Rich <someone@somewhere.com> writes:

In semi-infinite wisdom Louis Scheffer answered:

Andrew Nowicki <andrew@nospam.com> writes:

If your message lasts 10
seconds and you are sending it to one million stars,
the same star receives the message about once every
3 months. 

Perhaps I was not clear.  All the beams, shining on all the stars,
are on all the time, 

Just where do you plan to place your transmitter so that it
will not be obstructed by a planet or it's star periodically?

For stars near the ecliptic (ours or theirs), you will get 
occasional eclipses, but a very small fraction of the time.
For most stars, 100% coverage is posible.  A northern hemisphere
transmitter can send to stars near Polaris, for example,
full time with no eclipses or outages.  A southern
hemisphere transmitter can so the same for the stars
near their pole.

Whenever the receiver looks, they will see it.  This
is precisely to avoid the problem you just stated.

Gee, that technology would come in handy on Earth. Maybe
you should develop it?

Good idea, but it's already developed.  The IEEE database 
has more than 500 papers on phased-array radars alone.

As I understand it, a phased array can only transmit or
receive in one direction at a time.

This is incorrect.  A phased array can transmit (or receive)
in as many directions as it has elements, within the field
of view of a primary element (which can be very large for
transmitters).

The math is identical.  However, the phased array
is able to cope with changes much better (as the stars move
with respect to each other, or you add more stars, you 
would need to change the hologram.)  With a phased array
you just update the coefficients with no physical changes.

Have you contacted the SETI people?

Yes.  I'm on the Science and Technology Working Group of the
SETI Institute.  I'm a co-author and editor of the book SETI 2020,
and wrote the appendix on microwave transmitters for SETI.
I'm on the technical advisory board of the Allen Telescope
Array.  I wrote the band-splitting code used in the SETI@home
servers.  So I've contacted most of the serious SETI folks.

    Lou Scheffer