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?
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%.)