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
13.5.2Infrared 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.
