Extraterrestrial Civilizations - Isaac Asimov [134]
All objects that are not at absolute zero in temperature radiate photons over a wide range of energies. There are relatively few at either end of the range, and a peak somewhere in the middle. The peak represents photons of a certain energy, and as the temperature rises, the peak is located at higher and higher energies.
For very frigid objects near absolute zero, the peak radiation is far in the radio-wave region. For objects at room temperature, like ourselves, for instance, the peak is in the long-wave infrared. For cool stars, it is in the short-wave infrared, though enough photons of visible light are radiated to give the stars a red color. For Sunlike stars, the peak is in the visible-light region. For very hot stars, it is in the ultraviolet, although enough photons of visible light are produced to give the star a blue-white appearance.
Most of the range of electromagnetic radiation cannot penetrate our atmosphere, but visible light can, and most organisms have evolved sense organs that can respond to these photons. In short, we can see.
On Earth, at least, we have the aid of our other senses, but for any object beyond our atmosphere, the only information we have ever received (until very recently) is through the visible-light photons that have reached us from those objects.
It is natural, therefore, that we would think of signals from outer space in terms of visible light. We see the Martian “canals” and extraterrestrials watching Earth would see any markings we deliberately drew on the planetary surface, or the lights of our nighttime illumination.
Signaling by light represents a vast advance over signaling by neutrinos or gravitons. Light is easily produced and easily received. We can imagine some civilization setting up an exceedingly intense beam of light, and flicking it on and off in some way that would make it instantly recognizable as the product of intelligence. For instance, if we represent each flick as *, we might receive, over and over again, **—***—*****—*******—***********—*************—*****************— We would recognize that at once as the first members of the series of prime numbers and could not doubt that we were dealing with a signal of intelligent origin.
There are difficulties, though. A light beam intense enough to be seen at interstellar distances would require vast energies, and even then the light beam would be completely drowned out by the light of the star that the planet circles.
A Level-II civilization might conceivably know of ways to make a star bright and dim in such a way as to make a signal of undoubted intelligent origin, and a Level-III civilization might make a whole group of stars do so. This, however, is pure speculation. Nothing like it has ever been observed and it would certainly be unnecessary to make use of so heroic a signaling device if we can find something simpler.
For instance, what if the signal beam were a kind of light that was not produced in nature? This suggestion might have seemed silly prior to 1960, but in that year the laser was developed by the American physicist Theodore Harold Maiman (1927–), and within a year it was suggested as a possible carrier for interstellar messages.
All light produced in ordinary fashion is “incoherent.” It comes in a wide band of photon energies, and the different photons are generally heading different ways. A beam of such light quickly spreads out no matter how we try to focus it; and to keep it intense enough to be detectable at interstellar distances requires almost stellar energies.
In a laser, though, certain atoms are lifted to a high energy level and are allowed to lose this energy under conditions that produce “coherent” light—light that is composed of photons that are all of equal energies and are all moving in the same direction. A laser beam scarcely