In article <409A7C1D.6000100@somewhere.com>,
 Rich <someone@somewhere.com> wrote:

They need to detect us. If they've not detected us, why would they
waste the time and energy to broadcast in our direction? Would you,
in your attempt to contact ETI's, broadcast to apparently empty
space, or stars with no detected planetary systems? Perhaps you would.
I would focus my efforts on stars with planetary systems.

The problem is that you don't seem to realize that the detectability
issue works both ways. If ET can't detect our planetary system, why
would ET broadcast at our star?

If ET in our galaxy exists at all, it knows darn well that are here, 
because it almost certainly colonized the entirely galaxy many millions 
of years ago.  Yet no ETs seem to be in our solar system, which 
indicates that either (1) there are no ETs out there, or (2) they have 
specifically avoided our solar system.  In the case of option (2), I 
think it is by far most likely that our system has been avoided because 
it harbors life.  It's possible that there is something else special 
about our solar system that makes it worth preserving in its natural 
state, but I doubt it.

Now, SETI proponents often seem to imagine a third option, which is that 
there is an ET civilization out there which just happened to evolve at 
exactly the same instant in time as us.  Out of the billions-of-years 
history of the galaxy, our own civilization has arisen in the last few 
tens of thousands of years -- and as a technological civilization, 
really just the last couple of thousand years.  That is not even an 
eyeblink on geological time scales (to say nothing of astronomical time 
scales!).  To propose that two civilizations would arise in exactly the 
same eyeblink, such that they might be right over there and capable of 
sending and receiving messages but have not yet colonized the galaxy (in 
short, at exactly the same stage we're in) -- this is ludicrous.

A fourth proposal, that ETs are out there and much older than us but for 
some reason are all stay-at-homes with no interest in colonizing the 
galaxy, is just as ludicrous.  Civilizations are composed of living 
things, and living things are subject to selective pressure.  Expanding 
into new niches is strongly selected for, and while random 
counterpressures can slow down expansion briefly, no technological 
civilization arising in a resource-rich unpopulated galaxy would stay 
contained for long.

So.  Either ET's not out there, or it's right next door and knows very 
well that we're here (and probably speaks all our major languages too). 
In the latter case, if it wanted to contact us, I don't think it would 
require scanning millions of channels with sensitive detectors -- there 
would be no missing it.




 It's a simple concept, I have no idea

why it's not getting across. If you still don't get it I give up.

R> (and note that much of the galaxy is occulted and not directly
R> viewable).

This is manifestly not true.  At radio wavelengths, there is
effectively no absorption.

R> So?

So at radio wavelengths, one can see all the way through the Galaxy.
In other words, at radio wavelengths the entire Galaxy is viewable.

For radio astronomy, that's good. SETI has different goals.

I have observations of other galaxies seen through the disk of the
Galaxy.  Indeed, whole surveys have been conducted in the radio to
try to find background galaxies seen through the plane of the
Galaxy.

R> But since you can't see stars behind it, where are you going to
R> point your receiver to look for ET? Where are the ETI's going to
R> point their transmitters?

I'm sorry, these questions make no sense.

Why should an ETI point their shiny new gigawatt transmitter at
a gas or dust cloud that they can't see behind? They don't know
where any stars are unless the emit copious RF, and such stars
would not seem likely candidates for ET life. The sensible thing
to do would be to point it at where you think ET life might exist,
say, at a star with a detected planetary system.

At radio wavelengths, one
can see all the way through the Galaxy.

So do you expect ET to be broadcasting at us from behind the dust?
They can't even see our star, our planetary system, or measure our
stars motion to estimate where to point the beam. They have no target,
same as us looking the other way.

A sufficiently strong
transmitter can be detected by one of our radio telescopes, no matter
where the transmitter is in the Galaxy.  

So? Since they can't see our star or planetary system, they would
have no incentive to point their transmitters in our direction (or
where we would be when their beam reached our distance (which they
also could not know)).

Again, I don't understand what the issue is.

R> 4. Performance of experimental tests of the predictions by several
R> independent experimenters and properly performed experiments.

Otherwise known as SETI.

R> Indeed.  But with all the radio surveys done of the sky, with 30
R> years or so
R> of SETI searches of ever greater sensivity, one might question why
R> any nearby ET civilization's broadcasts have not already been
R> picked up.

Well, there are two possibilities, right?  As you note above, maybe
we haven't looked at the right frequencies or maybe there are no
radio transmitting ET civilizations nearby.

R> As for the issue of frequency, there seems to be some disagreement
R> in the views of the authority-type posters in this thread.

R> Dave Woolley just posted this...

R> --- to look, and you need to know what frequency at which to look.

DW> Most searches cover a wide range of frequencies, so frequency is
DW> not critical.

Yes, I saw Dave's post.  I'm not entirely sure I understand what he
means, but I suspect we're simply defining "wide" differently.  

I don't read the word 'wide' in "so frequency is not critical" at all.

That doesn't change the truth of my statement: The absence of a
detection either means that something doesn't exist or you weren't
looking for it in the right way.

You mean like looking behind dust clouds or gas opaque at optical
wavelengths?

The apparent issue is that you say frequency is critical and Dave W
seems ot be saying that it is not critical. Hopefully Dave will post
clarification, the words may be getting in the way of the message
somehow, as happens so often in the net.

R> It has been claimed that a 1 watt transmitter would be sufficient
R> with modern receivers.

Umm, have you read the FAQ?

R> Have you read this (also from Dave Woolley)?

R> It has been claimed that a 1 watt transmitter would be sufficient
R> with modern receivers.

DW> I don't know where you got that figure.  You need very
DW> approximately 1GW EIRP from the nearest star to get the S@H
DW> threshold of 22 times mean noise power in 0.075Hz and time *
DW> bandwidth = 1.

Yes, I read this statement by Dave Woolley, and I see him asking the
same question.  Where did you get this 1 W figure?  It's wrong.  As
Dave states, a transmitter power closer to 1 GW (= 1,000,000,000 W) is
required.

Looks like the 1 watt argument was made by Lou Scheffer.

---

Lou Scheffer wrote:
 > This is a reasonable question.  Why look for a signal that no one is
 > motivated to send?  However, it turns out this signal is not very
 > expensive to transmit. (see the appendix on beacon construction in the
 > book SETI 2020.)  If you target your beam so it only covers the
 > targeted solar system, it takes less than 1 watt per system covered.
 > So for 1 MW of power, costing at current rates about $700,000 per
 > year, you could hit each of the nearest 1 million stars with a beam
 > strong enough that we ourselves could detect it.  Although building
 > the transmitter costs somewhat more, it's still on the order of
 > existing SETI expenditures.

---

Rich

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