Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization
© 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
Virtually all higher lifeforms on planet Earth have some optical sensing capability, testimony to the tremendous advantage in being able to see. Light is most familiar to us, but there are also many other forms of electromagnetic radiation. Likewise subsumed under "vision" must be eyes that see by gamma rays, x-rays or ultraviolet rays in the higher frequencies, and by infrared (heat), microwave or radio waves at the lower end of the spectrum.
There are many advantages to sight. While all but radio frequencies cannot diffract around obstacles or turn corners, vision provides the greatest accuracy, highest directionality, and the finest resolution of any sense available. Since photons travel faster than any material pulse -- as with olfactory or auditory signals -- vision permits virtually instantaneous response times. And light can carry far more bits/second than any other stimulus.980
One possible disadvantage of vision is that it requires elaborate whole body motions,2534 body color changes,2506 or complex biochemical mechanisms such as are found in bioluminescent insects and "flashlight" fishes2522,2526,2544 for social interaction to take place. So visual messages may be an inferior mode of communication between individuals unless there are overriding environ mental factors at work -- such as an unusually thin atmosphere which transmits sound poorly.
But this difficulty is more than offset when we consider vision as a means of sensing the surroundings generally. The sighted animal has a tremendous advantage enjoyed by no other: An external source of illumination.
It has been suggested in earlier chapters that most lifeforms will probably evolve on planets circling other stars. If this is a valid assumption for the majority of extraterrestrial races in the Galaxy, it follows that most of these environments will also be reasonably well-lighted. While osmic alien must emit their own pheromones and auditory beasts must radiate homemade sonar pulses if they desire high resolution, light falls from the sky like sensory manna from heaven. Photons bounce off everything and thus collect information which is free for the taking by any organism equipped with eyeballs.
Naturally, there are a few restrictions. Despite the fact that stars emit radiation at all frequencies in varying intensities, planetary atmospheres tend to absorb a great deal of it. Depending on the composition of the air, its pressure and a hundred other factors, there will exist one or more "windows" through which environmental illumination may pour.
As a rule, gamma rays and x-rays are absorbed by energy-level jumping in the atoms of air. It’s true that snails are known to be especially sensitive to x-ray’s,551 and Arthur C. Clarke has speculated on the possibility of an x-ray sense,81 the fact is that such high energy photons will probably be unable to penetrate any planetary atmosphere of reasonable density.
Ultraviolet (UV) is absorbed too, although to a lesser extent. In a thin or unshielded (e.g., ozone-free) atmosphere like that of Mars, ultraviolet rays might reach the surface and become useful for vision. It is a surprising fact that the human retina is quite sensitive to UV down to at least 3300 Angstrom.2528 This soft-ultraviolet radiation normally never reaches the retina because it is filtered out by the lens. A few persons have undergone lens replacement operations to remove cataracts, and since the artificial lens passes UV the full potential of the retina is finally realized. Such people can see a "color" that the rest of us cannot!* Unfortunately, on most planetary surfaces any scene viewed in ultraviolet light would probably be quite dark.
Near-infrared (IR) radiation is partly removed by the vibrations in molecules of water, carbon dioxide, methane, ammonia and a variety of other atmospheric constituents, and far-IR is absorbed by molecular rotational transitions common in the air of all planets. Still, some infrared does get through and may become useful for seeing.
We are left with "visible" light, some near-infrared, and some radio frequencies to which normal atmospheres are transparent. Thus there are three main bands of electromagnetic radiation which may profitably be exploited for vision on the surface of any world: The visible, the infrared, and the radio.
* During World War II, senior citizens who had undergone lens replacement operations were used by the Office of Strategic Services to pinpoint the flashing UV signals from agents stationed on enemy coasts. Secrecy was maintained be cause these messages were completely invisible to anyone else.2529
Last updated on 6 December 2008