Optical SETI Map Conferences Map Illustrations Map Photo Galleries Map Observations Map Constructing Map
Search Engines Contents Complete Site Map Tech. Support Map Order Equip. Map OSETI Network

Search WWW Search www.coseti.org Search www.oseti.net Search www.photonstar.org Search www.opticalseti.org

colorbar.gif (4491 bytes)



The Search for Extraterrestrial Intelligence (SETI) in the

Optical Spectrum


Barney Oliver: What do you mean by short? Short compared to what?

Monte Ross: One nanosecond comes out to be the value that, for a given reasonable spectral filter width, the star background photoelectrons are quite low. They are in the range of one or less per nanosecond. This means that you have a certain number as Jim was just talking about, you have to have a certain number of signal photoelectrons anyway. You want to get the background down to a level below that to compare with. You want to choose your values, from a system engineering viewpoint, to not lose information or detectability because of the star's background. But you do not want to go any further than that because of other system parameters such as longer search times, etc. That's how one nanosecond came about. There may be some better times, but short pulses are still good. They're just like chickens soup!

Charles Townes: Thank you. Would you like to make some comments?

Neil Tennant: I am not a scientist. I am a philosopher, so this contribution may be very different from the other two. I have been very interested in this session, listening to scientists talk to one another at such high levels of technical expertise, and getting a feel for how detailed the debate is. At the same time, I've been quite distinctly impressed by how their being party to common assumptions, in some crucial cases seems to leave certain assumptions still covered; not brought out explicitly, and not examined to see whether they really are conditioning the whole discourse. I have been reading a few more papers in this area, and time and again I come across this assumption that communication is possible just as soon as you detect a signal; that you can interpret what may be said; that you will be able to tell an alien civilization by responding to them appropriately with a message that has a content that has to be agreed on by international protocol. It is all these philosophically loaded notions that I want to draw to your attention as scientists. I think you move too easily from the realm of discourse of the Shannon-Weaver theory of communication in bits-per-second, to the area of semantically structured and semantically laden messages. It's a very important juncture in the collaboration between scientists and philosophers.

I am reminded of two past episodes in the history of thought and intellectual endeavor generally. First there was the foundation of mathematics. When mathematicians and logicians in a philosophical way first started examining the very limits of the axiomatic and deductive method, there was a widespread assumption at the beginning of this century, that all the truths about certain mathematical structures could be deduced formalistically from certain specified axioms that were clearly true about those structures.

In a revolutionary piece of conceptual analysis, Kurt Gödel showed the incompletability of formal arithmetic. So, there was a mathematician doing a piece of conceptual work that showed that arithmetical truth outstripped what could be done in principle by any machine. This then led, because of the methods employed, into further investigations into the notion of mechanically computable function. Once, again there was a prevailing dogma to the effect that virtually anything could be done by a machine so long as it was combinatorially specified, and so on. But, then the diagonal methods involved in Gödel's proof allowed him to show that there were certain things that machines could not do; solving the halting problem for example, about the machines themselves. All I am suggesting is that the time may be ripe within the SETI community for a philosophical "fronting up" to some of the underlying issues about communication, about decodability, about "contact" and "content". And I would like to suggest that the search for extraterrestrial intelligence, "SETI", with a sibilant "s" at the beginning may be a "starter" in that you can go for detection. But when you start talking about communication with extraterrestrial intelligences, with a hard "c", "CETI", then you are WAY out of left field as far as most of my philosophical colleagues would see you.

Charles Townes: Alright, let's hear comments on any of the last three speakers. You had an earlier question?

Floor: I want only to comment on the definition of signal-to-noise ratio. It seems that in the systems I have dealt with that noise usually encompasses both the background and the signal fluctuations. In some situations the background dominates, while in other situations it's the signal fluctuations that dominate. A good definition of noise should include both types of fluctuations.

Charles Townes: Any other comments? Anyone solve the question of how we communicate? I've heard solution as to how we decide whether the signal is from an intelligent group, but I have never heard a solution as to what it says. Somebody back here?


Copyright ©, 1993, SPIE

Home Glossary
SPIE's OSETI I Conference SPIE's OSETI II Conference
SPIE's OSETI III Conference
The Columbus Optical SETI Observatory
Copyright ©, 1990-2006 Personal Web Site:
Last modified:  10/28/06
Contact Info