Summary of the sections on 08-06-96

Communication with extraterrestial civilizations

How could we communicate with extraterrestial civilizations? (list was put together by students)

• interstellar travel
  Advantages:
  • direct physical contact with other life-form
  • first-hand experience
  Disadvantages:
  • time problem
  • fuel problem
• visible light signals
  Advantages:
  • humans have used visible light signaling for communication purposes for a long time ...
  Disadvantages:
  • competition with visible light from Sun
  • energy per photon is high (-er than for radio waves)
  • interstellar absorption
• x-ray signals
  Advantages:
  • less competition from solar radiation
  Disadvantages:
  • energy per photon is very high
  • interstellar absorption
• radio signals
  Advantages:
  • interstellar absorption not a problem
  • energy per photon is low
  • even with current technology radio beams can be produced that have a higher intensity than the radio emission of the Sun.

Can we make an even more precise guess about where a search for signals from extraterrestial civilizations might have the greatest chance for success?
The single most important radio emission is the "spin-flip" transition of neutral hydrogen in the ground state at a wavelength of 21 cm (1420 MHz). It's emitted by cool hydrogen gas at low densities which is very common throughout in the Milky Way (and other galaxies). It's plausible that other civilization recognize this and use this frequency to communicate. It also lies in a region that is relatively free from Galactic background "noise" (which increases towards lower frequencies) and absorption by the Earth's atmosphere (which increases towards higher frequencies). If water plays as important a role for other civilizations as it does for us, the most likely region for interstellar communication may be the so-called "water hole" (1420-1721 MHz). This is the frequency range in which the most important transitions of OH^- and H are found. It's called the "water hole", because H + OH^- = water.

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