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The Planetary Society - 2

From: The Planetary Report, Volume XVIII, No. 3, May/June 1998, Questions and Answers, page 20.

Could extraterrestrial signals be detected in any range of frequencies?  Maybe the way extraterrestrials communicate is completely different from ours and they transmit in the visible spectrum instead.

Maybe this is why we have not received a strong signal yet.

Carlos A. Correa and Vincent Lopez, Buenos Aires, Argentina.


Response by Tom McDonough, SETI Coordinator for The Planetary Society.

Nature places some limits on the frequencies a civilization could use, if they are electromagnetic signals such as radio or light waves.  Very low frequencies are reflected by the thin ionized gas in the Milky Way galaxy and are also subject to interference from cosmic rays, which generate a lot of radio noise.  Light waves, which have higher frequencies, can be absorbed by dust clouds in space, although these are patchy, allowing us to see much of the Milky Way with our eyes.

All known forms of radiation decrease with distance because the energy spreads put over a greater and greater area, even if you sue a laser to beam the energy.  Any real laser will have a beam in the shape of a cone, which causes the energy to decrease as the inverse square of the distance, the same as with radio.  (In a laboratory, the laser beam can be made fantastically parallel, but over the huge distances of outer space, the beam spreads out in a cone.)

However, you are right that civilizations might use light to communicate, and several searches for extraterrestrial intelligence have looked for light or infrared signals from space, with no luck so far.


Response from the Director of The COSETI Observatory:

Tom has given a very good reply to the readers' question, and has been more honest than the SETI Institute in flagging the fact that although there are patches of dust clouds in space, it doesn't prevent us seeing most of the Milky Way galaxy.  In other words, the fact that dust clouds in M16 (7,000 light years from Sol), for example, would prevent visible laser communications through the dust, such dust clouds only block a very small part of the celestial sphere at that range and at further distances.  Besides, most Microwave SETI research is presently geared towards looking at nearby stars, so there is no problem with interstellar absorption at optical wavelengths in the range less than 100 light years or even significantly further.   Simply, you cannot base an argument against the optical approach to SETI on the occasional pockets of interstellar dust in deep space, at least, not today while most SETI searches are directed at nearby stars.  To use such an argument is just "clutching at straws" and is made out of desperation in place of more logical and scientific arguments.

While all electromagnetic beams spread out, being governed by diffraction theory, the divergence of laser beams from even 10-meter diffraction-limited telescopes is so much smaller that at radio frequencies, that the energy density at distances as great as ten thousand light years, can in general, be much greater than is possible at radio frequencies.  One could even take a considerable"hit" with interstellar absorption and still have a significant gain and signal-to-noise ratio advantage over microwave links.

One other comment.  In keeping with my definition of "optical", I do wish that writers would not speak of infrared as being distinct from light.   Talk about visible and infrared lasers or signals.  Today the terms "light" and "optical" are a superset of that part of the electromagnetic spectrum covering the far-infrared to the ultra-violet.  See the glossary for more details.

Stuart Kingsley
May 9, 1998

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