Pulsars flash over the entire spectrum. No matter where you tune your radio telescope, the pulsar can be heard. That's bad design, because if the pulses were intended to convey some sort of message, it would be enormously more efficient (in terms of energy costs) to confine the signal to a very narrow band. Even the most efficient natural radio emitters, interstellar clouds of gas known as masers, are profligate. Their steady signals splash over hundreds of times more radio band than the type of transmissions sought by SETI.

Imagine bright reflections of the Sun flashing off Lake Victoria, and seen from great distance. These would be similar to pulsar signals: highly regular (once ever 24 hours), and visible in preferred directions, but occupying a wide chunk of the optical spectrum. It's not a very good hailing-signal or communications device. Lightning bolts are another example. They produce pulses of both light and radio, but the broadcast extends over just about the whole electromagnetic spectrum. That sort of bad engineering is easily recognized and laid at nature's door. Nature, for its part, seems unoffended.

So it sounds like they're looking for a narrow band transmission, because it would be more energy efficient to produce one. Have I got that right?

http://www.space.com/searchfor...

Bilbo: I thought I had a read an explanation by a SETI researcher, who said the narrower the band, the less energy needed to send a strong signal. Is that correct?

There are a variety of known sources of radio noise, including synchrotron, thermal, plasma oscillations and spectral line emissions. Each has a characteristic spectrum. A narrow-band signal has a high energy for a given region of the spectrum, so have been traditionally used by humans as carriers for radio communications.

Bilbo: And why is it quiet at the "water hole"

The water hole has two important characteristics. It is relatively quiet. And it is associated with the spectral lines of H and OH, the components of water, water considered essential for 'natural' life.

There is significant low frequency noise due to galactic synchrotron radiation, which drops in a linear fashion as the frequency increases. The upper end of the spectrum is blocked by the Earth's atmosphere, primarily water and carbon dioxide.
http://www.setileague.org/gene...

Rock: I expect that SETI researchers will accept as a candidate just about any really intriguing signal regardless of bW.

Well, SETI researchers would be intrigued by Gort doing semaphore in front of the White House. But that reminds us of why SETI is considered a weak hypothesis. We have to make several assumptions to make the search practical and finite; but then any falsification reveals little.

(New technology has allowed a much broader range of frequencies to be studied, including optical searches, but no technology is infinite in power.)

Rock: The key here is that the wave is band-limited and modulated in a way that SETI researchers think is due to the transmission of encoded information (a message).

SETI is not searching for a modulated signal (though any serious candidate would certainly be tested for modulation). A simple beacon"”a lighthouse"”would be significant.

Rock: As I suggested above, Bilbo, its not really a "narrow band radio signal" that is being searched. It is any sign of intelligence.

More precisely, *artifice*.

Okay, SETI's looking for ET, Hsu and Zee are looking for God. Apparently, both quests involve 'science' enough to qualify. We're just not allowed to ask such questions about life.

Come on, that's just whining. There's enough cash around to investigate your questions. Ask the Disco Institute. It's just that some people prefer to spend the dough on propaganda movies.

The key here is that the wave is band-limited and modulated in a way that SETI researchers think is due to the transmission of encoded information (a message).

Well, is that really any sillier than expecting God to inform the theoretical physics guys which of their ToEs (Theory of Everything) is the right one?

Okay, SETI's looking for ET, Hsu and Zee are looking for God. Apparently, both quests involve 'science' enough to qualify. We're just not allowed to ask such questions about life.

I expect that SETI researchers will accept as a candidate just about any really intriguing signal regardless of bW. Narrowing the bW narrows the search and may prove exhausting. It's a limitation imposed by our state of knowledge and technological advance.

The key here is that the wave is band-limited and modulated in a way that SETI researchers think is due to the transmission of encoded information (a message).

As I suggested above, Bilbo, its not really a "narrow band radio signal" that is being searched. It is any sign of intelligence. What they mean by "narrow band radio signal" is any one (or a small range) of w that has been selected as a communications protocol for transmitting signals over long distances, from source to destination.

That is what the narrow band signal is conveying minimally to us.

Intelligence.

It sounds confusing and maybe because they don't want to say that what they are looking for is a "message from the heavens," or the "signs of intelligence," etc.

And I would argue that a "strong signal" is to be determined purely semantically, i.e., if the we can share any meaningful information with the source. (E.g., we may test the source by requiring it predict with some arbitrary degree of precision the value of some physical constant known to us.)

That could take a long time. It could never end, and endlessly into the future we may never know if we have actually identified an ETI.

(And I didn't mean to tell you what you already knew.)

Ha! You could fill numerous large libraries with what I don't know, Rock. But why should I look for a book, when I have you and Zachriel to ask (and Joy).

So I'm still not quite understanding the point of why a narrow band radio signal. I thought I had a read an explanation by a SETI researcher, who said the narrower the band, the less energy needed to send a strong signal. Is that correct? And why is it quiet at the "water hole"

This is why scientist make limited and specific claims rather than general broad-sweeping claims.

But planetary science and SETI are both motivated by the conjecture that life and intelligent life probably exist somewhere else in the universe. This conjecture at the moment is a broad and sweeping, if not basically metaphysical. Of course, specific methods need to be developed to detect ET life and intelligence. But without the freedom to make broad conjectures there would be no specific experiments. IOW science usually begins with broad sweeping conjectures and narrows the search by developing more specific tests and experiments.

The important requirement is that you need to be able to make specific testable predictions.

Hasn't SETI also developed specific methods for detecting ETI's– looking for specific electromagnetic signals on specific bandwidths etc. I would argue that this is no different that what the Viking probes did in their search for Martian life. Viking scientists consevatively hypothesized that life might be very similar to terrestrial life. However, it may be that ET life is quite dissimilar from terrestrial life. In the same way ETI's may be very similar to human intelligence or very dissimilar. Without a broader conjecture that life and intelligence of some kind exists elsewhere in the universe there would be no reason to continue the search. IOW there would not be an ongoing search for ET life and ET intelligence if it wasn't driven by a broad and basically metaphysical conjecture. These kinds of conjectures (as well as the intellectual freedom to make them) are absolutely necessary for the continued advancement of scientific knowledge.

What SETI researchers are searching for is the most common means of conveying messages, at least the most common one we use. (Maybe that's what Zachriel means by "artifice" and "clear chain of causation.")

It makes sense. It is not unreasonable to expect that just about any civilization attaining a certain level ("human-like") of scientific and technological knowledge and skill would realize (as we did even before quantum theory and EM theory) that messages could be effectively transmitted (over any distance) in this way.

No small complications are entailed, mostly due to "local" conditions, such as the characteristic absorption, emission, reflection features of atmospheres, or just generally the ISM (interstellar media), which are highly variable and constraining of the effectiveness of the specific means of communications chosen. (Effective communications are always "context dependent.") But these complications are simply a test of our (and LGMs') ingenuity.

(By measuring the Doppler shift of such a signal we can approximate its distance, and a periodic Doppler shift would be a good indication of an orbiting source, such as a source originating from a planet orbiting a star.)

Hopefully I got that right, Bilbo. (And I didn't mean to tell you what you already knew.) I'm sure there is a book in the library (or on the Web) that can provide a more lucid and accurate explanation.