The Quest for Extraterrestrial Intelligence Chapter 19 The Quest for Extraterrestrial Intelligence 1

19-1 Radio Searches and SETI The great number of galaxies, stars, and planets in the universe leads us to believe that life – maybe even intelligent life – can evolve elsewhere. SETI is based on the detection of radio signals from intelligent life forms. Between 1000 and 10,000 MHz there is little radio noise from other sources. 400 MHz, which is 21 cm, may be the one of choice. It would be well known as a line from hydrogen. 2

In 1960, Frank Drake first used a radio telescope to search for signals from other stars. Several other searches have been conducted since. No extraterrestrial signals have been detected. The Allen Telescope Array under construction is a SETI-dedicated array of radio telescopes. SETI@home uses personal computers to search through radio data for extraterrestrial signals. 3

19-2 Communication with Extraterrestrial Intelligence The search might detect stray signals. We have transmitted signals from radio and TV for decades. Radio telescopes used to detect signals can also transmit signals. Communication is unlikely to be a dialogue – it takes years for signals to reach even the nearest stars. Communication could be based on mathematics and physics, which are the same throughout the universe. 4

In 1974 a message was transmitted from the Arecibo radio telescope toward a star cluster. The message was a series of pulses that could be arranged into an image. Even if extraterrestrials can not decode it, they will still know that it came from an intelligent source. The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the National Science Foundation. 5

19-3 Letters to Extraterrestrials The Pioneer Plaques 6  9 inch plaques were attached to Pioneer 10 and 11, which are on trajectories that will take them out of the solar system. The picture describes humans and the location of the Earth. Courtesy of Pioneer Project, ARC, and NASA. 6

The Voyager Records Voyager 1 and 2 carry copper phonograph records with sounds and pictures from the Earth. Courtesy of Pioneer Project, ARC, and NASA. 7

Will the Message Be Found? The messages on the Pioneers and Voyagers are like messages in bottles thrown into the sea. The soonest any of these craft will pass within 2 light-years of a star is 40,000 years. When the Earth has been reduced to a charred cinder by the expanded and brighter Sun, these messages will continue to travel through space. 8

19-4 The Origin of Life Evolution explains how higher forms of life evolve; not how life began. The Miller-Urey experiment showed simple compounds could form organic molecules. Electric sparks were sent into a mixture of water, hydrogen, ammonia, and methane to simulate the early Earth. 9

Some amino acids, fatty acids, and urea formed. It is now thought the early Earth was primarily carbon dioxide, water, and nitrogen, but similar results are obtained with that gas mixture. UV light (which strikes the Earth from the Sun) has the same result as electricity. This experiment did not create life, only organic molecules. 10

N = R*  fP  ne  fl  fi  fc  L 19-5 The Drake Equation Frank Drake proposed a way to estimate the number of technologically advanced civilizations we might detect. N = R*  fP  ne  fl  fi  fc  L R* = rate at which solar-type stars form in our galaxy fP = fraction of those stars having planetary systems ne = the average number of planets per each system that are Earth-like enough to support life 11

fi = the fraction of those life forms that evolve to intelligence fl = the fraction of those Earth-like planets on which life actually develops fi = the fraction of those life forms that evolve to intelligence fc = the fraction of those intelligent species who are interested in interstellar communication and develop and use the necessary technology for it L = the average lifetime of a technologically advanced civilization 12

R* : The star formation rate is known to be between 1 and 10 solar-type stars per year. Stars more than 1.5 solar masses, about type F5, have lifetimes that may be too short. Stars less massive than about type M0 are dim and planets would have to be too close to the star to be suited for life. Similarly, the stars too close to the Galactic center or too far away may not have suitable planets. 13

fi , fc , and L are much less well known. Evidence is accumulating that planetary systems are common, so perhaps fP 1. Our solar system almost has three planets capable of supporting life and a number of satellites with liquid water. Perhaps ne is between 0.1 and 1. Given the right conditions and enough time, life may develop from simple compounds, so fl 1. There is no way to test this. fi , fc , and L are much less well known. Estimates of N can range from only a few to millions. 14

19-6 Where is Everybody? The great size of the Galaxy and number of stars with planets suggests that N could be large. But then there could be many civilizations like ours. Many of them should be older than ours. Older ones would be more advanced and could travel to other stars, including here. If life evolves readily then the Galaxy should be teeming with life. 15

Then where is everybody? This suggests either: The Earth has been visited by extraterrestrials who do not reveal themselves. 2. Intelligent life is rare. If the lifetime of a civilization is shorter than 10,000 to 1,000,000 years, we would not expect to find another one in the Galaxy at the same time as we are here. 16