The SETI League, Inc., a membership-supported, non-profit {501(c)(3)}, educational and scientific organization Searching for Extra-Terrestrial Intelligence

Ask Dr. SETI ®

Chapter 6: Technology

Can A Narrow Signal Carry Intelligence?

Dear 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:
To understand how ET thinks, proud Buffalo, we have to walk a parsec in his moccasins. Let's assume you're an intelligent extraterrestrial, wishing to make your presence known to the inhabitants of a distant and primitive planet (let's say, Earth). You know that the electromagnetic spectrum is a noisy place, and that your intended communications partner is going to have to separate your signal from the background noise. You also know that natural astrophysical radio emitters are inherently broadband. What to do? You could readily produce a signal that's spectrally narrow (your technology is good at that). This would clearly stand out as being artificial. It would also make it easier for the distant Earthlings to intercept, since the narrower they make their receiver, the more of the cosmic background noise they exclude, and hence the higher their received signal to noise ratio. Well, the narrowest possible signal is a pure CW (continuous wave) carrier, so that's what you send.

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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