| Subject: Re: What is SETI? was->>Re: How smart are SETI@homers? - ScientificAmerican |
| From: Joseph Lazio |
| Date: 07/05/2004, 14:35 |
| Newsgroups: sci.astro.seti,alt.sci.seti,sci.space.policy |
"R" == Rich <someone@somewhere.com> writes:
R> In infinite wisdom Joseph Lazio answered:
[O.k, this is quickly descending to "Yes, I did!" "No, you didn't!"
I'm going to try to summarize where I think it stands and then leave it.]
R> 3. Use of the hypothesis to predict the existence of other
R> phenomena, or to predict quantitatively the results of new
R> observations.
Observations at radio wavelengths can be conducted at a certain
sensitivity levels (call this S). These sensitivity levels
translate to a distance for an assumed transmitter power (roughly d
~ sqrt{S/P} for an assumed transmitter power P). If one conducts a
search at a sensitivity level of S and finds no examples of ET
broadcasts, the number of radio transmitting ET civilizations in the
Galaxy cannot be more than roughly (D/d)^2, where D is the diameter
of the Galaxy.
[...]
R> My question was which part of the paragraph you were disagreeing
R> with. Since I answered both of them, I do not understand the
R> reason for your condescending attitude.
You claimed that (1) Time lags are important; and (2) That more
distant civilizations are less likely to broadcast in our direction.
At best, neither is relevant to my original statement; at worse, I'd
argue that (2) is wrong. (See below.) You asked for a prediction
regarding SETI. I showed that one can make a prediction regarding the
number of radio transmitting ET civs. in the Galaxy resulting from a
set of observations. QED.
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> They need to detect us. If they've not detected us, why would they
R> waste the time and energy to broadcast in our direction? Would you,
R> in your attempt to contact ETI's, broadcast to apparently empty
R> space, or stars with no detected planetary systems? Perhaps you
R> would. I would focus my efforts on stars with planetary systems.
Suppose I wanted to broadcast a message to any potential ET civs. in
the Galaxy, but I didn't know where any were. What would I do? One
stategy would be to aim my telescope in the direction of the largest
number of stars, toward the inner Galaxy. Suppose we have a large
telescope at our disposal, like the Green Bank Telescope. (The GBT
can't transmit a signal, but this is just for illustration.) Its
*beam* or the angular extent of the transmitted signal is about 0.1
degrees. By the time the signal has travelled half-way across the
Galaxy, it covers about 20 pc (~ 60 light years). There are many
stars within such a region.
More generally, of course, one has to consider the volume covered by
the signal. The situation becomes even better for a smaller
telescope, as its beam is larger.
The important point is that the size of the region covered expands as
the signal travels deeper into the Galaxy. That means, in this
strategy, one is *more likely* to be in the beam of a transmitter, or
another civ. is more likely to be pointed toward us, the farther away
they are. (The detectability of course is another matter.)
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.
R> For radio astronomy, that's good. SETI has different goals.
Umm, some SETI is conducted at radio wavelengths. Ergo, seeing
through the Galaxy is a good thing.
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> You mean like looking behind dust clouds or gas opaque at optical
R> wavelengths?
sigh< No. If aim a radio telescope at the inner Galaxy, and I'm
tuned to the correct frequency and my telescope is sufficiently
sensitive, I can pick up any transmitter within my beam no matter
where it is. We detect radio sources at the center of the Milky Way
Galaxy, where the optical absorption is huge. Why would ET signals
not be able to make it to us if natural radio signals do?
R> It has been claimed that a 1 watt transmitter would be sufficient
R> with modern receivers.
[...]
Where did you get this 1 W figure? It's wrong. As Dave states, a
R> transmitter power closer to 1 GW (= 1,000,000,000 W) is required.
R> Looks like the 1 watt argument was made by Lou Scheffer.
R> 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. (...) 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.
Re-read what he wrote. He's saying that with 1 MW of power one could
transmit to 1 million systems so that the signal would be potentially
detectable once it gets there. That doesn't mean you build 1 million
1 W transmitters. It means you build one 1-MW transmitter and use it
for 1 million systems.
This is an economic argument. More powerful transmitters can reach
more distant systems, but they also cost more. How powerful does a
transmitter have to be in order to reach an "interesting" number of
systems?