is also a detectable technosignature of the first kind. Individual extrasolar planets can be analyzed for signs of technology. Avi Loeb of the Center for Astrophysics Harvard & Smithsonian has proposed that persistent light signals on the night side of an exoplanet can be an indication of the presence of cities and an advanced civilization. In addition, the excess infrared radiation and chemicals produced by various industrial processes or terraforming efforts may point to intelligence. Light and heat detected from planets need to be distinguished from natural sources to conclusively prove the existence of civilization on a planet. However, as argued by the Colossus team, a civilization heat signature should be within a "comfortable" temperature range, like terrestrial urban heat islands, i.e. only a few degrees warmer than the planet itself. In contrast, such natural sources as wild fires, volcanoes, etc. are significantly hotter, so they will be well distinguished by their maximum flux at a different wavelength. Other than astroengineering, technosignatures such as artificial satellites around exoplanets, particularly such in geostationary orbit, might be detectable even with today's technology and data, and would allow, similar to fossiles on Earth, to find traces of extrasolar life from long ago. Extraterrestrial craft are another target in the search for technosignatures. Magnetic sail interstellar spacecraft should be detectable over thousands of light-years of distance through the synchrotron radiation they would produce through interaction with the interstellar medium; other interstellar spacecraft designs may be detectable at more modest distances. In addition, robotic probes within the Solar System are also being sought out with optical and radio searches. For a sufficiently advanced civilization, hyper energetic neutrinos from Planck scale accelerators should be detectable at a distance of many Mpc. Fermi paradox Italian physicist Enrico Fermi suggested in the 1950s that if technologically advanced civilizations are common in the universe, then they should be detectable in one way or another. According to those who were there, Fermi either asked "Where are they?" or "Where is everybody?" The Fermi paradox is commonly understood as asking why extraterrestrials have not visited Earth, but the same reasoning applies to the question of why signals from extraterrestrials have not been heard. The SETI version of the question is sometimes referred to as "the Great Silence". The Fermi paradox can be stated more completely as follows: There are multiple explanations proposed for the Fermi paradox, ranging from analyses suggesting that