In infinite wisdom Joseph Lazio answered:
"R" == Rich <someone@somewhere.com> writes:
R> In infinite wisdom Joseph Lazio answered:
R> [...]
R> And lots can happen in the 100,000 years it takes light to traverse
R> the galaxy, there's a bit of a lag. I suggest that the probability
R> of said civilization broadcasting in our direction is inversely
R> related to it's distance from us
That's a hypothesis. The evidence that you have to support it is?
R> What? That lots can happen in 100,000 years? [...]
No, the statement, "I suggest that the probability of said
civilization broadcasting in our direction is inversely related to
it's distance from us" is a hypothesis. I had put forward some
evidence, and you were attempting to refute it with this hypothesis.
You need to state the evidence in favor of your hypothesis.
My question was which part of the paragraph you were disagreeing with.
Since I answered both of them, I do not understand the reason for your
condescending attitude.
R> As for the probability argument, seems to me that there are many
R> issues here, the detectability of a star (...) is a matter of
R> distance. [...]
The detectability, yes. That's not what you wrote. You wrote that
the probability of "broadcasting in *our direction* (emphasis added)
is inversely related to it's distance from us." I can easily imagine
why the probability might *increase* with distance from us.
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? 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