Extraterrestrial Civilizations - Isaac Asimov [133]
We are now left only with the subatomic particles without mass, and there are only three known general classes of such particles:* neutrinos, gravitons, and photons.
Being massless, all these particles travel at the speed of light and there can be no faster messengers. That is one point in their favor.
Moreover, no massless particle carries an electric charge, so none is affected by electromagnetic fields. They are affected by gravitational fields, but detectably so only in regions where such fields are very intense. Even there, beams of massless particles would bend in unison and would not be dispersed. Since the intensity of the gravitational field in space is negligible almost everywhere, all massless particles reach us in essentially a straight line and essentially un-dispersed and undistorted, even though their origins are billions of light-years away. That is a second point in their favor.
In the case of neutrinos, however, reception is extremely difficult, since neutrinos scarcely interact with matter at all. A stream of neutrinos could pass through many light-years of solid lead without more than a small fraction of them having been absorbed.
To be sure, a very small fraction can be absorbed even in relatively small samples of matter, and so many neutrinos can very easily be produced that such a very small fraction might suffice to carry a message.
However, the type of nuclear reactions that go on in the interior of stars produces neutrinos. In a Sunlike star, vast numbers of neutrinos are produced in this fashion.* A civilization is not likely to produce more than an insignificant fraction of the neutrinos their own star will be producing, so that there will be the danger that whatever message the civilization sends out will be swamped by the much greater volume of neutrinos the star is emitting. (It is a general rule, perhaps, that the medium you use for your message should be easily distinguished from the background. You don’t whisper a message across a room in a boiler factory.)
There is a possible way out of this. While the fusion reactions involving hydrogen nuclei at the center of the stars produce neutrinos, the fission reactions involving the breakup of massive nuclei such as those of uranium and thorium produce related particles called antineutrinos.
Antineutrinos are also massless and chargeless but are, so to speak, mirror images of neutrinos. When absorbed by matter, antineutrinos produce different results than neutrinos do, and if a civilization is careful to allow a stream of antineutrinos to be the message carrier, it could be read even in the presence of a vast flood of neutrinos.
Nevertheless, the difficulty of intercepting such particles is such that no civilization would use this method if something better were available.
Gravitons, which are the particles of the gravitational field, are certainly not better. Gravitons carry so minute a quantity of energy that they are even more difficult to detect than neutrinos. What’s more, they are far more difficult to produce than neutrinos. To produce even barely detectable gravitational radiation, using the technology currently at our disposal, huge masses must be made to accelerate—through rotation, revolution, pulsation, collapse, and so on—in some pattern that will serve as a code. We can fantasize a civilization so advanced that it can make a giant star pulse in Morse code, but even that advanced a civilization wouldn’t bother if something simpler were available.
That leaves the last category of communications systems—photons.
PHOTONS
All electromagnetic radiation is made up of photons, and these come in a wide variety of energies,* from the extremely energetic photons of the shortest-wave gamma rays to the extremely unenergetic longest-wave radio waves. If we consider any band of radiation in which energy doubles as we pass from one