| Subject: Re: What is SETI? was->>Re: How smart are SETI@homers? - ScientificAmerican |
| From: Joseph Lazio |
| Date: 06/05/2004, 18:23 |
| Newsgroups: sci.astro.seti,alt.sci.seti,sci.space.policy |
"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.
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.
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.
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. At radio wavelengths, one
can see all the way through the Galaxy. A sufficiently strong
transmitter can be detected by one of our radio telescopes, no matter
where the transmitter is in the Galaxy.
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.
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.
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.