Technosignature or technomarker is any evidence of the operation of advanced technology by an extraterrestrial civilization, with the exception of the radio messages which are traditionally searched for in the search for extraterrestrial intelligence (SETI). First covered in Paul Davies's 2010 book The Eerie Silence and later developed by Iván Almár, technosignatures are analogous to the biosignatures that signal the presence of life, whether or not intelligent. Various types of technosignatures, such as radiation leakage from megascale astroengineering installations such as Dyson spheres, the light from an extraterrestrial ecumenopolis, or Shkadov thrusters with the power to alter the orbits of stars around the Galactic Center, may be detectable with hypertelescopes.
A Dyson sphere, constructed by life forms not dissimilar to humans 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. Hence, Freeman Dyson selected the title "Search for Artificial Stellar Sources of Infrared Radiation" for his 1960 paper on the subject. SETI has adopted these assumptions in its search, looking for such "infrared heavy" spectra from solar analogs. From 2005, Fermilab has conducted an ongoing survey for such spectra, analyzing data from the Infrared Astronomical Satellite.
Identifying one of the many infra-red sources as a Dyson sphere would require improved techniques for discriminating between a Dyson sphere and natural sources. Fermilab discovered 17 "ambiguous" candidates, of which four have been named "amusing but still questionable". Other searches also resulted in several candidates, which remain unconfirmed. In October 2012, astronomer Geoff Marcy, one of the pioneers of the search for extrasolar planets, was given a research grant to search data from the Kepler telescope, with the aim of detecting possible signs of Dyson spheres.
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 in a similar fashion to the transiting extrasolar planets searched by Kepler. Unlike planets, though, the thrusters would appear to abruptly stop over the surface of a star rather than crossing it completely, revealing their technological origin. In addition, evidence of targeted extrasolar asteroid mining may also reveal extraterrestrial intelligence.
Various astronomers, including Avi Loeb of the Harvard-Smithsonian Center for Astrophysics and Edwin Turner of Princeton University have proposed that artificial light from extraterrestrial planets, such as that originating from cities, industries, and transport networks, could be detected and signal the presence of an advanced civilization. Such approaches, though, make the assumption that the radiant energy generated by civilization would be relatively clustered and can therefore be detected easily.
Light and heat detected from planets must be distinguished from natural sources to conclusively prove the existence of intelligent life on a planet. For example, NASA's 2012 Black Marble experiment showed that significant stable light and heat sources on Earth, such as chronic wildfires in arid Western Australia, originate from uninhabited areas and are naturally occurring.
Atmospheric analysis of planetary atmospheres, as is already done on various Solar System bodies and in a rudimentary fashion on several hot Jupiter extrasolar planets, may reveal the presence of chemicals produced by technological civilizations. For example, atmospheric emissions from industry on Earth, including nitrogen dioxide and chlorofluorocarbons, are detectable from space. Artificial pollution may therefore be detectable on extrasolar planets. However, there remains a possibility of mis-detection; for example, the atmosphere of Titan has detectable signatures of complex chemicals that are similar to what on Earth are industrial pollutants, though obviously not the byproduct of civilisation. Some SETI scientists have proposed searching for artificial atmospheres created by planetary engineering to produce habitable environments for colonisation by an ETI.
Interstellar spacecraft may be detectable from hundreds to 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. Such a signal would be easily distinguishable from a natural signal and could hence firmly establish the existence of extraterrestrial life were it to be detected. In addition, smaller Bracewell probes within the Solar System itself may also be detectable by means of optical or radio searches.
One of the first attempts to search for Dyson Spheres was made by Vyacheslav Slysh from the Space Research Institute in Moscow in 1985 using data from the Infrared Astronomical Satellite (IRAS).
In 2005, Fermilab had an ongoing survey for such spectra by analyzing data from IRAS. Identifying one of the many infra-red sources as a Dyson Sphere would require improved techniques for discriminating between a Dyson Sphere and natural sources. Fermilab discovered 17 potential "ambiguous" candidates of which four have been named "amusing but still questionable". Other searches also resulted in several candidates, which are, however, unconfirmed.
In 2012, a trio of astronomers led by Jason Wright started a two-year search for Dyson Spheres, aided by grants from the Templeton Foundation.
Other projects includes work by Lucianne Walkowicz from Princeton University, which aims to detect artificial signatures in stellar photometry, and Geoff Marcy who uses data from the Kepler Telescope to search for Dyson Spheres and interstellar communication using lasers.
Another search for technosignatures involved an analysis of data from the Compton Gamma Ray Observatory for traces of anti-matter, which, besides one "intriguing spectrum probably not related to SETI", came up empty.