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In the 1930s, astronomer Edwin P. Hubble and others claimed that we lived in an expanding universe filled with galaxies containing billion of stars. At about the same time, Karl G. Jansky invented the radio telescope that detected radio waves naturally emitted by astronomical bodies. Pioneer radio astronomer Frank Drake realized that radio telescopes could also be used to search for radio messages transmitted by distant, advanced civilizations. He then proposed Drake’s equation for calculating the number of civilizations in our galaxy able to transmit and receive radio signals. These developments raised questions about the proper radio frequencies to use in searching for extraterrestrial messages (Giuseppe Cocconi, Philip Morrison), the identification of astroengineering projects of alien civilizations (Freeman Dyson), and the ranking of extraterrestrial civilizations according to the amount of energy they controlled (Nicolai S. Kardashev).

The universe is certainly contrived in such a way as to make life possible within it. The tantalising question that arises then is whether life exists on planets other than Earth. The astronomer Frank Drake came up with an equation to summarise the important factors affecting the chances of detecting radio signals from alien worlds. The Drake equation yields a rough guess of the number of civilisations existing in the Milky Way from which we might get a signal. This chapter also discusses the Fermi paradox, as well as the doomsday argument and its use of the language of probability theory.

Frank Drake devised the Drake equation to estimate the number of advanced civilizations in the galaxy with the aim of gathering support for SETI (the Search for Extraterrestrial Intelligence). The earliest attempts to detect radio signals from extraterrestrials date back to the 1960s. Paul Allen has funded the Allen Telescope, Array which is dedicated to searching for such signals. When complete it will include 350 radio dishes. The citizen science project SETI@Home allows anyone with a home PC to participate in analysing the data amassed by the SETI project.

The vastness of the Universe gives strong support for the possibility of other intelligent life around other stars. At the same time, anthropic balances in the laws and circumstances of the Universe are a reminder of how special life may be. This chapter explores the tension between these arguments, reflecting on the Drake equation's attempt to give a probability to the existence of extra-terrestrial intelligence. In addition, the size of the Universe poses a fundamental problem for SETI—that is there may be civilizations out there who are too far away for us ever to know of their existence.

This chapter briefly reviews some the challenges encountered in the search for extraterrestrial life. So far, no signs of extraterrestrial life have been found. The search started with radio telescopes, looking for technology-based civilizations, but new strategies have emerged that take on the primary challenges in this search: the enormous distances to exoplanets and the question of the true nature of life. The author outlines the development of new tools for the search, and why the present focus is on Earth-sized exoplanets with a potential for liquid water on their surfaces. Not having been visited by an alien civilization presents us with a paradox: if life develops as quickly elsewhere as on Earth, then why have we not been contacted? Is the speed of light too slow to cross interstellar distances, is life intrinsically rare, or should we conclude that civilizations are intrinsically short-lived?

This chapter explores the concept of life across traditions, from science to philosophy to theology. The term “life” covers at least three constellations of meaning or life-concepts: biological life, internal life, and rational life. Biological life shares traits with all cellular life on Earth (archaea, eubacteria, and eukarya). Internal or conscious life shares subjective interiority with humans. Rational life shares intellect with all minds that can distinguish truth from non-truth. These three lives possess different origins, extents, and futures. The chapter then identifies three distinct “hard problems of life” relating to the origin and extent of biological organization, consciousness, and reason: moving from non-life to life, from life to sentience, and from sentience to rationality. The Drake equation, the Fermi paradox, and the anthropic principle provide concrete examples in astrobiology.