Lecture 22 Life beyond our Solar system SETI

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Lecture 22 Life beyond our Solar system(SETI)
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SETI

• Search for Extra Terrestrial Intelligence
• What is intelligence?
• In reality we search for some signs/manifestation
  of another technology
• What types of technology?

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Visible signs of technological civilization
(based on ours)

• Energy production
• Waging wars
• Transportation (including interstellar travel)
• Communication (unintentional leakage or
  deliberate beacons)

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The best information carrier for interstellar
communications

• Minimum energy per bit of information
• Max possible velocity
• Easy to generate, launch, and capture
• Not be appreciably absorbed by the interstellar
  medium
• Go where aimed
• Go photons!

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Nine-dimensional cosmic haystack

• Direction (any)
• Distance (100,000 light years)
• Time continuous signal vs. periodic
• Polarization right and left
• Frequency frequencies lower than 100 kHz are
  absorbed by interstellar medium
• Modulation
• Signal strength

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Background

• Ultraviolet, X-ray, gamma-ray blocked by
  atmosphere (absorption on ozone, oxygen, nitrogen
  etc.) space based telescope
• IR and mm effectively absorbed by water vapor
  telescope on the mountain-top
• SETI concentrated on 1-10GHz (30-3 cm in
  wavelength)
• Recently communication via visible light was also
  considered

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Visible vs. Radio search

• Radio photons are less energetic ? less energy to
  generate a signal
• Visible photons are easier to focus

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Arecibo radio telescope (Puerto Rico) diameter
305 m
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Signal and noise

• PR (PE x AR) / 4?r2
• PR the amount of signal power collected by
  receiver
• PE effective isotropic radiated power
• AR effective area of the receiver
• PN k x T x B
• B 0.01 Hz k 1.381 x 10-23 Joules/Kelvin
• We want PR 10 x PN
• r ?

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Drakes equation

• The number of civilizations, N, broadcasting
  their existence in our Galaxy
• N Rb T
• T time over which civilizations broadcast
• Rb R pp nE pl pi pc
• R rate at which suitable stars are formed
• pp probability of planets forming around a
  suitable star
• nE average number of suitable planets in HZ
• pl probability of life
• pi probability of intelligence developing in
  life
• pc probability of intelligent life broadcasting
  across space

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Drake equation (terms)

• R - 20 stars per year are born in our Galaxy
  but only 10 are solar-like.
• pp 0.1-0.2
• ne 1 and lt10
• pl ?
• pi ?
• pc ?

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Fermi Paradox

• Our planet is not special
• If even one civilization had originated before us
  then it should have already colonized the entire
  galaxy
• So where is everybody? (Enrico Fermi, 1950)

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• Size of the Milky Way galaxy 100,000 light
  years
• Speed of the Voyager 1 (launched in 1977) is 17
  km/sec
• Speed of light 300,000 km/sec
• How long (in years) will it take to go across the
  galaxy at the current Voyagers speed?
• Age of the galaxy oldest stars 13 Gyr ago
  Galactic thin disk formation 10 Gyr ago

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Possible solutions to the paradox

• We are alone
• Civilizations are common but no one has colonized
  the galaxy
• a) technological difficulties
• b) sociological considerations
• c) self-destruction
• There is a galactic civilization, but it has
  deliberately avoided contact

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Sociological considerations

• Civilization has to maintain desire and
  enthusiasm for colonization for millions of years
• SETI does not have a government funding NASAs
  SETI was terminated 1993
• Unless extra funding provided NSF will close
  Arecibo in 2011

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Self-destruction

• Nuclear weapons
• Global warming
• Population growth
• Diseases

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Galactic civilization

• No interest in us we are too primitive
• Zoo hypothesis (wildlife refuge) humans like
  other animals are supposed to have their own
  habitat