Why the 21 cm line for SETI?

[bcrowell]

Why is the 21 cm line considered favorable for SETI? The WP article http://en.wikipedia.org/wiki/Hydrogen_line explains why it exists (it's a hyperfine transition of hydrogen) and states that it was hypothesized to be a natural place to search for extraterrestrial signals, but it doesn't explain *why* it would be considered a natural place to search. Is the galaxy any more noisy or any less transparent at 20 cm? Is the idea that alien astronomers would naturally be observing at 21 cm to find out about the structure of the galaxy, and that therefore that's also a natural frequency to transmit on, because someone's likely to be listening?

On a related point, why isn't most hydrogen in the interstellar medium molecular? Hydrogen gas is molecular at room temperature, and yet WP describes http://en.wikipedia.org/wiki/Interstellar_medium the cold neutral medium as atomic...?

 

[Chronos]

The 21 cm [HI] line is used because it can penetrate dust and gas clouds that are opaque at most other wavelengths. More importantly, it is the frequency an intelligent alien civilization should expect to receive [or send] a signal. The 21 cm line is extensively monitored by earthbound scientists for a number of reasons. Modulations in that frequency range would be easily noticed by alien or human scientists.

 

[PhilKravitz]

Jodi Foster in the movie "Contact" offers pi*21cm