Extraterrestrial Civilizations - Isaac Asimov [132]
Freeman J. Dyson suggested that if human beings began to exploit and explore space, they might wish to expand their numbers to the utmost that can be sustained by the Sun’s energy. At the present moment, the Earth stops only a tiny fraction of sunlight, and almost all the solar radiational energy slips past the cool bodies of the Solar system to streak into and through interstellar space. Human beings might therefore eventually break up the various outer bodies of the Solar system to make up a group of free-worlds that will be placed in a spherical shell about the Sun at the distance of the inner edge of the asteroid belt.
All the Sun’s energy would be absorbed and utilized by one or another of the free-worlds. The energy would, of course, be reradiated into space from the dark side of each of the free-worlds, but only as infrared radiation. Viewed from another star, then, the Sun’s radiation would seem to change its character from one in which a major portion was emitted as visible light to one in which almost all was emitted as infrared. The changeover would take perhaps a couple of centuries, the barest instant of astronomical time.
If, then, from our own Earth we should see some other star, which has been shining steadily as far as our records tell us, suddenly begin to lose brightness and after a while blink out, we can be reasonably sure we have seen intelligence at work.
Well, perhaps—but we haven’t seen anything of the sort as yet.
We must come to the conclusion, then, that (1) we are hopelessly inept at detecting signals and might as well not bother; or that (2) no signals are being sent out and that we might as well not bother; or that (3) signals are being sent out but at much less than heroic energy content, and as a result of much less than heroic civilizational activity, and that in order to detect them we will have to make a considerable effort.
Clearly, we cannot accept the first or second conclusions until we have made an honest attempt at the third.
Then let us consider signals of low-energy content (but high-energy enough to detect) and see what they might be like.
They would have to consist of some phenomenon that could cross vast reaches of space, and these can be divided into three classes: (1) large objects such as plaques, probes, and free-worlds; (2) subatomic particles with mass; (3) subatomic particles without mass.
The large objects we can eliminate at once. They move slowly and are extremely inefficient as carriers of information.
The subatomic particles with mass can be divided into two subclasses, those without electric charge and those with electric charge. Subatomic particles with mass but without electric charge generally move slowly and can be eliminated as impractical for that reason.
Subatomic particles with both mass and electric charge can move quickly because they are accelerated by the electromagnetic fields associated with stars and with galaxies as a whole. Therefore, in crossing interstellar and intergalactic spaces, they achieve very nearly the speed of light and, in consequence, enormous energies.
Such subatomic particles do indeed occur everywhere and they are constantly and eternally bombarding the Earth. We call them cosmic rays.
The difficulty here, though, is that the mere fact that these particles are accelerated by electromagnetic fields means that they experience an attraction or a repulsion and that, in either case, their paths curve. As the particles gain increasing energy, their paths curve more and more slightly, but over vast distances even the slightest curve becomes important. What’s more, a beam of particles is gradually dispersed, since those with more energy are curved less than those with less energy.
The cosmic-ray particles bombard us from all sides, but because of their past experiences with electromagnetic fields, there is no way of telling from the direction of their arrival where they came from. Nor can we tell whether a particular