox.