"Neil_J" <my_antispamaccount@yahoo.com> wrote in message news:2040a962.0407072117.7a386e1a@posting.google.com...

Clive wrote:

....

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 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 so weak
that the noise floor 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?

We CAN, but we dont (at least not consistently). There is no reason (yet)
to transmit such strong signals consistently.

Interestingly enough, there is a n.g. thread on sci.astro.seti which is
discussing if we would be able to detect (with certainty)
an earth-like civilisation around Alpha Centauri ("call to arms!" on sci.astro.seti )
if it existed. Also seti@home FAQ discusses this.

So far, my conclusion is that we can't (prove or dis-prove the
existance of a earth-like civilisation around Alpha Centauri).
At least not without building a very, very large receiver antenna
(size of the moon or so), and listening to Alpha Centauri for a
long time, collecting all the signals in the entire microwave spectrum.

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.  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?

You need a terra-watt laser to blast over the 'noise' of our sun and
a big telescope around Alpha Centauri to receive enough photon's
for a reproducable signal.

Using Arecibo to point a radio transmission at
them would not be sufficient.  Not enough power, not enough
directionality.

Actually Arecibo can do it, but does not. At least not on a consistent basis,
and certainly not beamed consistently at Alpha Centauri.
So they do not receive a consistent, reproducable signal, so they will
discard it as noise (just like seti@home discards incoherent signals as noise).

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 any particular radio frequency, 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.

All correct.

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.  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-