Andrew Nowicki <andrew@nospam.com> writes:

Mark wrote:

At least we'll know that aliens with big non-directional
radio transmitters are rare. 

We have already learned that.

Not really.  First, we have only looked a small portion of the 
frequency spectrum.  If you turn on your TV, and see nothing
on the first two channels you try, you can't tell if there
is any signal (even a very strong one) on another channel.  You
need to try all the channels (or at least most) to conclude
signals are rare.

Second, our searches tell us nothing about the directionality
of the source.   We can't tell a bright omni-directional
source from a less bright but directional one.

What we need is a non-directional microwave receiver
on the far side of the Moon. It would be a sort of
phase-array radar in reverse -- lots of small receivers
but no directional antennas. To reduce the noise, it
would be used only during lunar night, when the surface
of the Moon is cold.

This is sound in theory, but has a bunch of practical problems.
The biggest is that looking in all directions at once takes
an enormous amount of computer power, far more than we
have right now.  (Basically, you need to phase and add all
the elements, and do this as many times as there are beams from
an equivalent sized conventional scope.  There are shortcuts
but not enough to make it practical.)  Also, given the
current technology, the lunar night is still not cold enough.

The advantage of the non-directional receiver is that
it can detect signals coming from a broad solid angle.

Very true, plus it can study transients, which is very hard for
conventional radio telescopes.  The problem is that for the same
amount of money, it is far less sensitive than a conventional
telescope.  So if you expect huge but infrequent signals, it's
better, but if you are looking for lower power but always on
signals, a conventional telescope is a better bet.  It would
make sense to try both.

     Lou Scheffer