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


13.5.2  Infrared Vision

The second range of electromagnetic radiation generally considered useful for vision is the near-infrared. While about 45% of Sol’s energy arrives as visible light, and 10% as UV, most of the remaining 45% comes in as infrared of various wavelengths. But seeing in the IR has another great advantage which is independent of atmospheric conditions. Every material body in the universe, provided its temperature is above absolute zero, emits a continuous spectrum of "black body" radiation (Figure 13.1). Simply because they have some heat, all objects "shine" in the infrared. So to infrared eyes, all the world illuminates itself.


Figure 13.1 Peak of Blackbody Radiation at Various Temperatures2524

WAVELENGTH OF RADIATION emitted by heated bodies varies with the absolute temperature. The wavelength of the most intense emission is given by Wein's displacement law, which states that for a thermal radiator the product of the peak wavelength and the absolute temperature is a constant. In the units used here the constant is about 3000 micron-degrees. Thus the sun, whose temperature is about 5700 K, has a peak emission near 0.5 micron. The eye responds only to the narrow band of wavelength between 0.4 micron (violet) and 0.7 micron (red). The arrows along the bottom of the diagram indicate the center of narrow infrared "windows" where the atmosphere allows the passage of 50 percent or more of the incident radiation. There are poor windows near 35, 350 and 450 microns that can be exploited by observatories in dry locations. The atmosphere becomes essentially transparent to incoming radiation at about 800 microns. Astronomers use photographic emulsions and photomultipliers out to one micron in the near-infrared. Infrared techniques are used between one micron and a few thousand microns, where they overlap with radio techniques.


The rattlesnake is remarkably good at sensing heat waves. This creature has four eyes -- two imaging eyeballs operating in the visible, and two conical pits on either side of the head which house its binocular infrared cameras. Each one measures 2 cm2 in area and is packed with 150,000 receptors sensitive to the near-IR from 15,000-150,000 Angstrom.

To this reptile, the world of heat is as important as the visible world. The typical rattler can sense a mere 0.005 watt/cm2, equivalent to the infrared radiated by 1 cm2 of human skin.2525 Temperature differences of only 0.002°C can be seen by the snake. This allows it to hunt mammalian prey even in darkness, and to penetrate protective coloration ruses and camouflage ploys during the daytime.

What about visual acuity in the IR? Would the accuracy of sight be much impaired? Arthur C. Clarke has suggested that infrared eyeballs would give "coarse and fuzzy pictures, for the images they produced could not be sharply focused." Furthermore, since "a typical heat wave is about a hundred times longer than a wave of visible light, infrared eyes with vision as sharp as ours would have to be a hundred times larger." Clarke then conjures visions of monstrous two-meter-wide alien eyeballs which, he then concludes almost casually, "would certainly be inconvenient!"81

This result is extremely misleading. Extraterrestrial infrared sensors need not be grotesque at all. For example, let us consider an intelligent ET peering at a crowd of Homo sapiens through infrared eyeballs. The black body radiation emitted by a warm human body peaks at about 93,000 Angstrom. How large must the sensor be?

At the stated wavelength, the aperture* of the alien eyeball only needs to be 3.9 cm to enable it to resolve one minute of arc -- about as good as a man’s eye. This size is comparable to the diameter of the eye of the horse (5.0 cm) and the Indian elephant (4.1 cm), and is nowhere near as large as the eyeball of the largest cephalopods (up to 37 cm) or even the largest mammal, the blue whale (14.5 cm).1697 So infrared eyes have to be just a few times larger than our own to achieve comparable optical resolution.

An extraterrestrial stargazer with IR eyeballs would not see what we see, however. Most of the familiar constellations would be gone because most of the brightest stars radiate only faintly in the infrared. New constellations would appear, comprised of stars visible to humans only with telescopes in the visible range but which burn brightly in the IR.

Beings capable of seeing down into the infrared may have divided the spectrum into heat-colors. The coldest objects would appear "red," the hottest "blue." This is easy for us to conceptualize, even though we are incapable of making such fine distinctions of temperature and have no absolute sense of temperature. That is, the perception of a given temperature is variable, depending upon whether our skin was previously hotter or colder.

But there are creatures indigenous to Earth that do have an absolute temperature sense. Many insect, birds, fishes and rodents possess the ability in a limited way. Fish have been trained to respond to a specific temperature -- say, 14 °C -- regardless of whether they had previously been kept in warm or cold water.2543 Honeybees are known to have an exact thermoregulatory mechanism by which they maintain the hive at a constant temperature. And one large bird -- the Australian bush turkey or "incubator bird" -- can keep its nest within 0.1 °C of precisely 33°C.2542

With this kind of absolute heat sense, an intelligent alien brain might interpret an entire rainbow of color in the heat spectrum. Since electrical permittivity varies widely in the infrared from substance to substance, "seeing" the chemical composition of the surroundings should not be too difficult for these beings.1703 With both visible and IR vision, like the rattlesnake, complicated visual messages and intricate works of interactive thermal art would be possible.

Heat, of course, is a persistent phenomenon. The footprints of a barefoot person across a cold floor or the jet blast on an airport runway well after departure remain visible in the infrared long after the source of these traces has departed. Like olfaction, thermosensitivity produces a world of echoes. The past gradually melds into the present as the traces of hot objects that passed by earlier begin to dissipate. Not only would such a clear view of the immediate local past be extremely useful survival-oriented information, but ETs with this sense would perceive reality and the flow of time in ways we can scarcely imagine.


* Calculated using usual approximation to Rayleigh’s criterion for optical resolvability of two point sources in air (diffraction pattern overlap): qR = 70l/A, where qR is resolution in degrees, l is wavelength and A is eye aperture both in meters.


Last updated on 6 December 2008