Transmission: The Other Side of SETI

Transmission: The Other Side of SETI

David F Mayer, Advanced Computer Consultants, Columbus, OH

Abstract: The thesis of this paper is that the best way to answer the question of how to search for extraterrestrial intelligence is to look at SETI from the perspective of the civilization which is attempting to TRANSMIT to another. It is concluded that the visible band presents the most viable medium of contact, since it offers both the greatest bandwidth and the most narrow focusing, permitting the most information to be transmitted to potential targets at the lowest cost. The essential problem of defining the meaning of a message to an unknown civilization is solved by the concept of the SELF-DECODING MESSAGE. The problem of the selection of potential targets is discussed and criteria for optimal choice are given. Finally, the essential question of the PRUDENCE of such a transmission program is presented and discussed.

0. Introduction, Overall Strategy: Reverse the Perspective

In order to search for extraterrestrial intelligence, it is essential to look at the problem from the point of view of the transmitting society. Transmission is clearly a bigger and more expensive challenge than receiving a signal. Examining the problem from the side of the transmitting civilization reveals the characteristics of the signal which we should expect to receive, and thereby shows us how and where to look. Therefore, the rest of the paper, until the last section, proceeds under the following hypothesis.

Hypothesis: The governments of the world have authorized a budget of $10,000,000,000 per year on an ongoing basis to establish a link with some extra-terrestrial civilization. Some form of message is to be constructed and transmitted to likely targets in nearby regions of the Galaxy. I have been put in complete charge of the project. How should I proceed? This is the question which this paper tries to answer, and in doing so, attempts to delineate how prospective recipients of such communications should maximize their chances of receiving them. In other words, only by putting ourselves in the position of someone transmitting an inter-stellar message can we be prepared to receive one.

It is a standard of the news business to ask the questions: "Who, what, where, when, why and how?" In addition, to these, I add, "whether or not we should transmit at all". There are certain non-trivial risks associated with such transmission. I propose that the analysis of these questions is the best way to examine the SETI issue

Who should be the target of our transmission?

What should we transmit?

Where should we aim our signals?

When is the best time to do it? How often?

Why is this to be done?

How is the signal to be created, modulated and aimed?

Whether or not transmission is prudent?

The scenario assumes that we on Earth are to transmit. The immediate criticism is "What grounds do we have for believing that the civilizations that we assume are transmitting resemble us?" My answer: None at all! However, ours is the only technical civilization that we know of, so I use it as an example by default. Any time an arbitrary assumption is required, I will base this on the characteristics of our own Earth and its civilization. The reader is free to make up his/her own assumptions if she finds these disagreeable. But, the basic principle of 'turning the tables' on the transmitting civilization is, I believe, crucial to success of SETI

Another assumption I make here is that the transmitting civilization and the receiving civilization both have limited budgets. That does not mean necessarily small budgets, but it assumed that any civilization has finite means. (I exclude any science-fictional super-civilizations that can harness the entire power output of a star.) Therefore, the most efficient method must be used by both. Another assumption is that the transmitting civilization will assume that the recipient will be roughly like itself in terms of resources.

Finally, I want to say that I believe that the combination of very high bandwidth and very short wavelength makes the visible spectrum the band of choice. Microwaves are a poor second best. The ability of a visible light source to carry vast quantities of information in a narrow beam makes the use of the visible spectrum far more efficient than any longer wavelength.

1. Who should be the target of our transmission?

We are seeking a target species that has:

· A fundamental curiosity similar to humans. A civilization lacking in basic curiosity would never bother to look for our signals.

· Vision roughly comparable to ours. It is unlikely that any creature has a visual band very far outside of ours for this reason: An oxygen atmosphere (assumed to permit multicellular animal life) would generate an ozone layer which would greatly reduce the penetration of the atmosphere by ultra-violet light. Furthermore, ultra-violet light breaks many molecular bonds. Light very far below the red end of the spectrum in energy would not have sufficient energy per photon to cause photochemical changes.

· A planet with sufficiently clear atmosphere to make astronomy feasible. This speaks for itself.

· A culture which promotes scientific research by both professionals and amateurs. If nobody is looking at the sky, our signals will never be detected.

· Cooperation between professional and amateur astronomers. Because amateurs have so many more telescopes and scan so much of the sky, they are likely to be the first to detect our signals.

Also, our target civilization must have developed the following technologies:

· Amateur telescopes of aperture at least 100 millimeters, with high quality precision optics throughout. These are needed to have sufficient gain to detect our signals.

· Professional telescopes of aperture at least two meters and of very high quality. These are needed in order to obtain sufficient gain to receive our signals clearly so that they may be interpreted.

· Electronic image detection. Without this technology, it is doubtful if our signals could be received in sufficient detail to be interpreted.

· Electronic computers. These are needed to do the signal processing required for interpretation.

· Interpretation of ancient written languages of its predecessors. It would be much easier for a civilization with a scholarly tradition of linguistics and language interpretation to understand our signals.

Obviously, our target must be some civilization having the technology to seek, detect, receive and interpret our signals. More specifically, within such a civilization, there are many potential targets. I would like to suggest that there are at least two separate target groups we should have in mind:

Amateur astronomers

and

Professional astronomers

I make this distinction because there is a big difference between these two processes

seeking and detecting a signal

and

receiving and interpreting a signal.

I need to explain how I am using these terms:

seek actively search for a signal

detect find a signal that is neither background noise nor natural

receive demodulate a signal into a bit stream

interpret decode a signal into its intended meaning

It is assumed (by default) that the intended recipient civilization has roughly the same number of professional and amateur observatories as exists on Earth. The sum total of area scanned at high resolution by professional telescopes per day is a very small fraction of the entire celestial sphere. (At least that is true on Earth.) Therefore, it simply will not be possible to depend upon the signal being detected by professional astronomers in our target civilization. We would need to depend on a fortunate coincidence that our solar system is being observed by one of a few large telescopes while our signals are arriving, and that is unlikely. We need to increase the odds.

The hundreds of thousands of amateur astronomers on Earth scan a vastly larger fraction of the sky than do the professionals. There are three reasons for this: (1) many more amateur telescopes than professional telescopes exist; (2) the dwell time of the typical amateur on an object is usually much less than the dwell time of a professional (who is usually getting spectra, not shooting photographs) so the amateur looks at many more objects per hour; (3) the professional telescope is seldom used by the eye but more often by light collecting instruments that are insensitive to periodic signals. For these reasons, amateur astronomers who usually detect new comets and other sporadic phenomena in the skies.

Even if a few large 'full sky' scanning telescopes are built, the total light collecting area of these can not compare to the total light collecting area of hundreds of thousands of modest telescopes. A 1,000-mm professional wide scan camera has only 64 times the collecting area of a 125-mm amateur telescope. To maximize the chance of detection by the target civilization, it will be more effective to try to make a signal detectable by amateur astronomers.

By hypothesis, we do not know whether a potential target stellar system actually contains a technical civilization. Therefore, we must cover as many potential target stellar systems to have a reasonable chance of getting at least one hit. We also want to make significant contact, not just say "Hello there!" In order to send significant amounts of information, we need to transmit at very high bit rates so that we can alternate transmission of our complete message among many potential target systems. Making the plausible assumption that the eyes of our intended recipients will have cycle times of not less than 1/100 second, they would not be able to see a high capacity signal as anything but a dot of light, which could easily be misinterpreted as a background star despite its "pure" (laser generated) color. It is therefore necessary to interrupt our signal transmission with periodic breaks of length on the order of 1 second in groups of at least three or four. Presumably, a light source that flashed on and off three seconds out of every minute would attract attention. Then, hopefully, the amateur who detected the signal but was unable to process it further, would contact the professional who could make a more detailed study with a much more powerful telescope and much more sophisticated instrumentation and a much larger staff and budget.

There is another requirement for making our signal visible to amateurs and that is high power at the receiving end. Clearly, the amateur telescope will likely have sensitivity far less than the professional. I therefore propose that the smallest practical transmitter would have a signal strength resulting in a light level permitting detection by the human eye aided by the types of telescope in common use.

I am assuming the aperture of the (perfect, lossless) amateur telescope to be 100 to 200 millimeters. This permits us to calculate the power required for transmission when the distance to the target civilization and the angular radius of the signal beam are both known. The power required clearly depends upon how far we wish to transmit our message.

The last item is quite important. If we suddenly were to receive a message from an extra-terrestrial source, who would be the first group that we would contact? Clearly, experts in understanding ancient texts, since they have done this kind of thing before.

OSETI Conclusion 1: We should expect a signal devised for detection by amateur astronomers.

2. What should we transmit?

This is the opposite problem of cryptography. Cryptography is concerned with preventing the receiver from interpreting a message. We must create a signal that is easily interpreted by anyone who receives it.

For hints in how this should be done, we should consult not cryptographers, but archeologists, in particular those who interpret forgotten languages, and language teachers, who teach languages. Whatever characteristics enhance the interpretation of lost languages should be included in our messages. The single most important trait which facilitates understanding of forgotten languages is the occurrence of pictograms. [1] After all, from nothing you can get only nothing. The target civilization of our messages must have some hints, some frame of reference, to interpret our signals.

There is a price in redundancy for this ease of interpretation. We shall need to send far more data bits than we would if we were sending a signal to a recipient who had some pre-arranged compression algorithm. We can not expect our target civilization to be able to decompress any information unless we include a "Rosetta Stone" in uncompressed form elsewhere in our message.

There are two constituents of a message: Form and Content. (Transmission technology is discussed elsewhere) Form comes first. To accomplish this, I propose the self-describing image. This image is designed to be as easy to interpret as possible. In particular, it includes repetitive frames which impose a regular order on the message and force proper alignment. These next three examples show the same data displayed with different line widths. Clearly, examples 1 and 2 are misaligned, while example 3 is correctly aligned. Forcing alignment in this way makes the interpretation of the pictures, text symbols and accompanying sound track easy.

The following shows a crude oversimplification of the format of message we should expect to receive.

Example 1

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If the reader received the message above, she would clearly interpret it as being a picture of some creature whose name or description was written in the symbols below. The zig-zag line suggests a sound track, and that is exactly how I intended it to be interpreted. The key to both marking a signal as the product of intelligent life and facilitating its speedy interpretation is regularity.

Generalizing this technique to real pictures of some realistic size, say 1024 * 800, and replacing the letters by pixel values from 0 to 255, we could easily encode a picture. Following it with a sound track, we would have an effective "slide show" to carry our message to our audience.

Now that we know how to send pictures and sounds in an unambiguous manner, we must consider the content of our message. The content consists of two parts: Definitions of primitive terms and the use of these primitive terms to convey information.

To define our primitive terms, we must utilize the method by which we all learned to talk and read: By watching and listening to someone. Therefore, I propose a sort of cosmic cross between Dick and Jane and Sesame Street, with perhaps some Captain Kangaroo thrown in.

Consider the problem of an explorer who comes across a tribe that was previously unknown (at least to him) with whom he or she has no common language. They inevitably communicate with gestures while speaking, to associate the gesture or object with the words. A pair of moderators, a man and a woman, would begin by speaking very slowly while gesturing. It sound corny, but remember, our target civilization knows nothing whatsoever about us or our cultures. The moderators would point to themselves and say "Hello, my name is Fred; and my name is Gertrude" while pointing to themselves. They would identify objects by name: "This is a rubber ball" while bouncing it. They would then illustrate and mention actions such as walking, sitting, and drinking water. Concurrently, the written text of the dialog would be written on the bottom of the picture with sub-titles, while the audio track would be transmitted between picture frames.

In this way, we can build up an understanding in our recipients' minds of the language and symbolism of the transmission. Which brings up the extremely delicate question of "Which language should be used in the message?" This is where a very fine international political fight will undoubtedly break out with many countries agreeing to compromise with English and a few other languages (perhaps Latin), while France threatens to withdraw from the program entirely unless French is used exclusively. However, since most scientific work is published in English, it may be the better choice. I would suspect that the highly political language issue will be resolved in only a few hundred years which will give the rest of the project divisions plenty of time to get ready.

For want of a better term, I refer to this explanation of our language and symbols as the primer. This primer must be transmitted at the beginning, end, and at regular intervals in the middle of our principal message. Secure in the knowledge that our primer will be understood, we can then transmit our compressed encoding scheme immediately after the primer. The encoding scheme permits us to use more concise coding to transmit our messages, such as ASCII codes instead of pictures of letters. This coding scheme will therefore permit us to transmit much more information that could be transmitted in the self-describing format of the primer.

OSETI Conclusion 2: We should expect a message that carries within it a primer containing a regular and orderly set of marking signs which is very easy to interpret, and when interpreted, gives the key to understand of the entire transmission.

3. Where should we aim our signals?

Since our resources are finite, we must choose our target systems wisely, so that we do not waste our efforts on systems that do not bear intelligent life. Our use of visible light permits us to transmit narrow beams which can be aimed directly at individual stellar systems and not waste power on the voids between systems. We must choose our targets wisely. In order to do this, we first restrict the types of stars under consideration, and then try to delve into planetary data

There are some 250,000,000,000 stars in our Milky Way. Most of these are no larger than our own star, Sol. In order for life (as we know it) to develop, a planet must have significant amounts of the elements heavier than hydrogen and helium. This requires that the original nebula from which the stellar system condensed be enriched with the products of supernova explosions. Assuming that the planets have condensed from the same nebula as the parent star, then only stars that contain significant quantities of post-helium elements could have planets that support life. This means that the older Population 2 stars that form the Galactic Halo are not likely candidates for our quest, so we must look to the Population 1 stars of the Galactic Disk

In addition, for life to develop, the temperature and pressure on the surface of the planet must support the presence of liquid water. (At least that is true on Earth.) This means a temperature of from 0° to perhaps 50° C. This condition needs to persist for a long time, at least tens, probably hundreds, of millions of years. Only main-sequence stars of types F,G,K,M,N,S and beyond live long enough for this to occur.

As near as we can tell, life originated on Earth by the photocatalytic action of ultraviolet radiation upon the organic molecules deposited on the planet by impacting comets, or outgassed from the interior. No tiny energy source like lightning can compare in power with sunlight. Stars of type beyond S do not produce sufficient ultraviolet light to catalyze organic synthesis, and it is questionable whether type S stars do either. That truncates our list down to exactly F,G,K,M,N, and (perhaps) S type main-sequence stars.

Furthermore, a life-bearing planet needs a stable orbit. The tidal effects in a multiple star system would perturb orbits over the length of time we are discussing. This makes it highly unlikely that multiple star systems will be good candidates for our target. Most stars belong to multiple star systems, so they can be excluded.

Not all stellar systems need bear planets. In our own stellar system, while the star contains more than 99% of the mass, the planets (Jupiter, in particular) contain more than 99% of the angular momentum. Somehow, during the formation of our solar system, the angular momentum of the original rotating and collapsing cloud was transferred to the planets. Very fast-rotating stars, then, probably do not have orbiting planetary systems, at least no planet of the mass of Jupiter. This is important, because Jupiter has had the effect of ejecting bodies (asteroids, comets) with highly eccentric orbits, out of the Solar System. If this garbage removal process had not occurred, the continued bombardment of Earth would have precluded the development of complex life, and perhaps all life.

So far, the target stellar systems will be attached to Population 1, small (F,G,K,M,N,S), single, slowly-rotating, main-sequence stars. This narrows our list of potential target considerably.

Everyone has prejudices about the probability of the origination and development of life. I say prejudices, because not much is known on the subject other than the record of its occurrence on Earth. I have my own totally unsupported prejudices which I herewith detail.

I assume that Earth-type planets at the proper distance from the parent start and of the correct composition are common. I also assume that, given conditions favorable to the development of life as outlined above, life is virtually certain to originate. My conclusion is that life of some sort is very likely to be found about any star having the characteristics delineated above.

However, there is a very large gap between primitive life and the complex macro-organisms that spawned humanity. Life began on Earth only a few tens or hundreds of millions of years after the planet was formed and cooled sufficiently. It took an additional period for the compound cell structures represented by the eucaryotic cell to arise. Billions of additional years were required for conditions to become favorable for multicellular animal life. The key factor in this transition was free oxygen.

Free oxygen is essential for the development of animal life. In fact, despite the vast variety of animal life on Earth, nearly every known animal requires elemental oxygen to live. The reason for this is that an aerobic metabolism is much more efficient than an anaerobic metabolism. A reasonably high concentration of oxygen is needed to permit it to be carried by water or transport molecules (hemoglobins).

Free oxygen came into existence on Earth due to the action of cyanobacteria and (possibly) green algae. Oxygen was a by-product of carbon fixation which, being a gas, went into the atmosphere. It did not build up appreciably at first because of the very large quantity of easily oxidizable minerals at the surface and dissolved in the early seas. It required billions of years for the easily oxidizable minerals to be oxidized, so that a surplus of oxygen could begin to accumulate.

Supposing that it becomes possible to detect Earth-like planets with oxygen atmospheres, these would clearly be the best candidates for our project. The prospect of doing so is upon us, due to the work of Ronald N. Bracewell and Alain Léger [2]. They have proposed a telescope which could detect Earth-sized planets orbiting nearby stars.

The range of our signals depends upon three factors: bandwidth, power and mirror size. With a large enough mirror, it would not take nearly as much power as otherwise to reach a distant system with a recognizable signal. The gradual increase in both power and mirror size should enable us to target a progressively larger sphere of stellar systems as time progresses.

OSETI Conclusion 3: We should expect a message to come from earth-like planets in systems of slowly rotating F,G,K,M or N Population 1 stars.

4. When is the best time to do it? How often?

There are millions of potential target stellar systems, and only one (or at most a few) transmitting devices. Transmission time will be at a premium, so best use must be made of it. After a rational choice of star systems is made based on the criteria given above, a transmission schedule needs to be devised. We should start with the nearest candidate systems and move gradually to the more distant ones as our experience, our transmitting power and our mirrors size grow. There are so many potential target systems (unless some drastic qualification comes through) that we could only devote a few tens or hundreds of hours per system. We must rotate among all of the targets that we can reach with our hardware setup of the given moment. After all, what if such a signal were to have hit Earth 200 years ago. Nobody could have received or interpreted it. Similarly, we must return repeatedly to the same targets.

OSETI Conclusion 4: We must expect only a relatively brief message to be sent to us at extremely long intervals. We must therefore be alert to make the most of the small window of reception that will be available to us.

5. How is the signal to be created, modulated and aimed?

It is assumed that some form of photonic communications in the visible band is selected. We know of no other which has the range to succeed, and there is certainly none faster. The key to successful transmission of any information lies in the signal to noise ratio, denoted s/n. According to the classical result of Claude Shannon, the information carrying capacity of any medium is limited to (log₂(s/n)+1)*b, where b is the bandwidth. Therefore, if we are to transmit most effectively, we must try to minimize noise, n while maximizing both signal strength, s and bandwidth, b.

The maximization of b immediately shows us that a very high frequency should be chosen. The frequency chosen should have these qualities:

1 Relatively easy to generate at high power levels (to maximize s)

2 Of shortest possible wavelength capable of being focused (to maximize b)

3 Capable of penetrating a planetary atmosphere

4 Rather dimly found naturally in the background sky (to minimize n)

Clearly, visible and near infra-red light meet condition 2,3 and 4 above. Visible light also, as will be shown below, meets condition 1. Microwaves fail conditions 2 and 4, while UV, X-rays and Gamma rays fail conditions 2 and 3. So we are left with the visible (and near infra-red, near ultra-violet) band, hence our conference chairman, Dr Stuart Kingsley, is right about optical SETI after all. No one can doubt that our source must be coherent (in order to be focussed properly), so a laser is absolutely essential

Given limited resources, the source of our light should give us the highest power per unit investment. We must choose some form of gas, liquid or solid (glass or crystal) laser. While solid-state diode lasers are often highly efficient, they do not (yet) produce enough power to be useful in our project. However, this could change rapidly.

Every effort must be made to make the detection of our signal as easy as possible for the target civilization. This means that we must not assume that they have unlimited resources to scan the vast expanse of the skies. The power requirements for transmission are very large, but not prohibitive even by current standards. The fundamental factor is that the signal is near the star (Sun) about which the transmitting planet orbits. Therefore, as observed from the target planet, the signal should outshine the star in the band chosen.

This brings up one of the key ideas of this paper, the concept of preferred (magic) wavelengths. By this term, I mean those wavelengths that are inherently (relatively) easy and inexpensive to produce at high power. It is with these wavelengths that we need to transmit our signal, and it will be to these wavelengths that our recipients will tune their searches. We simply can not outshine a star over wide band, no matter how narrow our beam. This need to narrow the band means that we must assume that our recipients are monitoring the very band that we are using. Since we can not establish any such conventions in advance, we must let the laws of nature establish them for us. Clearly, not all laser media are equally efficient. Some are more efficient than others.

For example, xenon difluoride produces a very high power output for a given input, compared to many other amplification media. It can be pumped in many ways, electrically, optically and directly by nuclear fission.

There are two basic basing options for a transmitter, on planet or in space. On a planet's surface, the transmitter has easy access to power sources, heat sinks and maintenance personnel. However, there is a high price to pay, and that is the need to transmit out through the atmosphere. The most recent dynamically adjusted telescopes give hope that the atmospheric distortion might possible be compensated for, but there is a very significant difference between the ability to adjust for reception and transmission. The fact that a beam of such tremendous power must be used means that there will be a large amount of heating of the atmosphere by the beam, a problem not encountered in telescopes. This may cause so much perturbation of the atmosphere as to render a land-based transmitter unfeasible.

A space-based transmitter has the overwhelming advantage that it will experience no interference from the planetary atmosphere. The three pumping options, electrical, optical and fission can be used in space as well as on Earth.

A space-based laser can be pumped by means of electricity generated by means of photovoltaic arrays. This requires no explanation here.

A second pumping method is based on concentration of sunlight on a transparent tube containing the medium by means of a mirror which is a parabolic cylinder. Filters would reflect the unwanted wavelengths back towards the star while permitting the pumping wavelengths to enter the tube and activate the medium.

The third design would use a fission process to pump the medium directly. This eliminates the need for external power sources. For example, imagine a quartz tube lined with a transparent fluoride. Into this tube, we introduce uranium 235 hexafluoride, xenon difluoride and helium. The tube is folded to reduce the surface to volume ratio and surrounded by neutron-reflecting materials. When the U-235 becomes critical, fission begins on a large scale. Each fission releases at least two very energetic massive charged nuclear fragments, along with neutrons to continue the chain. These fission fragments, due to their mass and speed, ionize the helium gas along their paths. This helium then transfers its energy to the xenon difluoride, which is then activated to a state capable of coherent amplification. This may be the smallest and cheapest way to generate laser beams with very high power.

An often overlooked problem is that of heat dissipation. A laser medium must always be out of thermal equilibrium, otherwise a population inversion of high and low energy states is impossible. In order to maintain this situation, heat must be dissipated. This calls for a very large radiator, along with some heat transport mechanism. Since Plank radiation energy goes as the 4th power of the absolute temperature, a small reduction in temperature requires a very large increase in the area of the radiator. Therefore, a laser medium that can operate at a relatively high temperature has an advantage. In any event, a very large radiative heat sink will be required for any space-borne laser. This will be one of the largest cost components in the system, regardless of the type of laser chosen.

Therefore, I would propose an immediate search for the most efficient media and their corresponding wavelengths. I would assume that any civilization that would be interested in receiving our signal would be tuning in on exactly these wavelengths.

There is also the question of transmission rate. It would seem that the maximum rate technically possible should be used, but this is an error. The higher the bit rate, the greater the bandwidth. This limitation is fundamental. As the bandwidth increases, the ratio of signal to background noise (mainly from the parent star) increases. It simply is not possible to outshine an entire star over the entire visible band unless our final output mirror were impracticably huge. Furthermore, the use of a very wide band would cancel the advantage of using a preferred wavelength. Therefore, we must limit our transmission bandwidth. It will be a matter of practical compromise, and is vital to the success of the program. Too narrow, and much of the signal capacity will be wasted. Too wide, and the message will not be detected because of the unfavorable signal to noise ratio. As the power of the transmitter increases, so the bandwidth can permitted to increase.

OSETI Conclusion 5: We should expect a pulse modulated signal to be sent at naturally preferred wavelengths which are chosen for the ease of generation of a powerful light beam.

6. Why is this to be done?

There must be some form of payback, return upon investment, so to speak, to influence the political forces that will be required to finance the project. Even the smallest conceivable transmission project would cost billions of dollars each year, and more promising projects proportionately more. What is the payback and how long will it take?

Obviously, the payback comes only in the form of a return message from a recipient. Otherwise, we are truly getting back absolutely nothing except the technical knowledge developed in the course of the project. (This may itself be a good return.)

Is the return on investment worth it?

The main problem here is simply that a response may take centuries, millennia or we may never receive a response. This makes it hard to justify in dollars and cents. Even if we succeeded in our first attempt to contact a very nearby G-type star, and our target civilization sent back a reply immediately, we would still need to wait for 20 years to receive the reply. If you believe that intelligent life is as common as is portrayed on Star Trek, then this may be promising. However, if you believe that target civilizations are relatively rare, and that perhaps only one out of a million systems have such life (that would still give some 100,000 such intelligent life forms in our Galaxy), the distance to the nearest one would be very large, and the waiting time for a returning signal would be measured in centuries, millennia or longer. If intelligent life is very rare, perhaps only one per billion systems, then we might wait for hundreds of thousands of years to receive a reply. (We might never receive a reply.) Clearly, this is a long time to wait for a return on investment. Would the political establishment on Earth fund a project with such a long time scale? This is a very serious question for which I have no answer.

Finally, suppose that we do receive a return message. Of what value would it be? Here I need to admit my prejudice: I believe that such a return message would be of immeasurable value to humankind. What would be the value? First of all, we would know that we are not alone in the Universe. Next, we would learn about other civilizations, their problems and how they handled them. Thirdly, we might gain great scientific knowledge. What knowledge? If we knew, then we would not need to receive the return signal.

OSETI Conclusion 6: Many civilizations that have the ability to transmit a visible signal that we could detect may choose not to do so for lack of any clear benefit that might be derived.

7. Whether or not transmission is prudent?

What are the risks of transmission? For want of other cases, let us take Earth and human history as an example. The history of our species is filled with cases of one group invading and subjugating or exterminating another. The most obvious parallel with the SETI situation is the conquest of the Western Hemisphere by European invaders. The case will be made that there are stupendous risks associated with announcing ourselves to our fellow creatures in the universe. After all, they might just be as cruel and wantonly destructive as ourselves.

In order to make the situation clear, let me introduce the Graphs of Good and Evil:

Rose Colored Glasses

Real Universe

Evil

Good

Evil

Good

          1234 5  6    7 8                9      1234 5  6    7  8                9

Since it is hard to define units of Good and Evil, I use these civilizations as markers:

1. Nazi Germany, Conquistadors, Aztec Empire, Cambodia under Pol Pot

2. Soviet Union under Stalin, China under Mao

3. Roman Empire under the Mad Emperors (most of them), Iraq today

4. Soviet Union after Stalin, China after Mao & before reforms

5. China today, Iran today

6. Imperial US & Western Europe before World War One, Russia today

7. US and Western Europe today

8. US and Western Europe in 50 Years (optimistic?)

9. World Ruled by rational and compassionate persons such as Albert Schweitzer / Mother Teresa

The history of mankind does not tend to promote optimism concerning our species' behavior when two civilizations come into contact. We have no reason to suppose that any civilization that we should contact would be much better than ours, and might be much worse. If it is better, then it may be able to teach us something about how to get along. If it worse, perhaps it could cause us great harm. What harm could a distant civilization possibly cause to us? The answer is frightening.

I assume, based upon the general consensus of physicists who work in the area, that a controlled fusion power plant is possible, and furthermore, that such a power plant could be used to propel a spacecraft to reach speeds of 1% to 2.5% c for an interstellar journey and to brake to zero (relative to the destination system). If an extraterrestrial civilization can not reach us, then the harm that can be inflicted on us is limited to the contents of messages. However, I assume the contrary, that interstellar travel is possible at 1% to 2.5% c. This technology has a potential for great destruction.

When the Europeans first heard about the New World, the first thing that they did was to set forth expeditions to conquer it, destroy its civilizations, annihilate its culture, pillage its wealth, and enslave its people. These were not mere by-products of contact, but were the original intentions of the Conquistadors and those who underwrote their expeditions.

Based upon ample precedent of human behavior, suppose that we announce our presence to some civilization that is as evil as that of the Conquistadors or Nazi Germany. It is possible. If such civilizations could develop here, they could develop elsewhere. Suppose further, that this civilization can mount an expedition of a half-dozen spacecraft at 1% to 2.5% c. There is no need for these invaders to carry sophisticated weapons along. The mere fact that they possess interstellar spacecraft would enable them to inflict incalculable destruction upon us.

Imagine a fleet of six interstellar spacecraft bound towards our Solar System at 1% to 2.5% c. At a distance of perhaps 1/10 to 1/4 light-year out, one of these vessels is singled out for a special purpose. The others dock with it to unload from it crew and useful stores, and load their waste products aboard it. Then the selected vessel is set upon a collision course for Earth. The other five craft then begin deceleration while the selected one continues on at 1% to 2.5% c, or perhaps resumes acceleration to double its speed to 2% to 5% c. This missile needs no warhead. An object moving at 1% to 2.5% (or maybe 2% to 5%) c possesses a kinetic energy roughly equivalent to a thermonuclear warhead of the same mass. Now, supposing the spacecraft has a mass of 10,000 tons (a modest size for an interstellar craft), with each ton equivalent to millions of tons of TNT. This amounts to tens of billions of tons of TNT, and could be much larger as our estimate of 10,000 tons is rather minimal for an interstellar spacecraft. A 100,000 ton spacecraft would produce an explosion ten times more energetic, or some hundreds of billions or even trillions of tons of TNT equivalent, depending on the impact speed.

This spacecraft would impact on Earth long before the rest of the fleet. We could not possibly detect and intercept such a missile. Upon hitting the Earth, it would produce such havoc and chaos that civilization as we know it would come crashing to a halt. Essentially all of the energy in the original load of thermonuclear fuel (less waste heat) will have been converted into kinetic energy. This kinetic energy will then be converted into thermal energy upon impact. If it impacts on land, a cloud of dust would be raised the like of which has not been seen since the extinction of the dinosaurs. The air would become nearly opaque, making plant life impossible. Furthermore, the blast effect would be so large as to knock down every tree (and nearly every building) on Earth. An impact on water holds no better promise as the resulting tsunami would inundate nearly the entire land area of the Earth.

Humanity would be either exterminated utterly, or reduced to a few pitiful survivors. No radioactivity would be produced, so the invaders would have a nice, clean, safe planet to seize and colonize. By the time the rest of the fleet arrived, weather conditions would have returned to normal. No great war machines would be required to secure the planet for the invaders. They would need, at most, a few tanks, helicopters and small arms to hunt down the few (if any) remaining humans. A few invaders could easily secure the Earth for the colonists to follow in later trips. The proof that this is not fantastic is that, with a few changes, this scenario has occurred on Earth many times. A few hundred Conquistadors did indeed conquer the bulk of the Central American aborigines. They had a 'super-weapon', smallpox, and it killed millions of American aborigines in a few decades, resulting in a population reduction of 90% in Mexico alone. This reduced population was easily conquered. A relativistic spacecraft collision would be even more destructive.

The area under the curve to the left of line 4, 5 or wherever one draws the 'cutoff' line of tolerable civilizations, represents the probability of transmitting to an 'unacceptably evil' civilization and thereby exposing ones own civilization to catastrophic harm. No matter how large the benefits may be, this risk is still non-negligible. It must be weighed against the possible benefits. It seems highly unlikely to me that any conceivable benefits could outweigh the risks.

Is Transmission Worthwhile? The Benefits of transmission.

The project director, having admitted the risks, will need to testify before the oversight committee to demonstrate the positive value of the Transmission Project. What can she say?

We will not even know whether or not we have succeeded in finding a real target for centuries, and perhaps millennia. When we do find out, the answer may come in several forms. We may receive a polite response acknowledging our original message and giving details of the responding civilization. This is clearly the hoped-for reply. We may be told to get lost. Or, the response may be an invading fleet. The polite response would initiate a dialog between our two civilizations that could bring us information that we could not otherwise obtain. We could learn about new technologies. We would certainly learn details about different biological and social systems which we could never even imagine. We could join a network of such civilizations.

Or we could be exterminated by a single interstellar spacecraft. Which would it be? Who knows?

As far as a network or federation of civilizations is concerned would we really wish to join it? To paraphrase Julius (Groucho) Marx, perhaps we should never even consider joining any confederation that would have a species like us as a member.

Is it worth the risks?

If any civilization considering transmission ponders arguments like these, it would very possibly choose not to transmit. This may mean that few, if any, technological civilizations will transmit signals at any wavelength for our detection because of such trepidations. If this is so, then that explains why we have never detected such a signal, and may never detect one. The risk of transmission may be too great to take. I realize that this is a very pessimistic conclusion, but the realities seem to point in that direction.

OSETI Conclusion 7: We should expect many civilizations to be reticent about revealing themselves out of fear of harm due to interstellar spacecraft impact.

8. Results: A Strategy for SETI Emerges

Reversing our perspective to a receiver, we can draw some rather definite conclusions and use them to develop a strategy. Despite the negative results given above, I personally feel that a search for extraterrestrial life is worth the trouble. Perhaps this is pathological optimism. Here is an outline of a SETI strategy:

0. Forget about microwave or longer wavelengths. No intelligent civilization would attempt to use them when the visible wavelengths are available. Therefore, no intelligent civilization should bother to look for them.

1. Set up an organization that is dedicated to Optical SETI (the only kind worth doing) and to coordinating the search in a rational and efficient way.

2. Find out what the preferred (magic) wavelengths are that permit transmission with high efficiency. This may be an expensive piece of research. This is one of the cornerstones of my strategy.

3. Develop a set of inexpensive notch filters which will permit only the preferred wavelengths to pass. There must be several for each magic wavelength each with a different combination of width and offset to compensate for various expected bandwidths and Doppler shifts due to stellar motion.

4. Use these notch filters as the centerpieces of a standard kit for amateur astronomers. Kits should include filters, photon detectors, amplifiers, computer interface hardware, software and whatever else is needed to turn an amateur astronomer with a PC into an OSETI observatory. Kits should be sold a cost to all qualified amateur observers.

5. Set up a network of these amateur astronomers which will use the kits and their own telescopes to monitor stellar systems that have been assigned by the coordinating organization.

6. Have the major observatories on instantaneous standby to use a positive detection report to direct an immediate effort to receive and record the signal for later interpretation. Interpretation need not be done at once, but the data must be captured when it is available. We might not be on the receiving end of such a transmission again for thousands of years.

7. Be prepared to wait for centuries or even millennia for a positive result. If this mind-set is not firmly entrenched, the search will almost certainly fail. Perhaps the hundredth generation will be the first to receive a signal. This is not a short-term project.

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Reference:

[1] Extinct Languages; Johannes Friedrich; Barnes & Noble Books; translated from German by Frank Gaynor; Copyright 1957 Philosophical Library; translation copyright 1993 Barnes & Noble; ISBN 0-88029-338-1

[2] Searching for Life in Other Solar Systems; Roger Angel and Neville J. Woolf; Scientific American; March 1998