Intelligent Life

1. Intelligent Life

2. What’s your best guess for the number civilizations that currently exist in the Milky Way and that have the technology needed (radio transmitters) to communicate with other civilizations? (The Milky Way contains roughly 200 billion stars.) • 1 • 100 • 10,000 • 1 million • 100 million

3. 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

4. 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.

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

7. Ncivil = N*fpnp flfifcfL The fraction of stars that have a rocky planet in the habitable zone is unknown. What’s your best guess for that fraction? • 0.001 • 0.01 • 0.1 • 0.5 • 1

8. 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.

9. 100 M 1 M 0.1 M 1 trillion years 1 million stars 10 billion years 50,000 stars 1 million years 1 star

10. Ncivil = N*fpnpflfifcfL What’s your best guess for the fraction of habitable planets that produce life? • 0.001 • 0.01 • 0.1 • 0.5 • 1

11. 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.

12. Ncivil = N*fpnpflfifcfL What’s your best guess for the fraction of life-producing planets that eventually produce intelligent life? • 0.001 • 0.01 • 0.1 • 0.5 • 1

13. 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.

14. Ncivil = N*fpnpflfifcfL What’s your best guess for the fraction of intelligent civilizations that are able to communicate with other planets? • 0.001 • 0.01 • 0.1 • 0.5 • 1

15. 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

16. Simultaneity Imagine that the Milky Way has produced 10 million intelligence civilizations during its existence (10 billion years). Each civilization survives for 1000 years before going extinct. If those civilizations are uniformly spread across the 10 billion years of the Milky Way’s history, at any given point in time, how many civilizations are alive in the galaxy? • 1 • 10 • 100 • 1000 • 10,000

17. Civilization A Civilization B Civilization C Civilization E Civilization D Civilization F Civilization G time 0 years 1 million years

18. What’s your best guess for how long a typical civilization exists while its able to communicate with other planets? In other words, how long do they survive after first developing the technology for radio communications? • 100 years • 1000 years • 10,000 years • 100,000 years • 1 million years

19. Ncivil = N* fp  np  fl  fi  fc  fL N* = the number of stars in the Milky Way = 200,000,000,000 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 =/ 10,000,000,000 years =civilizations communicating in Milky Way now

20. 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.

21. 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.

23. 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.

24. 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

25. 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

26. Locations of Pioneer & Voyager in 2007 Pioneer 10 Voyager 2 Pioneer 11 Voyager 1

28. Voyager’s Golden Records

30. 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”.