24.2.2 - Electromagnetic Waves and Frequency Selection

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

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

Based on the historical development of human communications technology, it is probably fair to say that electromagnetic radiation represents the most primitive technique for signaling across interstellar distances. Advanced cultures may possess particulate, neutrinic, gravitic or tachyonic communication channels, or they may have knowledge of only a few of these, but it is difficult to imagine any technical society in possession of any of such sophisticated technologies without at least having an awareness of photonic communication techniques. Electromagnetic waves are probably the easiest information markers physically to generate, and they are ubiquitous throughout the cosmos. Photons are most likely the most primitive communication technology available for interstellar discourse.

If this is true, which photons are the best to use? The electromagnetic communication band spans a useful frequency range of at least 16 orders of magnitude -- gamma rays, x-rays, ultraviolet, visible light, infrared, radio and so forth. Unless we can identify a preferential region of the spectrum, our search for intelligent alien signals will be frustrated by the enormous number of possibilities.

The most logical place to begin is with the question of energy efficiency. Following Drake’s Principle of Economy, we should expect ETs to select those photons which transmit the greatest number of bits per second for the least cost in joules of transmission energy. Unfortunately, both bit rate and photon energy are proportional to frequency, so when the two are divided the frequency dependence drops out. Hence, the criterion of energy efficiency is incapable of distinguishing between photons of different frequencies.

If we look back at Oliver’s criteria (Section 24.2), we see that there are really only two of them which are useful in making a choice between photons: Noise and absorption. Signals transmitted over channels which are too noisy are not detected. Messages transmitted through a medium which absorbs them do not reach the receiver.

Consider the graph in Figure 24.2. The vertical axis represents the total electromagnetic flux density in watts/meter², assuming a detector which is looking at approximately 8% of the entire celestial sphere (1 square radian of sky, or "steradian"). Noise from the most important sky sources are plotted over the various frequency ranges in which they occur. Coverage stops at 10⁷ Hz because electromagnetic waves with frequencies less than this are heavily absorbed by planetary ionospheres, and in any case, waves below 10⁶ Hz are absorbed by the interstellar medium.

Figure 24.2 Integrated Flux Density of Background Radiation Likely to Obscure Interstellar Electromagnetic Communications³¹²⁹

What conclusions may we draw from the data? First, it appears that the noisiest part of the electromagnetic spectrum is the region from 10¹¹ Hz up to about 10¹⁶ Hz. Any signals sent by photons within this range must compete with starlight, the 3 K cosmic background, and a variety of other emissions and absorptions. Although interstellar communications seem least likely in this portion of the spectrum, a few xenologists have suggested making use of the laser’s ability to produce highly directional, extremely monochromatic beams. If the laser is tuned to emit in a stellar absorption band (a narrow frequency band where the sun is about an order of magnitude darker than normal), distant alien observers would observe an artificial spectral line winking on and off in a clearly intelligent pattern.¹⁰³⁹

A better choice is the part of the spectrum which lies above 10¹⁶ Hz. X-rays and gamma rays may be useful in interstellar communications because they are not absorbed by the interstellar medium. (See Elliot,³¹⁴⁴ Fabian,³¹³⁷ and Kuiper and Morris.²⁶⁰⁸) But attempting to search the entire range from 10¹⁶-10²³ Hz is hardly going to be easy. If we could check a 1 MHz band for ET signals every 10 seconds, the search time to cover the entire high frequency region would require a period of time on the order of the age of the universe. Somehow the possibilities must be narrowed. One way to do this is to look at "magic frequencies" derived from universal physical constants or defined by well-known physical phenomena.

The possibilities are endless.²⁶⁰⁸ For example, the Compton Wavelength* of the electron is 2.420 x 10^-12 meter, corresponding to a frequency of 1.239 x 10²⁰ Hz. This falls almost exactly into a noise minimum between hard x-rays and gamma rays on the flux density curve on the preceding page. The Compton Wavelengths of the proton and neutron, respectively, are 1.32134 x 10^-15 meter and 1.31952 x 10^-15 meter, which define a "narrow" 10²⁰ Hz band between the frequencies 2.26885 x 10²³ Hz and 2.27198 x 10²³ Hz. This band conveniently lies in another local noise minimum on the flux density curve. Since it is defined by the two major particles from which all stable matter is constructed, this "baryon gap" may be the preferred region for interstellar communications using high-energy photons. Yet another approach -- one which fills in the third local noise minimum between soft and hard x-rays -- is somewhat biochauvinistic. It assumes that ETs will transmit signals between favored spectral lines of atoms or ions that are biologically important to them. For instance, silicon-based sentients may transmit between Si- Ka1 and Ka2 lines, located at 4.20736 x 10¹⁷ Hz and 4.20591 x 10¹⁷ Hz, defining sharply what might be called the "silicon hole" with a width of 1.45 x 10¹⁴ Hz in which signals might be detected. Similarly, we might define a "sulfur hole" between 5.58048 x 10¹⁷ Hz (Ka1 line) and 5.57758 x 10¹⁷ Hz (Ka2 line); a "chlorine hole" between 6.34108 x 10¹⁷ Hz (Ka1 line) and 6.33718 x 10¹⁷ Hz (Ka2 line) for chlorine-breathers; a "germanium hole" from 2.9464 x 10¹⁷ Hz to 2.9428 x 10¹⁷ Hz for germanium xenobionts; and so forth.

It is clear, however, that on the basis of noise/absorption criteria alone the low frequency end of the spectrum (below 10¹¹ Hz) should be optimal for long-distance interstellar communications. The weight of xenological opinion today is that radio frequencies are the preferred mode of photonic information transmission between the stars.^57,22

But exactly which radio frequencies are best? The graph in Figure 24.3 represents an expanded view of the quietest portion of the electromagnetic spectrum. The vertical axis is no longer energy density. Rather, intensity is expressed as sky brightness (blackbody) temperature which is what radio-astronomers actually measure.

There are three fundamental sources of noise associated with all highly sensitive radio receivers. First there is "galactic noise," caused by synchrotron radiation arising from free electrons orbiting magnetic field lines in space. From the graph we see that this noise rises steeply below 1 GHz, depending very slightly upon the galactic latitude toward which we point our receiver. Second, there is "thermal noise," caused by the 3 K cosmic back ground, the relict radiation from the Big Bang. Third, there is "quantum noise" (spontaneous emission or shot noise), representing a fundamental quantum mechanical limitation on receiver sensitivity.

Above 1 GHz galactic noise falls below the isotropic cosmic background, and beyond about 60 GHz quantum noise exceeds the cosmic background and in creases indefinitely with frequency. (It is the dominant form of noise at optical wavelengths.) Thus from any point in interstellar space the sky is likely to be quietest from about 1-60 GHz. This is the microwave window out in free space.

Now look at the graph in Figure 24.4 on the Terrestrial Microwave Window. Atmospheric absorption must be added for receivers located on a planetary surface under a sea of air. If signals from the stars arrive at frequencies above 10 GHz, they will be strongly absorbed by water vapor molecules, oxygen molecules, and many other molecules not shown. These substances are likely to be present in the air of any terrestrial world that resembles Earth even remotely. We see that the Terrestrial Microwave Window is closed virtually for all radio frequencies save those few between 1-10 GHz. The great majority of xenologists agree that this is the range where alien electromagnetic signals most profitably may be sought.

There is considerably less consensus on exactly where to search within the Window. Philip Morrison and Guiseppi Cocconi first suggested that the search should be made at or near the natural emission peak of neutral inter stellar hydrogen gas (the most abundant element in the universe). According to these early pioneers in SETI, the preferred frequency was 1.42 GHz:

As the hydrogen line itself is rather noisy, a few scientists responded that searches ought to be made at integral multiples of 1.42 GHz.¹⁰⁵⁴

Since 1959, the science of radioastronomy has made tremendous advances. It is now known that there are many other elements and molecules with emission lines within the Window. The hydroxyl (OH) radical has emission lines at 1.612, 1.665, 1.667, and 1.720 GHz. Spectral lines of molecular species are also quite popular in the speculative literature. For instance, some have proposed listening in at 4.83 GHz -- the natural formaldehyde emission line -- because it is comparatively less noisy than many other natural lines.³¹²²

In the early 1970’s, interest turned to what is commonly called the "water hole." Much as with the x-ray bands described above, the water hole is the band of radio wave frequencies lying between the H and OH emission lines. Readers familiar with chemistry will recognize that H plus OH equals water, the basic solvent for all life as we know it on Earth. Dr. Bernard Oliver, who originated this idea in connection with his work on Project Cyclops (see below) in 1971, explains the rationale for the water hole in a particularly poetic fashion:

During the mid- and late-1970’s there has been an outpouring of new ideas and proposals for preferred frequencies in SETI, so the water hole concept today has a great deal of competition. Drake and Sagan suggest using an "average value" of the H and OH natural emission lines, obtaining a kind of "molecular center of mass" frequency of 1.65 GHz as the favored interstellar channel.³¹²⁸ A related proposal is that in space, where the Free Space Microwave Window allows greater leeway, we should search the water line itself at 22 GHz²⁸⁶⁵ (or perhaps the ammonia line at 24 GHz, if we are looking for ammonia-based beings¹⁵).

We could adopt the "magic" frequency of 56 GHz, the point at which the blackbody 3 K background "thermal noise" curve intersects the "quantum noise" curve.²² Argue Drake and Sagan: "The 56 GHz channel has the provocative property of being determined simultaneously by quantum mechanics and cosmology."³¹²⁸ Using another combination of basic physical constants, Kuiper and Morris have derived a "magic" frequency of 2.56 GHz.²⁶⁰⁸ Then there is the intriguing suggestion of Soviet SETI researcher P.V. Makoveskii of the Leningrad Institute of Aviation Instrument Manufacture, that the most probable frequencies for interstellar radio traffic will be the natural hydrogen line frequency alternatively multiplied and divided by such constants as p, 2p, and SQRT(2).³²⁶¹ This identifies several "uniquely artificial" frequencies, including 0.23, 0.45, 1.0, 2.0, 4.5, and 9.0 GHz.

However reasonable they may seem, each of the above proposals rests on a plausibility argument whose conclusions perhaps are suggested but certainly are not compelled by the basic facts and assumptions of xenology. A few scientists have attempted to predict the optimum interstellar signaling frequency based solely upon fundamental physical laws and conditions expected to apply to all communicative civilizations in the Galaxy.

Using his Principle of Economy, Dr. Frank Drake points out that the best radio frequency is the one in which transmission power is minimized -- that is, where noise is lowest. This is customarily described in terms of the "brightness temperature" of the sky -- the temperature a black body would have to have in order to duplicate in brightness the observed radio radiation coming from a given spot in the sky. Following Drake, we write the noise temperature as a function of celestial right ascension a and declination d (the astronomers’ way of specifying sky position) as T(a,d), This temperature is not constant for all radio waves, but varies as a function of frequency n. Typically the variation follows a "power law" -- frequency raised to some variable exponent g -- of the form n^-g. Since g is also a function of sky position, we shall write it as g(a,d). Both T(a,d) and g(a,d) can and have been measured very precisely by terrestrial radioastronomers for every point in the sky.

Finally, Drake derives the following equation for no the frequency of maximum economy (of maximum communication range):

What does all this mean in plain English? Simply this: For each position in the astronomers’ sky there exists a unique frequency of minimum noise and maximum economy. Whatever direction you point your radiotelescope, range will be greatest if the radio frequency determined by the above equation is used. Best of all, the numbers that must be plugged into Drake’s formula are already known, so n₀ theoretically may be computed today for any star system in the heavens with whom we may wish to enter into communication. Drake calculates that the range of frequencies of maximum economy span the Terrestrial Microwave Window from 3.75 GHz out to about 10 GHz.

* The Compton Wavelength is the change in wavelength corresponding to a loss of energy suffered by a photon whenever it collides with matter.