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Searching for Alien Intelligence

Searching for Alien Intelligence. The Drake Equation. In 1961, Frank Drake synthesized an equation to estimate the number of civilizations currently communicating in our Galaxy. N civil = N *  f p  n p  f l  f i  f c  f L where N * = the number of stars in the Milky Way

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Searching for Alien Intelligence

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  1. Searching for Alien Intelligence

  2. The Drake Equation In 1961, Frank Drake synthesized an equation to estimate the number of civilizations currently communicating in our Galaxy. Ncivil = N* fp  np  fl  fi  fc  fL where N* = the number of stars in the Milky Way fp = the fraction of stars that have “habitable planets” np = the number of habitable planets per system fl = the fraction of habitable planets where life evolves fi = the fraction of life-planets that evolve intelligence fc = the fraction of civilizations that communicate fL = the fraction of the star’s life that the civilization exists

  3. Ncivil = N* fp  np  fl  fi  fc  fL The number of stars in the Milky Way is relatively well-known. It contains roughly 200 billion stars.

  4. Ncivil = N*fpnp flfifcfL Mass Fraction of stars with rocky planets in the habitable zone is unknown

  5. Ncivil = N* fp  np fl  fi  fc  fL Our solar system has 1 planet in the habitable zone right now (np=1), but 2 others are just outside of it, and may have been within the habitable zone in the past (np=3). Most stars probably do not have np>3, otherwise the planets would be too close and they would disrupt each other’s orbits.

  6. Ncivil = N* fp  np  fl fi  fc  fL There may be other factors that limit the development of life. For example • Planets without large moons may have the direction of their spin axis shift over time. This may produce long term climatic shifts. • Planets with very large moons may have an unstable crusts due to tides. Mars 0 2 4 6 8 10 Million Years ago

  7. Ncivil = N* fp  np  fl fi  fc  fL Massive stars probably die before life can form. But most stars are M stars, which have very long lives. So most habitable planets should have billions of years to produce life. Whether that’s enough time depends on how long it typically takes for life to arise, which is unknown.

  8. Ncivil = N* fp  np  fl  fi  fc  fL What fraction of planets with life produce intelligent life? Perhaps evolution inevitably leads to at least one intelligent organism on a planet. If so, then fi=1. But for 2.5 billion years, life on Earth did not evolve past single-celled organisms. Perhaps the development of complex (and intelligent) life is very rare. In that case, fi<<1.

  9. Ncivil = N* fp  np  fl  fi  fc  fL What fraction of intelligent life communicates? Some may not develop technology for it, or may not want to communicate.

  10. Ncivil = N* fp  np  fl  fi  fc  fL Our Sun spent its first 4.5 billion years without hosting a civilization. We only achieved the technology to communicate with radio transmissions ~ 70 years ago. How long will we maintain this ability? • Extreme Optimistic Case: We continue as a civilization for the rest of the lifetime of the Sun: fL = 1/2 • Extreme Pessimistic Case: We destroy ourselves in the next 50 years: fL = 100 / 10,000,000,000 = 0.00000001

  11. Ncivil = N* fp  np  fl  fi  fc  fL Now make your best guess at each number and multiply them. What do you get? N* = the number of stars in the Milky Way fp = the fraction of stars that have “habitable planets” np = the number of habitable planets per system fl = the fraction of habitable planets where life evolves fi = the fraction of life-planets that evolve intelligence fc = the fraction of civilizations that communicate fL = the fraction of the star’s life that the civilization exists

  12. Ncivil = N* fp  np  fl  fi  fc  fL Now make your best guess at each number and multiply them. What do you get? N* = the number of stars in the Milky Way = 200,000,000,000 fp = the fraction of stars that have “habitable planets” = 0.5 np = the number of habitable planets per system = 2 fl = the fraction of habitable planets where life evolves = 0.5 fi = the fraction of life-planets that evolve intelligence = 0.01 fc = the fraction of civilizations that communicate = 0.01 fL = the fraction of the star’s life that the civilization exists = 10,000 / 10,000,000,000 = 10-6 = 20 civilizations communicating in Milky Way now

  13. Distance to the Nearest Civilization 2000 light years 50,000 light years The volume of the Milky Way is V=1.5  1013 cubic light years The density of communicating civilizations is Ncivil / V The distance between civilizations is If N = 20, then nearest civilization is 10,000 light years away

  14. Extraterrestrial Communication Radio waves are not blocked by interstellar dust, so they can travel through the entire Milky Way. They also require the least energy to transmit because they have low energy. The atmosphere is transparent to radio waves. As a result, we can easily receive radio signals from space, and our transmissions escape into space as well.

  15. Extraterrestrial Communication Through our radio and TV broadcasts, we have been transmitting radio signals into space for 70 years. Aliens around other stars would see these transmissions increase and decrease regularly because of the Earth’s rotation.

  16. SETI: The Search for Extraterrestrial Intelligence Rather than send a transmission into space and wait for a reply, we can listen for other civilizations who are already broadcasting.

  17. Which wavelength to listen to? Somewhere between 21 cm (H I) and 18 cm (OH) might be a good choice because the background static from the galaxy is low here, and any intelligent civilization should know the wavelength of hydrogen emission, the most common element in the universe.

  18. Deep Space Explorers: Pioneer 10 and 11 Launched in 1972-73 Visited outer planets in 1970’s Beyond orbit of Pluto in 1980’s Now 80 AU from Earth Traveling at 10 km/s = 3 light years in 100,000 years Last contact in 2002

  19. Deep Space Explorers:Voyager 1 and 2 Launched in 1977 Visited outer planets in late 1970’s and 1980s Beyond orbit of Pluto in 1990’s Now 100 AU from Earth Traveling at 15 km/s = 5 light years in 100,000 years

  20. Voyager’s Golden Records

  21. Current Locations of Pioneer & Voyager

  22. The Pale Blue Dot On February 14, 1990, Voyager 1 turned around and photographed the planets as it sped beyond the orbit of Pluto. This famous image of the Earth from a distance of 4 billion miles is known as the “Pale Blue Dot”.

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