Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm


13.5.3 Radio Vision

In so many ways the visible portion of the electromagnetic spectrum seems ideal for use by living beings. The atmosphere conveniently allows these wavelengths to pass, and most photosensitive chemical substances respond well in this region. Photons of visible light are energetic enough to excite our senses, yet not so energetic as to damage our tissues. Wavelength is small enough to permit the resolution of extremely small objects without distortion.

As if this was not enough, we find that our sun emits its maximum power smack in the center of the visible range. Our sight spans just those frequencies in which the greatest power is available for illumination -- a most auspicious arrangement. Let’s take a closer look at this.

The peak wavelengths in the power spectra of all classes of stars are shown in Table 13.5. Note that all stars of xenobiological interest have peak emissions well within the visible range. So choosing a star other than Sol won’t alter our conclusions -- alien worlds will be impinged with quite similar wavelengths of bright visible light, though the mixture of colors may vary slightly.

The fact that F0 through K5 stars peak in the human-visible optical window argues strongly for the evolution of visual, rather than radio, eyesight. It has also often been asserted that radio eyes would be difficult if not impossible from a bioevolutionary point of view. To achieve the same resolution as the human eye, a radio eyeball using 1-meter radio waves (300 MHz) would have to be several kilometers in diameter.1338 If 1-centimeter waves (30 GHz) are used, the radio eye need only be about 40 meters wide, but Carl Sagan’s cryptic remark is still appropriate: "This seems awkward."20


Table 13.5 Peak Emission Wavelength of Radiation from Various Stars
Photosphere Temperature (K) Approximate
Amax (Ang)
Visual Color
of Maximum Wavelength


No organisms on Earth are known regularly to use radar as a functional part of their normal sensorium. Of course, we cannot exclude such a possibility elsewhere on this basis alone. Electrical senses are quite well-developed on this planet, and there’s no reason why alien creatures could not learn to manipulate kilocycle and megacycle signals with ease. Recordings have been made of electric fish sputtering along at 1600 Hz for brief periods,2516 and experiments conducted by Clyde E. Ingalls, Associate Professor of Electrical Engineering at Cornell University, have demonstrated the ability of some humans to actually "hear" radar waves beamed at the head.79

And there really is no need at all for a single, localized viewing organ. To demand such is to become an "eyeball chauvinist." For instance, the entire body of a moderate-sized macromorph could be used for this purpose, much as arrays of electric field sensors are embedded in the outer skin of sharks and the electric ray.

The central, most critical test is to find a good reason to evolve a radar sense. Certainly it is not an easy matter, and the rewards appear marginal because of the low radiative power of most natural sources of radio waves. In other words, if we hope to find radar beings elsewhere in the Galaxy, we must think up some excellent reasons why nature should go to so much trouble.

The power output of Sol in the visible is compared with its radio emissions in Table 13.6. The radio window, while far broader than the visible, has more than ten orders of magnitude less energy available for sight! With hotter class-F stars the disparity is even greater.


Table 13.6 Radiative Power Output at the Surface of Sol
----- Visible Window -----  ----- Radio Window -----
Power Output
Wavelength Power Output
l (A)
1 cm
2 10-12
10 cm
9 10-16
1 m
1 10-18
10 m
4 10-22


If radio is to be competitive with sight as a primary sensory modality, the levels of environmental illumination should at the very least be roughly equivalent in magnitude. This guesstimate is probably a trifle optimistic since the information-carrying capacity (bit rate) of radio is about a million times lower than for visible light -- but we’ll stick with it anyhow.

Where might we find the proper conditions? The planet Venus has a surface temperature of about 750 K. The hot rocks emit blackbody radiation in the radio at roughly the same intensity as in the visible. This would give radar eyes a fighting chance, except that we still have the enormous influx of visible light from Sol to contend with. It appears that if radio is to win, the visible window somehow must be closed.

It was once believed that the thick cloud cover over Venus would preclude the illumination of the planet’s surface by Sol’s rays. But on-site measurements made by the Russian space probes Venera 9 and Venera 10 have shown that such is not the case.2386 Sunlight penetrates the Cytherian gloom rather easily, providing light equivalent to Earth’s on a rainy, overcast day.

It’s doubtful whether cloud covers very much more opaque can feasibly be designed. Unless we resort to such planetological oddities as chlorine or sulfur atmospheres to blot out the light, vision in the visible range must still be preferred over radio in any reasonable solar system.

If we cannot easily close the visible window, only one alternative remains: We must turn out the lights!

Probably the best environment for the evolution of radio-sensitive creatures would be the surface of starless, self-heating planets. Ensconced in the cold, dark blanket of the interstellar void, these worlds might sustain life and intelligence. Radar beasts with 40-meter-wide sensors may well be found on subjovian, jovian, or superjovian planets in the dead of space. No sun shines; all illumination must percolate up from below, and most of that is in the radio.

What might extraterrestrial radio eyes see on such a world?

From orbit, the night sky would appear suffused with a faint radio glow. Normal stars -- as we know them -- would all be gone. All constellations would vanish. In their places would be found a few brightly flaring pulsars and quasars, visible to the naked radio eye because of its greater sensitivity. Tiny creases and splashes of multicolored light would mark the titanic explosions in distant radio galaxies. The Milky Way itself would cut a bold, wide circular swath around the field of view, a glorious aurora-like ribbon of brilliance punctuated by a giant "radio moon": The Core of the Galaxy. Probably visible even from the ground as a spot of light about the size of a dime held at arm’s length, it would inspire the awe and curiosity of the inhabitants of the starless world below.

On the surface of the planet most of the astronomical universe visible from the orbital platform would be blotted out by ground and sky glare. Since the ground is the source of all energy, it will appear the brightest. The atmosphere, in close equilibrium with the ground temperature but slightly cooler, will be brighter but appear as a "colder" (redder) color. A radio snapshot would look surprisingly like a photographic negative of a scene on Earth: A brightly gleaming world nestled beneath a soft hazy blanket of swirling, cloud-pocked, multicolored, dully-glowing sky.

On Earth there are an estimated 100 lightning flashes each second worldwide. On a radio world, the atmosphere would be constantly flickering with "fireflies" dancing in the distance. Radio waves generated by electrical discharges any where on the planet would spread over great distances. The horizon would literally sparkle with light all around.

Water and many other liquid surfaces would appear shiny and bright, but outcroppings of dry, solid rock must look dark and foreboding. If there are any clouds hanging over the planet, they probably won’t be seen because they are generally transparent to radio waves. Radar sensing can be used to peer deep inside non-metallic objects heated nonuniformly1338 (such as a living body). "Radio blue" might be defined as the radio color coming from the hottest parts of such objects, whereas "radio red" would be perceived from the coolest regions. How can we comprehend what it means to simultaneously view all parts of a solid body, heated from within or with layered surfaces, which appears "red" on the outside, "green" on the inside, and "blue" at the center? This is three-dimensional see-through color vision with a vengeance!

If radio-sensitive extraterrestrials have developed some form of space travel technology, they might tend to beware the blinding radio brilliance of stars. Should they happen to approach Sol, our sun would not appear to be the well-behaved, steadily-shining object we know it to be. The total radio flux can wander over as much as five orders of magnitude during periods of intense solar activity (during which the variations in optical brightness rarely exceed 1%).1339

Enormous storms lasting many days would be observed near sunspots, releasing powerful blasts of radio energy resembling bombs bursting. A typical outburst raging across the shimmering surface of Sol would last tens of minutes, but the star might suddenly flare up unexpectedly, climbing several orders of magnitude in brightness in a matter of seconds. To radio-sensitive aliens, all stars must appear to be quite dangerous places indeed -- variable, inhospitable, random, violent, and very uninviting in comparison to the tranquil, stately quiescence of the home planet.

But suppose for a moment that some foolhardy adventurer ventures close to our solar system in search of life. The terrestrial worlds would be only faintly visible, so the alien astronaut would first be drawn to the gas giants. The jovians emit vivid and colorful flashes "as if an enormous electrical storm were raging over its entire surface."49

With proper shielding and access to powerful telescopes, however, the alien might journey to Earth at last. On our world, there would be three sources of radio-light by which our strange visitor could see: (1) Radio emissions from our sun (a kind of flickering daylight to the ET); (2) black body emissions (given off by all warm bodies -- air, ground, human bystanders, etc.); and (3) artificial sources.

It is the last of these which is most likely to cause an early breakdown in interstellar relations. Earth-based radiotelescope transmitters, TV and radio broadcasting antennae, and BMEWS defense radars would appear as hot and bright as the unshielded surface of the sun to the alien’s eyes. When our military radar first scans the incoming spacecraft, it may be interpreted as an act of war. A beamed radio "welcome" signal would fare no better.

After all, would we take kindly to a megawatt optical laser beam trained on our vessel as we came in for a landing on another world?


Last updated on 6 December 2008