The most common acoustical sense on Earth is 3-D sound perception. Such sound waves normally don’t carry much energy. For example, the pressure variations in a room due to people talking are only about 10^-6 atm. Yet our ears can detect waves with so little energy (10^-16 watts /cm²) that our eardrums only vibrate 10^-9 cm in response.⁷⁹ It is a fact that the primary human acoustical organ is extremely well-developed, able to distinguish at least 1600 discrete frequencies and to hear the quantum hiss of molecular motion of the air — the theoretical lower limit of sonic sensitivity.

A good sense of hearing is highly advantageous for a number of reasons. Much like scent signals, sound waves diffract around obstacles in total darkness — at night, in dark caves, deep underwater — or in blinding light to reach the recipient. And while pheromones are a less expensive way to transmit data over large distances, auditory signals are considerably more efficient than optical ones in biocommunication.

Unlike visual displays which require whole body motion or complex lighting patterns, or scent-talk which requires a separate glandular "voicebox" for each chemical letter in the odor alphabet, sound may be adequately generated by a single organ. A relatively simple system can produce wide variations in pitch, tone, intensity, wave shape, and timing. This means higher data flow between brain and terrain, significantly increasing the chances for survival.

Perhaps the only real disadvantage to audition is that the sonic alien must provide its own source, since natural sounds in the environment are rarely sufficient to permit a thorough surveillance of the surroundings. But sonic senses may be highly directional with fine resolution: Even humans, who have no echolocation system at all, can accurately separate distinct sound sources located only 10-20° apart.

Animals with built-in sonar fare much better.* Many can virtually "see" with sound. For instance, bats are small mammalian avians able to navigate at high speeds using ultrasonic echolocation. Although visually blind, these creatures easily avoid millimeter-wide wires strung across their path by investigators. Even when 0.3 mm wires were substituted the animals managed to avoid them more often than not. Only when tiny threads the thickness of human hair — about 0.07 mm — were used in the obstacle course were the bats unable to dodge them.^{219, 2514}

The upper limit for spatial resolution of targets is a function of the wavelength of sound. The tiniest object that can just be discerned has a size roughly equal to the wavelength. Smaller objects are too small to give an appreciable reflection and so remain invisible.

* Part of the difficulty is ear design — most of the energy in sound is reflected away at the surface of the structure. There are various solutions to this problem. Star Trek’s Mr. Spock’s ears are adaptations to the thin air of his home planet Vulcan. They permit greater directionality of sonic reception by utilizing a back-curving pinna ("pointed" ears). Another solution commonly found on Earth is the independently targetable ears of goats, cats, dogs, and others.

Table 13.1 Range of Hearing for Terrestrial Animals48,1698

Typically, the sonic beings of planet Earth use frequencies from 20-100 KHz or higher for echolocation (Table 13.1).

• This corresponds to a wavelength on the order of millimeters in normal dry air at a range of several meters.
• Much higher frequencies, say, 500-1000 KHz, would permit the resolution of targets 0.1 mm in size.
• This is quite enough to thread needles and soldier circuit boards at distances of about 10 centimeters from the creature’s face.

Of course, aliens using sound waves with that kind of resolution would take a great deal of energy to produce. They would be "decidedly dangerous to human explorers," according to science fiction writer Hal Clement:

A story could be built on the unfortunate consequences of the men who were mowed down by what they thought must be a death ray, when the welcoming committee was merely trying to take a good look.

But there is no fundamental reason why sentient extraterrestrials couldn’t use sound as their primary sense, and to build and create as well as any sighted being.

Audition has been developed as the primary sense modality among many aerial and land based animals on Earth. Creatures evolving on planets with unusually thick atmospheres with heavy refractive effects might tend to rely far more on hearing than on seeing. Also, there is much evidence that the sea is also a most auspicious environment for the development of sonic senses. It’s quite possible that hearing may even be the sense of choice for intelligent organisms residing in the murky oceanic media of other worlds. Why is this so?

Sound travels about four times faster in water than in normal air. This allows faster response times and greater ranges of communication and data collection. For example, at sonic frequencies of 500-1000 KHz, spatial resolution in ordinary seawater is equivalent to that in air at 100-200 KHz — but the range is about a hundred times greater.

Sound fares well against competing senses in the watery environment. Pheromones are relatively ineffectual in water, since diffusion in liquid media generally proceeds 10³-10⁴ times slower than in air. Smell signals are emitted, but creep slowly away from the source. Ocean currents are tamer than atmospheric ones, so pheromones would have to be about a million times more concentrated in water than in air to achieve comparable results.

Vision doesn’t compare much better. Light and other electromagnetic radiation cannot penetrate most liquids to any appreciable degree and are subject to countless distorting effects. In a tropical sea at high noon under very clear water, useful visual information can be gained only out to about 30 meters. Using acoustic channels instead, this range is extended to many kilometers — comparable to vision in air on a hazeless day.

These facts are reflected in the physiological differences between humans and dolphins. The eyeballs of a man receive an estimated 50 million bits of information each second, while his ears manage only 2 million bits/second. In porpoises the emphasis is exactly reversed: Dolphin sonar handles perhaps 40 million bits/second, while the eyes manage only 5 million bits/second of information.*²⁰¹We can easily imagine that sound may be the preferred sensory modality among many, if not most, intelligent aquatic races scattered throughout the universe.

There are many properties of sound that make it a totally unique window on reality. For instance, the ultrasonic world is very quiet in comparison to the normal range of human hearing. This is because of the relatively short range of ultrasound. There is little noise because sources remain localized.

In a dense fog, both sighted and sonic organisms are ill at ease: The former, because light is scattered away causing everything to appear visually white; the latter, because the droplets of moisture or particles are excellent ultrasound absorbers and cause the entire field of view to appear acoustically black.²⁵¹⁴

* Lilly’s figures are probably overoptimistic. Homer Jacobson and others have done a more careful analysis, and have concluded that the human eye is capable of transmitting only 4.3 million bits/second and the human ear only about 50,000 bits/second maximum.^{955, 979, 980}

Human vision is limited to the surfaces of objects, but sound penetrates and exposes the insides of targets to view. Objects may be scanned for composition and internal structure using almost distortion- and reflection-free sound waves. A pelagic sonic alien (perhaps modeled after the dolphin) thus views its fellows, not as a sharp contour of lines and edges and distinct boundaries, but rather more like an x-ray snapshot. Skin, muscles, and fatty tissues are virtually transparent to ultrasound. Bones and teeth, internally-trapped gas bubbles, and cartilaginous structures give good reflections. Hard parts, as well as the digestive and respiratory tracts, stand out in clear relief.

Might aliens with such sonic sight be more honest and less deceitful than humans generally? Physiological reactions to other individuals might be instantly perceived by those others, a kind of body-telepathy. At least one writer has remarked: "If your visceral reactions are obvious to everybody, you don’t waste much time trying to lie."²⁸⁵⁵

Dolphin echolocation may properly be compared to human vision. Just as people are able to see by the reflected white light of the sun, porpoises emit ultrasonic clicks and trills that illuminate the surroundings with white noise. Differences in texture and composition are as obvious to their sonic senses as to our visual ones. As Dr. Winthrop N. Kellogg points out; "Wood simply 'sounds different' from metal to a porpoise in the same way that it looks different to the human eye. It is the sound spectrum of the returning vibrations which gives the clue to the nature of the reflecting surfaces."¹⁶⁹⁸ Even more sophisticated, the dolphin has sound generators on either side of its head, which allows sonic depth perception¹⁷⁶ using binaural hearing — what Lilly has called "stereophonation."²⁰¹

Acoustically-oriented aliens will also have a rather clever use for color. The color of an object is just the frequency of radiation it emits, and we may expect that objects may take on various sonic colors. But there is more. Objects moving rapidly towards a sonic sensor will reflect sound at a higher frequency because of the Doppler effect. Likewise, objects in the field of view which are moving away will reflect lower frequencies.

If sonic beings divide their audible frequency spectrum into colors, then the sonic Doppler effect will cause approaching targets to appear "bluer" and retreating targets to appear "redder." As objects pass in front, their color alters noticeably depending on relative speed, angle of approach, and distance. If the medium is in motion, as with gusts of wind or surging water currents, the apparent color will flicker in frequency — a phenomenon quite alien to normal human visual experience.

The linguistic significance could be enormous. Much as the Eskimos have more than fourteen different ways to say "snow" (depending on its firmness, wetness, age, etc.), intelligent marine ETs will have words without analogue in our language. There might be terms describing approaching-red, receding-red, stationary-red, stereophonic-red, circulating-red, gulfstream-red, and dozens of other subtle distinctions we cannot begin to imagine. Or perhaps these sea-folk communicate in a manner which is possible only among creatures who use the same sense to see as to talk. Many researchers believe that the language of dolphins consists not of words as we understand them, but rather of a series of sonic images transferred into speech:

In this view a dolphin does not "say" a single word for shark, but rather transmits a set of clicks corresponding to the audio reflection spectrum it would obtain on irradiating a shark with sound waves in the dolphin’s sonar mode. The basic form of dolphin/dolphin communication in this view would be a sort of aural onomatopoeia, a drawing of audio frequency pictures — in this case, caricatures of a shark. We could well imagine the extension of such a language from concrete to abstract ideas, and by the use of a kind of audio rebus — both analogous to the development in Mesopotamia and Egypt of human written languages. It would also be possible, then, for dolphins to create extraordinary audio images out of their imaginations rather than their experience.²⁵⁵²

It is difficult to emphasize too strongly how unfamiliar the world may appear when viewed through other senses. Extraterrestrials will have experiences and capabilities that are difficult for humans to fully appreciate. A case in point in the perception of a peculiar sea phenomenon known to oceanographers as the sofar channel.

The speed of sound in water increases with pressure and decreases with temperature. Moving downward from the surface the temperature plummets, and about 100 meters down the speed of sound reaches a minimum value of 1480 m/sec. At greater depths the temperature remains fairly constant (close to freezing) but the pressure begins to build, causing the speed of sound to go back up. This region of minimum sound speed is called the sofar channel.

Sonic radiation, due to the physics of refraction, is actually attracted towards the channel. As with astronomical black holes (from which light waves cannot escape), acoustical signals sent from within the sofar channel likewise can’t get out. Transverse waves cannot cross it easily, which must cause it to appear sonically dark and foreboding when "viewed" from above or below.

If sufficient courage can be mustered, the channel has one very interesting property as regards long range marine communications. Sounds generated at the proper depth remain trapped in the zone, much like light in a strand of optical fiber. Messages may travel virtually unattenuated for literally thousands of kilometers in all directions. Sentient oceanic creatures of other worlds may regularly broadcast long-distance calls to other "universes" in the sea.

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" fishes^{2522, 2526, 2544} for social interaction to take place. So visual messages may be an inferior mode of communication between individuals unless there are overriding environmental 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

Table 13.2 Wavelength of Maximum Sensitivity to Visible Light

The "visible" range of light is actually slightly broader than the visual spectrum for human eyes. Our sight is normally limited to 4000-7000 Angstrom. Bees, however, are fully sensitive to ultraviolet radiation down to about 2500 Angstrom but they cannot see red. All animals have somewhat different reactions to visible stimuli, as shown in Table 13.2 at right.

Many have advanced the interesting notion that ETs evolving under the light of other suns will necessarily have eyesight which is most sensitive to the frequency of peak output of their home star. Hotter stars, under this scheme, would spawn creatures with bright-light accommodation and heightened sensitivity to blue light. Cooler stars would give rise to organisms more attuned to the red. (See especially Anderson,⁶³ Clarke,⁸¹ Clement,²⁹² Macvey,⁶¹ and Shklovskii and Sagan.²⁰)

Table 13.3 Electromagnetic Power Delivered to Planet, Orbiting in Earth-Normal Ecosphere, by Various Stars in Visible Wavelengths of Light

There are strong reasons to doubt the above hypothesis. Consider Table 13.3 at right, which shows the visible radiation from various stars incident on planets located near the center of the Earth-normal habitable zone. Note that among all stellar classes of xenobiological importance the shift in the visible power spectrum is not dramatic. While some small variation exists in the relative intensities of blue, green and red from star to star, the differences are decidedly underwhelming — hardly sufficient to represent a decisive evolutionary selective factor.

Furthermore, the peak sensitivity of terrestrial lifeforms is highly variable, ranging from 3600 Angstrom for bees (the equivalent emission peak of an A7 sun) up to 6500 Angstrom for seagulls (the equivalent emission peak of a K3 star).

And yet all of these creatures evolved under the same sun. It seems clear that eye sensitivity probably relates more closely to other environmental factors than the stellar class of the home sun. For example, it is often suggested that the wavelengths of light to which humans are most sensitive are virtually identical to the color of sunlight filtering down through a dense forest canopy of green vegetation and foliage, thus reflecting our arboreal origins. Aliens will answer in similar fashion to their own peculiar evolutionary heritages.

If sight is so important to living beings, then what kinds of photoreceptors might we expect to find on other worlds? Will ETs have eyes like ours, and if so, how many?

One of the most striking examples of parallel or "convergent" evolution in the terrestrial animal kingdom is the incredible similarity between the eyes of creatures with vastly different evolutionary histories. Animals in many separate phyla — Chordata (amphibians, fishes, reptiles and birds), Mollusca (octopus, squid), Annelida (the alciopid worms Torrea andVanadis²⁴⁸²), Coelenterata (the cubomedusan jellyfish²⁸¹⁶), and even Protista (the elaborate lensed eyes of dinoflagellates²⁸⁵⁶) — have independently evolved photoreceptors surprisingly similar in structure to the mammalian eye. There are some discrepancies; for instance, the photoreceptor cells in molluscs point towards the light, the opposite of vertebrates.¹⁶⁹⁷ Nevertheless, close comparison reveals that the adjustable lens, retina, pigments, focusing muscles, iris diaphragm, transparent cornea, and eyelids all are immediately recognizable. The ubiquity of the eyeball is perhaps the clearest indication that it’s simply the best design for the job. Similar evolutionary forces and physical laws should lead intelligent ETs down much the same path.

Of course, the "camera lens" eye used by mammals isn’t the only imaging system lifeforms can use (though it’s probably the best for larger organisms). The next most common — indeed more common if you just count species — is the compound eye of insects and crustaceans.²⁵³⁵ Each organ looks like a small multifaceted jewel, but is actually a bundle of optical tubes. Light is directed down each of these tubes onto a large matrix of individual photosensitive spots on the retina. The image appears as a composite mosaic of tiny light-bits. Dragonfly eyes, to take an example, have more than 28,000 facets each.⁷⁹ Motion may be discerned as far away as 12 meters.

Table 13.4 Resolving Power of Some Common Animal Eyes

Unfortunately, the compound eye has only very poor resolving power. (See Table 13.4.) It has been pointed out that an insect, poring over this page of print, would be quite unable to make out the individual letters.⁸¹ This is why larger creatures who need large amounts of accurate, well-resolved visual data will probably find the compound eye an unattractive alternative on any planet. Yet for smaller organisms it is ideal. Part of the reason for this is the laws of physical optics. If a flea had a spherical lens eye like that of humans, the pupil would be so minute that diffraction effects would make clear imaging impossible.⁹⁵⁸ Once again we see that the worlds of size are truly worlds apart.

Other image-forming techniques are of limited importance on Earth, but this is no guarantee that the emphasis on other planets will be the same. For instance, alien species may utilize the pinhole camera concept. Such a system uses a open optical pit without lenses that is exceptionally useful in water. The beauty of the pinhole eye lies in its simplicity, and it has been adopted by at least one group of animals on Earth: the chambered nautilus.⁵⁸⁴

Another curious mechanism is the scanning eye of the snail. The image formed by a simple crystalline lens is scanned by moving a single retinal nerve sensor over the field of vision. The entire picture is slowly reconstructed from a series of these scans. Scanning eyes aren’t particularly useful for spotting rapid movements, but may be of value in highly viscous environments.

The principle of the optical reflector telescope has never been directly employed to form images by terrestrial lifeforms. But it is clearly possible to do so. And while pinhole and compound eyes cannot gather light but merely serve to redirect it, both lenses and reflectors can.⁴³⁹

Some animals have developed biological reflectors for other purposes. The luminous squid has retractable reflectors which, when coupled with its bioluminescence, produce a kind of searchlight. This elaborate apparatus, constructed of lenses, concave mirrors, diaphragms and shutters, emits a beam of illumination with which it sweeps the vicinity in search of prey.¹⁰⁰⁰

The common house cat also makes use of curved reflectors. When light enters the feline eyeball not all of it is absorbed by the visual pigment in the retina. In most mammals this light is simply wasted. But the cat has evolved a mirrorlike coating on the inward-facing front side of its eye, called the tapetum. This specialized device reflects unused photons back into the retina for another try at absorption. Efficiency is increased and sensitivity heightened, although the image is blurred very slightly.

Another specialized adaptation is the split-pupil eyeball. This allows an organism swimming along the surface of a liquid body to enjoy bifocal vision.⁶¹The Four-eyed Fish, one of the largest tropical tooth carps (Anableps), actually has this remarkable feature. It is an elongate fish with eyes projecting upwards, each member of the pair divided into an upper part adapted for vision in air and a lower part adapted for vision in water. Any creature so equipped — alien or Earthly — can keep tabs on events above and below at the same time!

Another kind of visual perception is the sensation of polarized light. Many animals on Earth have this sense — insects, crustaceans, birds — although most mammals do not.*

Radiation coming from the sun is unpolarized. But when these rays are intercepted by a planetary atmosphere the plane of vibration is altered depending on the position of the star in the sky during the day. Honeybees, to take one example of many, monitor these shifting patterns of light and use this information to map out the shortest direct air route home. Sky polarization thus serves as a highly accurate navigational aid for these creatures.²⁵³⁰

Since sunlight reflected off a lake or ocean also is polarized, sentient avians of other worlds who must seek their prey by diving through a shiny water surface may be equipped with internal Polaroid filters. (Herons, kingfishers and other terrestrial birds have these.⁹⁶¹) Species inhabiting a "glassy" desert planet (with large expanses of volcanically or electrically fused surface sand) might also find this sense quite useful as sunlight bouncing off glass is polarized. And in constantly swirling irregular media, the detection of polarized light could help to maintain a proper orientation of "up" and "down."

We can imagine still more exotic schemes. Nature may have use for an eye with the ability to integrate light from a scene over a long period of time, much like photographic film. Such a system might be useful on a torpid, dimly-lit world (such as Venus) or in an environment populated by creatures with very slow response times. Since human rod cells are able to soak up dim light like a sponge for seconds at a time,⁸² there is no reason why aliens could not go us an order of magnitude or so better. This same ability could be used to design "wraparound eyes" for panoramic viewing.

What about eyes on stalks? Despite the attractiveness of this idea among devotees of the outré, both omnidirectional viewing (as found in some insects and terrestrial fishes) and independently swiveling eyes (like the chameleon) are generally seen by most xenobiologists as rather unlikely adaptations for thinking animals. It is said that eyes on mobile stalks are a feature more of animals likely to be preyed upon, and that "no superior intelligence could evolve in circumstances in which it lived in constant fear of being struck down and eaten."⁵⁰

Bonnie Dalzell has produced three additional arguments against eye stalks for intelligent ETs that are very persuasive:

1. Eye stalks require an hydraulic support mechanism, which is very inefficient except in small animals.
2. Eye stalks are too dangerous. Eyes are often the most vital sense, yet a predator could just clip them off with the stroke of a claw or pincer. And such organs are more prone to normal injury — in an accident, stalks could be bumped, slammed, or squashed rather easily.
3. Stalks, in fact all independently-targetable eyes, present severe problems for the brain in making the correct parallactic computations necessary for binocular vision.⁴⁰⁰ (However, the chameleon seems to triangulate on his food quite easily.⁴⁷⁴)

* Humans can see polarized light, but just barely. In natural skylight "Haidinger’s Brushes" appear as a small yellow and blue Maltese cross in the center of the visual field, the blue segment oriented parallel to the plane of polarization. The phenomenon is so close to the borderline of perception, so faint and unreliable, that it cannot serve a navigation function for man.²⁵³¹

How many eyes are best? It’s clear that nature usually chooses the cheapest way to do a given job. Certainly for senses that are not highly directional, it would seem that a single receptor organ should suffice. This is perhaps why most larger organisms have but one organ of smell and one of taste.

On the other hand, senses that are highly directional can make good use of the benefits of stereo. The ability to accurately triangulate a source depends on the simultaneous operation of at least two physically separated receptors. One organ of sight or sound gives 2-D resolution; a second organ, by making use of binocular or binaural sensing, gives three-dimensional coverage.

But there appears to be little to gain from using more than two sensors.

A single pair is sufficient to ensure spatial resolution with depth perception — a quantum leap over single organs but not much poorer than three or more. Notes one xenologist: "Three eyes represent not nearly the same improvement over two that two represent over one."²⁰

Since the Principle of Economy tells us that nature invariably chooses the cheaper of several ways to do the same thing, this may partly explain why we have exactly two eyes and two ears. The advantages of having more than this number are slight or nonexistent, a conclusion bolstered by the apparent convergent evolution of stereoscopic vision among mammals, birds, and other animal groups on this planet.²⁵²⁰ Still, there is plenty of biological precedent for alternatives.

Cyclopean organisms are common in the microscopic world, but these are mere eyespots, useless for imaging or any discrimination between all but fuzzy patches of light. Scorpions and water fleas have a pair of compound eyes set so close together that they appear almost as one — but these are not truly monocular.

It is possible that monocular vision might suffice even for large alien creatures. For example, a single eyeball that vibrated slowly from side to side could provide limited depth perception. Nearer objects would seem to move faster across the field of view than more distant ones, giving clues to their relative positions in space. (The human eyeball constantly quivers to prevent "visual accommodation," but the effect is too small to permit stereoscopy.)*

Very few animals have more than two imaging eyes, because third eyes don’t tell the being anything it didn’t already know. Reptiles (and man) have an ancient third eye called the pineal gland. In today’s reptiles it is only a day/night sensor and has no imaging capability whatsoever,²⁵²¹ and in humans it is fully degenerate. But such may not always have been the case. The skull of Cynognathus, an extinct Triassic Period theriodont reptile, definitely shows three eyes. This animal was mammal-like, possibly warm-blooded and probably hairy.⁶⁰⁰

Nereis, the common sandworm or clamworm, has four eyes. The Horseshoe Crab (Xiphosura) is also tetraocular, though two of the four are degenerate. Most insects are pentaocular, with three small eyespots called ocelli located on the upper part of the head between the two compound eyes. Once again, how ever, there are only two image-forming eyes.⁹⁶⁵

In a few cases, several batteries of eye-pairs are used during the successive phases of the hunt for food. Dr. Norman J. Berrill describes the dinnertime antics of the spider, which has four pairs of eyes:

The rear pair serve to watch behind for either food or danger. The other three pairs work together but in succession. If something comes within the range of vision of one of the outermost pair, the head turns until the object is brought into the field of the two pairs of eyes in the middle, and the spider then advances. When the object is brought into focus of the forward pair, the spider jumps to attack. The whole business is much like a self-operated mechanism with seeing instruments, and the eight eyes together do not compare with the camera or the compound eye of a bird or a bee.⁸⁹

The ultimate limit is probably reached by the scallop, whose literally hundreds of tiny, beautifully constructed "eyes" are spread around the circumference of its mantle like running lights on an ocean liner. These sense organs are very limited in function, however, as they cannot image. They serve only to initiate an automatic escape reaction at the approach of a hungry starfish (the scallop’s natural predator) heading in from any direction.

Specific environments can be imagined in which more or less than a single pair of eyes might be selectively advantageous. But the cost in added neurological equipment will usually be prohibitive. From the many examples above we know that more or less than two eyes have seldom proven more adaptive than exactly two.

Large sentient aliens, if they see by visible light, will most likely be binocular.

* It should be pointed out that the possession of two eyes does not guarantee stereoscopic vision. The rabbit and the woodcock, for instance, have eyes located on opposite sides of the head — which provides almost no binocular field.⁸²

Table 13.5 Peak Emission Wavelength of Radiation from Various Stars

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.
• 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

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.

Table 13.6 Radiative Power Output at the Surface of Sol

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.

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?