Extraterrestrial Civilizations (39 page)

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 group that arrives together left together. For a signal to be of any use, it has to come in a straight line and be neither dispersed nor distorted, and that eliminates all subatomic particles with mass.

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.

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 end of the band to the other (or the wavelength doubles in the other direction) then that is one octave. There are scores of octaves making up the full stretch of electromagnetic radiation, and visible light makes up a single octave somewhere in the middle.

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 spreads out at all, so that for a given energy it can remain intense enough to be detected at far greater distances than a beam of ordinary light. What’s more, a beam of laser light can be easily identified spectroscopically, and merely through its existence is satisfactory indication of intelligent origin.

With laser light we come closer to a practical signaling device than anything yet mentioned, but even a laser signal originating from some planet would, at great distances, be drowned out by the general light of the star the planet circles.

One possibility that has been suggested is this—

The spectra of Suntype stars have numerous dark lines representing missing photons—photons that have been preferentially absorbed by specific atoms in the stars’ atmospheres. Suppose a planetary civilization sends out a strong laser beam at the precise energy level of one of the more prominent dark lines of the star’s spectrum. That would brighten that dark line.

If we studied the spectrum of a star and discovered that it was missing one of the dark lines characteristic of a certain group of atoms in the star’s atmosphere, but that other dark lines also characteristic of that group were present, we would have to conclude that the missing energy level had been supplied by artificial means. That would mean the presence of a civilization.

Nothing like that has been observed—but before feeling depressed over that, let us see if perchance there are still simpler ways of signaling. After all, no civilization would be expected to use the harder method when a simpler is available.

Early in the nineteenth century, electromagnetic radiation outside the range of visible light was first discovered. In 1800, William
Herschel discovered the infrared range of sunlight by the manner in which a thermometer was affected beyond the red limit of the range of visible light. In 1801, the German physicist Johann Wilhelm Ritter (1776–1810) discovered the ultraviolet range of sunlight by the manner in which chemical reactions were brought about beyond the violet limit of the range of visible light.

These discoveries did not affect astronomy very much, however. Most of the range of ultraviolet and infrared could not penetrate the atmosphere, so that little of it reached us from the Sun and the stars.

Beginning in 1864, Maxwell (who had worked out the kinetic theory of gases) developed the theory of electromagnetism. This first identified light as an electromagnetic radiation and predicted the existence of many octaves of such radiation on either side of the visible light range.

In 1888, the German physicist Heinrich Rudolf Hertz (1857–1894) detected lightlike radiation with wavelengths a million times longer than light and with energy levels that were, therefore, only a millionth as high. The new radiation came to be spoken of as radio waves.

Radio waves,
because
of their low energy content, turned out to be easy to produce, and
despite
their low energy content, easy to receive. Radio waves could penetrate all sorts of material objects as light could not. Radio waves could bounce off layers of charged particles in the upper atmosphere as light could not, so that radio waves could, in effect, follow the curve of Earth’s surface. Radio waves could easily be produced in coherent fashion, so that a tight beam could go long distances, and could easily be modified to carry messages.

For all these reasons radio waves were clearly ideal for longrange communication, and that, too, without the wires that telegraphs and cables required. The first to make practical use of radio waves in this way was the Italian electrical engineer Guglielmo Marconi (1874–1937). In 1901, he sent a radio-wave signal across the Atlantic Ocean, a feat generally recognized as the invention of radio.

From that day on, with further improvements and refinements, radio became a more and more important means of communication. It was clear to many people that any technological civilization would surely make use of radio communication in preference to anything else.

Therefore, when the planet Mars made a closer than usual
approach to Earth in 1924, there was some attempt to listen for radio signals from the presumed civilization that had built its canals. Nothing was detected.

In a way that was not surprising. The layers of charged atoms in the upper atmosphere that reflected Earth-made radio waves and kept them in the neighborhood of the surface instead of allowing them to pass outward into space would also serve to reflect spacemade radio waves and keep them away from Earth’s surface.

In 1931, however, the American radio engineer Karl Guthe Jansky (1905–1950), working for Bell Telephone Laboratories, detected an odd signal when he was trying to determine the source of static that interfered with the developing technique of radio telephony. It turned out that the signal was coming from the sky. That was the first indication that there was a wide band of short-wave radio waves, called microwaves, that could easily penetrate Earth’s atmosphere. There were two types of electromagnetic radiations that we could get from the sky: a narrow band of visible light and a broad band of microwaves.

By December 1932, it was demonstrated that Jansky had detected radio waves from the Galactic center, and that made front-page headlines in the
New York Times
. Some astronomers, such as Jesse Leonard Greenstein (1909–) and Fred Lawrence Whipple (1906–), at once appreciated the potentialities of the discovery, but there was little that could be done about it. There were no decent instruments for detecting such radiation. One American radio engineer, Grote Reber (1911–), did take it seriously, however. He built a device to detect radio waves from the sky (a “radio telescope”) and from his back yard, beginning in 1938, studied as much of the sky as he could reach in order to measure the intensity of radio-wave reception from different areas.