"Russ Childers" wrote in message ...
Rob Dekker wrote:

Below, my thoughts on why I think 20 years is excessively optimistic for detecting an ETI beacon,



---
Beacons:

With 200 billion stars in the Galaxy, and 10,000 ETI civilizations, the average ETI neighbor
will be a few thousand lightyears away.
So even our ETI neighbors have not received any radio signals from us yet.

But we may be bathed in signals from lots of neighbors which
have been broadcasting for thousands of years.

Thus beacons. See below.


With 200 billion stars in the Galaxy, and 10,000 ETI civilizations, Each of these ETIs
need to send beacon signals to an average of 10million stars in its neighborhood continuously.
Otherwise, there is very little chance that there is any ETI beacon aimed at us.

There are about 41,000 square degrees in the sphere of the sky
(according to my back-of-the-envelope calculations). So if
a radio telescope has a beam size of one degree by one degree,
a civilization would only need 41,000 telescopes to cover the
entire sky. All stars, local and remote, would be covered.
Ten million beacon signals are not required, because there
would be overlap.


You seem to suggest a omni-directional beacon.
41,000 telescopes each receiving 1/41,000th the power, each transmitting
to 1/41,000th of the sky is similar to sending full-power to an omnidirectional antenna.

We had some talk about this earlier in the NG, and the main conclusion is that
omnidirectional beacons are not possible for any civilization. Just do the power-calculations.
A single omni-directional beacon which reaches a million stars or so would require more
energy than the entire ETI population is using.
Or, to word it differently : if the beacon targets each star system inividually, it
can obtain the same signal strength at each target star with orders of magnitude
lower power requirements.

Why would a civilization being called 'intelligent' spent enormous amounts of effort
to send signals continuously to millions of star systems for which they do not even
know when or if any communicating civilization will ever arise.

No one knows why an alien would do anything. I do not
play the lottery, but I know many who do.

I think it is more subtle than that. If there are multiple lotteries, you would choose the
one that has the highest probablility of return. That is intelligence.

If the goal is to contact an emerging ETI, and there are very few ETIs emerging in the galaxy
(1 per year or so using the Drake formula with reasonable numbers) then it is much more
efficient to wait until you detect an 'emerging civilization' leakage signal than it is to
keep on beaming to star systems that do not respond to beacon signals send in the past.

If you don't know if a star system has intelligent inhabitants, then it makes no sense to
send a beacon signal continuously. You might want to send a signal every 100 or 1000 years
(depending on the distance from them) or so, just to probe if some civilization emerged.
That should be more than enough as a beacon.

But if you detect an emerging civilization radio-leakage signal from a star system, then you
know that there is somebody there, so then you can turn the beacon on immediately,
and continuously.

Technology ETIs did not become technology ETIs by being extremely wastfull and
constantly choosing the most expensive solutions to problems.

Based on that, for a beacon to be turned on and aimed at us, it should make a big difference if the ETI has detected us or not.


Now I am a SETI enthousiast, but we need to be realistic.
Unless there are millions upon millions of ETI civilizations out there, the odds are
very much against us finding ANY beacon signal at all.

But if we don't look, we won't find anything.

Yep. That's why I'm cranking work units like a mad man.
We just need to be realistic about the odds. The odds of any beacon being there are extremely low.


We probably need to detect another civilization by other means (such as radio leakage),
which requires much larger antenna arrays than the ATA, and thus much more time than 20 years.
Or 'it' (detecting ETI) will happen totally different than we think.

I agree that there is likely more radio leakage than
intentional beacons. But we have NO idea of how powerful
this leakage is.

Well, we have one example of an 'emerging' technology civilization.

We leaked lots of narrowband TV carriers into space, and still leak a lot of radar signals.
This can be considered a 'wave' of emerging radia signals, which has a certain strength.
In this NG we even calculated how large antenna arrays would need to be to detect such a wave.

They must be large : (100s of km diameter) for detection of leakage at 1000 LYs, but this is certainly
within the possibilities of advanced civilizations. They might need these very large arrays
anyway for radio-astronomy purposes.

We are most certainly hundreds of years away from building such gigantic arrays.

For ETIs within 25LYs of us, if they have a 10km diameter antenna array,
they could have detected us starting some 25 years ago. If they turned on a beacon,
then we can receive it any moment now. This is very analogous to the movie "Contact",
and I still see that as a much more likely scenario than looking for 'blind' beacons elsewhere.

25LYs only encompasses a hundred stars or so, but the silence so far says something about
the prevalence of ETIs in the neighborhood. At least something about the prevalence of
ETIs that can and want to communicate with us.


Maybe ETI are doing powerful planetary
radar. Maybe they are communicating with a colony on a star
a couple of light years away. Maybe they are trying to
communicate with a space probe which has a damaged receiver.
If we happened to be in line with these powerful, beamed
broadcasts, we could conceivably receive a LOT of power.


True. But the further advanced a civilization, the narrower the beam to the probe will be,
so the chances of being in 'line' with that beam diminish.

It is a BIG space out there...

Russ

Rob