david@djwhome.demon.co.uk (David Woolley) wrote in message news:<T1075937111@djwhome.demon.co.uk>...

...snip...

The reason it is significant is nothing to do with life; it is that it
is the most abundant element in the universe and in inter-stellar space.
The 1.42...GHz hydrogen hyperfine line radiation is used to map the
distribution (and temperature etc.) of cold hydrogen between stars and
is therefore of fundamental interest to radio astronomers.  The theory is
that any attempt at interstellar communication will involve radio 
astronomers and this hydrogen radiation will be a fundamental concept to
all of them - even if they have exhausted experiments on these frequencies,
it will still be fundamental to the history of the science.

Perhaps some would think that the significance of that frequency has
everything to do with life, it being the bottom end of "The
Waterhole".  Indeed, back in 2000 David Woolley probably also wrote:

OPEN QUOTE
...(There are actually at least three water hole definitions: - 1 to
~20GHz where absorption/noise (they are aspects of the same thing) is
low; - between 1.42.. and 1.6.., strong lines associated with H and
OH, the constituents of water; - 1.42... +- a few hundred kHz, by
analogy with waterholes where animal gather to drink. SETI@Home uses
the third, SERENDIP, on which it is based, uses the second, and
Phoenix, the SETI Institute targetted search, uses a significant part
of the first. The second has, amongst other things, strong interfering
signals from GPS satellites and the Russian equivalent.)...
CLOSE QUOTE

David's third definition of 2000 seems to have something *to do with
life*, and his second definition is the dominant definition (IMO) for
the Waterhole that I've encountered over the years (the point being
that water-based LIFE would perhaps search in these frequency ranges.)
_________________
(I remembered David's 2000 post because I had responded to it with
OPEN QUOTE
...The second water hole definition you gave (above) is considered by
some to be more like 1.42 to 1.72 GHz because there is also a strong
OH emission at 1.7205299 etc. GHz.
CLOSE QUOTE)
_________________

Regards, Jason H.