Ask Dr. SETI ®
If SETI receivers have to be narrow band, how is intelligence carried in these signals? Also, why do they need to be narrow band?
Ron, Buffalo State
The Doctor Responds:
That's all very well and good, but communications theory suggests that a pure CW carrier contains no intelligence. But wait -- is that really true? It can be argued that the reception of such a narrow signal is in fact a one-bit message, conveying the information "here I am." Run that message through your Universal Translator, and out comes the more meaningful proclamation "you are not alone." So, information has been exchanged, in vanishingly narrow bandwidth.
OK, so maybe ETI want to communicate higher level concepts than our single-bit existence statement. Doing so will of necessity require them to transmit over a wider spectral bandwidth. But, if they want to ensure detection, they will make the job as easy as possible for the receiver, by employing a modulation mode that contains individual narrow-band components, strewn across a broader stretch of the electromagnetic spectrum. Most modulation modes used on Earth (even direct sequence spread spectrum, or DSSS) include just such narrow-band components. Initially, one such spectral component may be detected, and existence proof established. After that, follow-on analysis may well discern other spectral components, from which a coherent reconstruction of Encyclopaedia Galactica can be attempted.
But first, you have to get the attention of those primitive Earthlings -- and narrowband signals are a promising and effective way of doing so.
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this page last updated 18 October 2008
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