SETI: Search for Extraterrestial Intelligence

What Signals?

This is a project to search for intelligent signals impinging upon the Earth from outer space. What kind of signals should one expect to find?

• Electromagnetic signals: radio freqencies are probably the first electromagnet frequency that an intelligent society would discover how to use. So radio transmission is likely to be a top choice for communication. AM radio frequencies = about 1 MHz; FM radio frequencies = about 100 MHz. (See http://hyperphysics.phy-astr.gsu.edu/hbase/audio/radio.html)
• Neutrino signals: very much more energetic and technology intensive. Useful for communicating only with very intelligent civilizations. See: http://www.bigear.org/vol1no3/neutrino.htm and http://www.qsl.net/mac/neutrino.htm.)

Radio Signal Detection

• Carrier wave plus modulation (amplitude or frequency)
• Periodic pulses
• Intelligent pulses: much harder to detect.

Galactic "water hole": Frequency of transition of hydrogen atom for which the proton and electron spins are aligned to unaligned =
1420 MHz frequency = 21 cm wave length (freq)x(wave length)
= speed of light (1420x1,000,000)x(21 cm) = 2.98x10,000,000,000 cm
= 2.98x10¹⁰ cm.
The lifetime of the upper aligned state is 10⁷ years, so it is not easily observed in an earth laboratory.
This spin-flip transition is the most prominent frequency of state transition for the hydrogen atom in outer space, because most of the hydrogen is in the ground state. Hydrogen is by far the most abundant atom in the universe: 73% (See lhttp://hyperphysics.phy-astr.gsu.edu/hbase/astro/hydhel.html.) (Helium abundance is 25%; all other atoms are 2%.)

• For the detailed physics of this transition:
  http://tesla.phys.unm.edu/phy537/8/node2.html
• For information about the discovery of the 21 cm line of hydrogen:
  http://www.gb.nrao.edu/fgdocs/HI21cm/21cm.html
• More information about the 21 cm line of hydrogen in astronomy:
  http://www.iiap.ernet.in/answers/galactic.html
• Book: Confessions of an Alien Hunter: A Scientist's Search for Extraterrestrial Intelligence, Seth Shostak, 2009, National Geographics Society
• Book: Rare Earth: Why Complex Life is Uncommon in the Universe, Peter Ward & Donald Brownlee
• Debate: Are We Alone? Geoff Marcy vs Dan Werthimer