24.0 - Interstellar Communication Techniques

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

"I know perfectly well that at this moment the whole universe is listening to us -- and that every word we say echoes to the remotest star."
-- from The Madwoman of Chaillot, by Jean Giraudoux (1882-1944)

"The reader may seek to consign these speculations wholly to the domain of science fiction. We submit, rather, that ... if signals are present the means of detecting them is now at hand. The probability of success is difficult to estimate; but if we never search, the chance of success is zero."
-- Giuseppe Cocconi, Philip Morrison; "Searching for Interstellar Communications" (1959)¹⁰³³

"And who, in time, knows whither we may vent
The treasure of our tongue, to what strange shores
This gain of our best glory shall be sent,
T' enrich unknowingly nations with our stores?
What worlds may come refin’d with th’accents that are ours?
-- from Musophilus, by Samuel Daniel (1562-1619)

"The Pioneer plaques are destined to be the longest-lived works of mankind. They will survive virtually unchanged for hundreds of millions, perhaps billions, of years in space. When plate tectonics has completely rearranged the continents, when all the present landforms on the earth have been ground down, when civilization has been profoundly transformed and when human beings may have evolved into some other kind of organism, these plaques will still exist. They will show that in the year we called 1973 there were organisms, portrayed on the plaques, that cared enough about their place in the hierarchy of all intelligent beings to share knowledge about themselves with others."
-- Carl Sagan, Frank Drake (1975)³¹⁴³

"So deep is the conviction that there must be life out there beyond the dark, one thinks that if they are more advanced than ourselves they may come across space at any moment, perhaps in our generation. Later, contemplating the ¥ of time, one wonders if perchance their messages came long ago, hurtling into the swamp muck of the steaming coal forests, the bright projectile clambered over by hissing rep tiles, and the delicate instruments running mindlessly down with no report."
-- from The Immense Journey, by Loren Eiseley (1957)²¹¹⁰

Most xenologists today earnestly believe that millions upon millions of inhabited worlds pepper the Disk of the Milky Way. Innumerable intelligent extraterrestrial races may await contact and communication with humanity. How might scientists -- human or alien -- best bring this about?

More than a century ago the great mathematician Karl Friedrich Gauss set forth the startling suggestion that the only way to communicate with sentient beings on other worlds was by means of a "mathematical language." Gauss’ idea for contacting the inhabitants of Mars was to lay out the figure of a giant right-angle triangle in the middle of the Siberian forests. His plan called for 15-kilometer-wide strips of forest to delineate the lines and golden fields of wheat to fill the interior of the symbol. The geometric solution to the Pythagorean theorem, Gauss believed, would communicate the fact of terrestrial intelligence to any Martian astronomers who happened to be eyeing our Earth. (One writer has estimated that the scheme would have involved more than 4 million acres of forest and nearly 13 million acres of wheat, a total area roughly the size of the entire country of Ireland.⁴⁵)

In Vienna, the astronomer Joseph Johann Littrow, a pioneer in spectrography, is reported to have proposed that great canals be dug in the Sahara desert in the shape of a variety of geometrical figures -- circles, squares, triangles, and so forth. Each would measure perhaps 30 kilometers on a side and would be filled with water. For signaling purposes, kerosene was to be spread over the surface of the water and ignited. The flaming symbols, it was expected, would alert observant Martians to our presence on this planet. Assuming the ditches were 1 kilometer wide and the layer of petroleum many centimeters deep to permit a burn of several hours’ duration, roughly five million cubic meters of kerosene would be required each night (at a cost of $400 million at current world prices).

Another magnificently implausible project was put forth by Charles Cros, a Frenchman, in 1869.³¹⁰¹ Cros apparently spent much of his life trying to persuade the French government to help him construct a giant mirror to send messages to Mars. The device was planned to have a focal length equal to the Earth-Mars separation, thus permitting Cros to focus concentrated sunbeams onto the Martian desert. This would melt the sand, and Cros could then carve various figures and numbers on the surface. Another technique using mirrors involved the construction of a large checkerboard of shiny surfaces which could be covered and uncovered in sequence to describe patterns and shapes (a kind of slow-motion semaphore). Yet another scheme was advanced by one Schmoll, who wanted to establish mirror works at Bordeaux, Cherbourg, Marseille, Stockholm, Amsterdam, Copenhagen, and upon the shores of the Gulf of Bothnia to give the appearance of the Big Bear constellation (Ursa Major) as seen from space.*

It is hard for us today to fathom how anyone could have taken any of these proposals seriously. But a century ago the state of xenological know ledge was extremely poor. Little was known of the actual conditions on the surface of Mars or any other planet in our solar system; the size and extent of the Milky Way Galaxy was unknown and undreamed of; extrasolar worlds were believed extremely rare; and radio waves had yet to be discovered. Gauss and Littrow were forced to speculate within the bounds of their limited knowledge, and arrived at conclusions which seemed moderately plausible at the time. It is sobering to consider that hundreds of years from now the people of the future may regard our own current proposals with equal astonishment and disbelief.

Xenologists today believe that there are two fundamental avenues of contact that are feasible across interstellar distances: Probes (or artifacts) and signals. As one writer puts it:

Which is the superior mode of communication? That is, are probes or signals better? The answer to this seemingly innocuous question is not at all obvious. Both talk and travel are commonplace on Earth, and we cannot use as our guide any current human technology (which elsewhere in the Galaxy may be found in a variety of states of retarded or advanced development). Xenologists most properly must seek to address the question in a manner independent of the vagaries of human technological history. Judgment criteria must be selected which spring from those basic and nigh universal laws of existence which all sentient races and alien cultures share in common.

Certainly the most basic goal of life and intelligence -- the accumulation of information and complexity -- involves judicious energy-flow management. Evolution via natural selection generally favors those living systems which manage their limited resources most efficiently. In other words, of two organisms, one of which is less wasteful than the other, the more frugal but equally effective creature has a higher probability of being favorably selected for survival.

On this basis, astronomer Frank Drake has proposed what he calls the Principle of Economy. This is the notion that economy is practiced universally or nearly so, or at least represents the ideal. Drake argues that technological civilizations everywhere in the Galaxy will attempt to choose those alternatives which are least expensive:

It is quite possible that some races may not be subject (or as subject) to the same competitive rules of natural selection as are we -- recall, for example, Sneath’s "soil creatures" mentioned in Chapter 6. Still others may choose a shorter-lived, more profligate lifestyle than ordinary thrift would dictate. But general living systems theorists agree that Drake’s principle is applicable virtually at all levels of living systems from cells to societies, since it appears to be a manifestation of the well-known principle of least effort.³⁰⁷¹

If xenologists accept Drake’s Principle of Economy, then it follows that a technical communicative civilization will choose those means of communication which cost the least to do the job. The "job," of course, is the transfer of information and complexity across interstellar distances. The quantitative units of information are bits. (See discussion, Chapter 14.) Assuming that all data must be carried on markers of matter-energy, the units of "cost" are joules. Our criterion for judging whether signals or probes are superior thus reduces to the simple query: Which mode of communication maximizes bits/joule transmitted per unit time interval?

Unfortunately, to answer this question we need to do a little physics. Readers who are not mathematically inclined may skip the next three paragraphs.

Consider signaling by photons (radio, microwave, visible, ultraviolet, x-rays). According to Shannon’s classic theory,³¹⁸⁶ the rate of information transmission through a channel of frequency bandwidth nmax is n_max log₂(1+S/N) bits/second, where S/N is the signal-to-noise power ratio. This rate is a theoretical maximum when bandwidth equals carrier frequency; that is, when n_max = n. From quantum physics we know that the energy per carrier photon is equal to hn joules, where h is Planck’s constant. Hence we calculate that the maximum theoretical photonic information transmission efficiency e_g is:

Consider next the possibility of signaling with masses. Any self-contained material system of mass m is subject to a limitation in the maximum energy it can utilize as a marker for carrying information. That limit is the mass-energy of the matter, or mc² where c is the speed of light. If we use energy levels as markers, then Heisenberg’s Uncertainty Principle defines a minimum measurement accuracy for these markers: DE Dt > h, where DE is the uncertainty in energy and Dt the duration of the measurement. According to Bremermann,³⁰⁷² the optimal use of a given amount of mass-energy available occurs when (mc² / DE) markers with values between zero and DE are used (which permits the representation of mc² / DE bits). This means that no material mechanism of mass m using energy levels as markers can measure more than mc²Dt / h bits during the time interval Dt. Hence, the maximum rate of information transmission is mc² / h bits/second. From special relativity we know that the total energy (mass-energy plus kinetic energy) of any material body is mc² / (1-v²/c²)^½, where v is the velocity of the mass relative to a stationary frame of reference. Hence we calculate that the maximum theoretical information transmission efficiency for matter e_m is:

Maximum photon efficiency e_g and maximum matter efficiency e_m for the transmission of information are compared on the graph in Figure 24.1. Photon efficiency is represented by the vertical line at the right, which shows different efficiencies depending upon the signal-to-noise ratio achieved. (Note that the effect of S/N on efficiency is negligible at the upper end.) The matter efficiency function curves downward to the right, and lies below the photonic efficiency for all S/N > 1. Strictly speaking, then, photons would appear always to give the most bits/joule.

Figure 24.1 Comparison of the Theoretical Maximum Information Transmission Efficiency of Photonic and Matter Markers

But the truly amazing feature of the two curves is their unexpected convergence. If any reasonable velocity and signal-to-noise ratio are chosen for comparison, the difference in transmission efficiency is only an order of magnitude or two. To advanced Type II and Type III technical civilizations equipped with virtually perfect photon- and matter-handling technologies, the choice of communication mode probably will depend far less upon energy economy than on other considerations (see below). With few if any techno logical limitations on their activities, stellar and galactic cultures most likely will view signals and starprobes as energetically indistinguishable alternatives for interstellar communication.

Type I societies are another matter, however. Lacking advanced technological expertise, planetary civilizations will be forced to rely upon the most primitive, simpleminded, easy-to-construct communications devices imaginable. Across interstellar distances radio waves, which require nothing more complicated than a small metal wire-mesh dish and a few volts of electricity, seem to be the best bet for technological dullards.

Xenologists may draw two specific conclusions from the above discussion. First, since interstellar travel on a large scale is virtually impossible for any Type I civilization,³⁰⁷⁸ transmission of signals is probably the method of choice for such technologically-limited planetary cultures. Second, since starprobes and signals are energetically equivalent exercises for technically proficient Type II and Type III civilizations, both probes and signals probably will be utilized in interstellar communications depending on the particular purposes and needs of the societies seeking interaction.

* A modern version of the Gauss-Littrow proposals has been suggested by Kh. Geshanov, a Bulgarian, who recommends turning all the world’s radio stations on and off in careful synchrony, providing a regular "blinking" effect in Earth’s artificial radio emissions.¹³³¹ In March of 1964, two Russian science fiction writers put forth the idea that the Great Siberian Meteorite of 1908 was caused, not by a fallen object, but rather by an attempt at communication [a powerful laser beam) by the planets orbiting the star 61 Cygni.³²⁶⁷