Clive wrote:

We are doves in a Universe of sparrowhawks.

I'm not sure that's logical, Captain.

By definition, that statement states that we're the exception.  If you
accept the axiom that we are doves, why should we be the only ones,
and everyone else is a sparrowhawk?  Why not accept ourselves as proof
that life can evolve into something other than sparrowhawks?

If their sun is turning into a red giant, why must we assume they
would approach us as invaders?  They would have learned by now that it
would be to both our benefits to approach us as friends (an evolved
philosophy).

(Next thing you know, they'll be marrying your daughter.  Your
grandchild will have tentacles.)

Based on the limited examples of evolution that we see on Earth, it
appears that as life evolves and becomes more intelligent, it becomes
more distant from the "laws of the jungle."  (Obviously, we ourselves
could use a little more evolving.)  If that's the case, then we might
extrapolate that more highly advanced civilizations would be even less
predatory than are we.

So don't worry.  Be happy.  It could be worse -- you could be trying
to run BOINC.

There's another technological issue, regarding your concern about our
radio waves spreading like ripples throughout the universe, giving
away our location.  There are a couple of interacting concepts that
cut off the maximum distance that a radio signal could be received:

1) As the radio wave moves further away from the transmitter, it gets
weaker (by the square of the distance, methinks).

2) There is a background "noise-level" throughout the radio spectrum,
called the Noise Floor.  There are, for instance, [terrestrial]
scientists that make a study of the radio noise that is omnipresent
throughout space, which is supposed to be a remnant of the Big Bang
and provide clues about its nature.

At some distance from the transmitter, the radio wave gets weaker than
the noise floor.  Beyond that distance, the noise obliterates any
possibility of detecting the radio wave.

If you want talk to Alpha Centauri, you can calculate how much power
needs to be transmitted in their direction so that they could detect
our signal.  Does anyone know if we can transmit a powerful enough
signal to be heard at the nearest star?

Certainly, there are no broadcast transmitters (I Love Lucy, Star
Trek, Rush Limbaugh) that can be heard as far away as Mars, thank
goodness.  It is inconceivable how much further away is the closest
star, and generally, none of our terrestial broadcasts can be heard
beyond that distance.  The only chance our transmissions might be
detected at Alpha Centauri would be if we point a gigawatt laser at
them -- maybe that would be sufficient?  Using Arecibo to point a
radio transmission at them would not be sufficient.  Not enough power,
not enough directionality.

You can calculate how far the Jack Benny show has propagated over the
last 50 years (probably about 50 light years, right?), but that
doesn't mean that a receiver 50 light years away could actually detect
the signal.

Since there is noise on all radio frequencies, I don't see how any
technological improvement will eliminate the need to overcome the
noise floor.  The best you can hope for is to clearly receive signals
that are closer and closer to the noise floor.  But at some
calculatable cutoff point, the signal is lost in the noise and cannot
be recovered.

I detect, Clive, that you were beginning to consider our radio
transmissions as a hazard to life as we know it.  I hope this post
begins to explain why they are not a problem -- at least not the way
you were thinking about it.  Since our transmissions are not going to
attract predatory (or other) ETs, you might take the opportunity to
get your Ham Radio license, and try a little transmitting of your own.
 However, further discussion on that point would be a big tangent --
don't get me started.  Just go to www.arrl.org.

Best luck,
                 -Neil-