a discussion on the recognition of alien life || the recognition of extraterrestrial intelligence

The Recognition of Extraterrestrial IntelligenceAuthor(s): C. SaganSource: Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 189, No.1095, A Discussion on the Recognition of Alien Life (May 6, 1975), pp. 143-153Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/76926 .

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Proc. R. Soc. Lond. B. 189, 143-153 (1975)

Printed in Great Britain

The recognition of extraterrestrial intelligence

BY C. SAGAN

Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14850, U.S.A.

[Plates 11-18]

A necessary but not sufficient condition for the recognition of extraterrestrial intelligence is a significant departure from thermodynamic equilibrium. This can be discerned clearly in the inverse problem of the detection of terrestrial intelligence from a distance. Photography of the Earth in reflected sunlight reveals no clear sign of life until 100 m resolution is achieved, at which point the agricultural and urban reworking of the Earth's surface in rectangular arrays first becomes obvious. This pattern is however detectable only over interplanetary distances. Mars exhibits no such patterns. The departure from radiative equilibrium - represented by radio, television and radar technology - in the micro- wave spectrum of the Earth is by contrast easily detectable over interstellar distances. Even with a technology no more advanced than our own, a civilization on a planet of a nearby star could easily determine, by auto-correlation techniques, the artificiality of these radio signals. Intentional interstellar radio messages should be detected and decrypted far more readily. Possible message contents for interstellar discourse of a modulated signal at any accessible frequency include (1) m-dimensional imagery represented by the transmission of numbers which are the products of m prime numbers; and (2) the use of a common mathematics, physics or astronomy to convey a range of information on more difficult subjects. The only direct attempts to date to communicate with extraterrestrial intelligence - the plaques aboard the Pioneer 10 and 11 spacecraft - are discussed briefly.

The search for extraterrestrial intelligence has recently evolved from a largely disreputable pseudoscience to an interesting although extremely speculative endeavour within the boundaries of science. This discussion meeting of the Royal Society is itself an indication of the growing respectability of the subject. Such a transition is due almost entirely to the technological advances of the last twenty years. We are now capable of receiving radio signals over immense interstellar distances; and we are on the verge of landing sophisticated instruments on nearby planets to search there for life. The search for extraterrestrial intelligence is about to enter an experimental phase, after several millennia in which the subject was amenable only to somewhat murky speculation. (Recent general discussions on extraterrestrial intelligence can be found in Shklovskii & Sagan (I966) and in Sagan (I973 a).)

The likelihood of extraterrestrial intelligence within the confines of the Solar System is generally taken to be small, independent of whether there is life on other planets of the Solar System. We cannot expect that life on separate planets

[ 143 ]



144 C. Sagan (Discussion Meeting)

will have closely synchronized evolutionary developments. A planet on which life has achieved a level characteristic of that on Earth some millions of years ago would be judged by many as lacking intelligent life. Yet a few million years is only 0-3 of the age of the Solar System. If our criterion of intelligence is a technical civilization, the odds are narrower still. A civilization 100 years behind us would be indetectable - for example by radio transmission. A civlization 100 years in advance of us would almost certainly be readily detectable in a variety of ways including radio observations, photographic investigations by our flyby spacecraft, and visitations from such a planet to the Earth. It is of course to some degree misleading to talk about an extraterrestrial civilization as so many years ahead or behind us, but such simple considerations suggest that the chance of there being an extraterrestrial civilization within the Solar System is about 10-7.

In addition, the subject can be approached observationally and recent data, described below, set some boundary conditions on the possibility of intelligent life on our neighbouring planet Mars.

The identification of life or intelligence is not in all instances easy, as, for example, the cases of (1) 'organized elements' in carbonaceous chondrites (Anders & Fitch I962); (2) antarctic ventifacts (Morris, Mutch & Holt I972); and (3) the Calico flints (Haynes I973).

In the absence of extraterrestrial civilizations within the Solar System, some insight into the problem can be obtained by investigations of the inverse problem, namely the detection by remote means of intelligent life on Earth.

Let us first imagine a photographic reconnaissance by orbiter spacecraft of the Earth in reflected visible light. We imagine we are geologically competent but have no prior knowledge of the habitability of the Earth. Photography of the Earth at a range of surface resolutions down to 1 km reveals a great deal that is of geological and meteorological interest, but nothing whatever of biological interest (figures 1-5). At 1 km resolution, even with very high contrast, there is no sign of life, intelligent or otherwise, in Washington, London, Paris, Moscow, or Peking. We have examined many thousands of photographs of the Earth at this resolution with negative results (Sagan et al. I966). However when the resolution is improved to about 100 m, a few hundred photographs of say 10 km x 10 km coverage are adequate (Sagan & Wallace I97I) to uncover terrestrial civilization (figures 6-8). The patterns revealed at 100 m resolution are the agricultural and urban reworking of the Earth's surface in rectangular arrays. Human beings have a simultaneous passion for territoriality and Euclidian geometry, but have been able to exercise this passion only on scales of about 100 m and smaller. Whether it is a characteristic of more advanced societies to rework their planets on a larger scale is a debatable question. These patterns would be extremely difficult to understand on geological grounds even on a highly faulted planet.

Such rectangular arrays are clearly not a thermodynamic or mechanical equilibrium configuration of a planetary surface. And it is precisely the departure from thermodynamic equilibrium which draws our attention to such photographs.



Sagan Proc. R. Soc. Lond. B, volume 189, plate 11

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FIGURE 3. Approximately half the photographs of the Earth are primarily of clouds. Here, turbulent eddies are produced by winds passing over Guadalupe Island, off the coast of Baja California, made visible by the clouds. The tether line which attached the astro- naut to the spacecraft during e.v.a. is visible at the right. Courtesy, N.A.S.A.

FIGURE 4. AS9-3303. This photograph shows the meanders and oxbow lak-es left by the Mississippi River near Memphis, Tennessee. Courtesy, N.A.S.A.



Sa.an Proc. R. Soc. Lond. B. volume 189. niate 13

FIGuRE 5. AS6-2-1435. This photograph of the mouth of the Colorado River contains two peculiar features. The first is the bright corkscrew in the upper centre. It is apparently a tidal drainage channel outlined by salt flats. The second is the two bright dots at the lower right, which may possibly be boats. Courtesy, N.A.S.A.

FIGuRE 6. This photograph shows the cities of Dallas and Fort Worth, Texas. Large urban areas such as these, and the straight lines of roads are among the major signs of intelligent life on Earth detectable at 100 m resolution. Courtesy, N.A.S.A.




FIGURE 7. A clear example of intelligent life on Earth. A rectangular grid of irrigated fields in the Western United States is strikingly illustrated in this photograph of the Imperial Valley, Mexicali, and part of the Salton Sea. Courtesy N.A.S.A

FIGURE 8. This photograph covers portions of Arizona and New Mexico. The bright area near the centre is Wilco Dry Lake, and is surrounded by the grid pattern of irrigated fields. At the left is another example of 'intelligent' life on Earth - atmospheric pollution in the form of a plume of smoke from a factory in Douglas, Arizona. Courtesy, N.A.S.A.



Saciqan Proc. R. Soc. Lond. B. volume 189. Plate 15

FiGURE 9. A dying oasis, inundated by dunes, in the Southern Sahara, north of In-Salah. The mechanical disequilibrium shapes of the palm trees are very evident. From H. Reich 1966. The world from above. N.Y.: Hill & Wang.

FiGURE 10. Cows casting long shadows, indicative of mechanical disequilibrium in California. Courtesy, Dr R. N. Colwell.




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The recognition of extraterrestrial intelligence 145

This principle has long been recognized in the debate over the Lowellian canals of Mars which were widely recognized to be of intelligent origin, the only question of debate being Mrhich side of the telescope the intelligence was on.

Detection of individual organisms on the Earth. even with long shadows, generally requires 1 m resolution, except in the somewhat dubious case of fore.ts. Here (figures 9 and 10) a departure from dynamical equilibrium is clearly discernible. Many organisms, detectable at such a resolution, have a topheavy appearance. They are in apparent dynamical disequilibrium and, with known aqueous and aeolian erosion rates on Earth, the narrow stilts supporting such organisms would be eroded in times short compared to the lifetime of the planet. Thus configurations such as those in figures 9 and 10 require a mechanism for the rapid and large-scale production of identical configurations in dynamical disequilibrium, and again geological processes are hard pressed to supply a solution.

These two cases suggest that a general approach to the question of extraterrestrial intelligence is the search for departures from thermodynamic equilibrium, a method first enunciated in a general way by Lederberg (I965). Thermodyniamic disequilibrium is a necessary but of course not a sufficient condition for the recognition of extraterrestrial intelligence. To take a homely example, ozone is present in the Earth's atmosphere because of departures from thermodynamic equilibrium, but of a photochemical rather than a biological sort. On the other hand a concentration of about 10-6 methane in the Earth's atmosphere, when the thermodynamic equilibrium mixing ratio in an excess of oxygen is less than 10-36 (Lippincott, Dayhoff, Eck & Sagan I967), is due to life on the planet - in this case methane bacteria. In addition signs of intelligent life on the Earth may be photo- graphed but not readily recognized: the outline of the North American continent in figure 11 is due to the lights of large cities; but without ground truth it is unlikely that an extraterrestrial observer would be able reliably to draw such a conclusion.

The Mariner 9 mission to Mars permitted for the first time a photographic investigation of the possibility of a technical civilization on that planet. A total of 7232 photographs were obtained, most of them at 1 km resolution, at which resolution the entire planet was mapped from pole to pole. Several percent of the planet were also observed at 100 m resolution and with adequate coverage to have detected a technical civilization on the Earth at that resolution. No signs

DESCRIPTION OF PLATE 16 FIGuRE 11. North America in emitted visible light at night. The bright band in Canada is the

edge of the aurora borealis. U.S. Department of Defense photograph. FIGURE 12. Mariner 9 geological map of a portion of Mars overlain by classical 'canals'

as drawn by Slipher. The correlation of 'canals' with topography is poor. After Sagan & Fox (I974).

FIGURE 13. Dune field in crater Proctor near Hellesponti Montes. Mariner 9 photograph DAS 9807429. The pattern is regular and quasi-linear, but there is an obvious abiological interpretation.




of such a technical civilization were uncovered. A few of the Lowellian canals were revealed to be large rift valleys, ridge systems, or crater chains, but the great majority of the so-called canals correspond (see figure 12) to nothing whatever on the planet (Sagan & Fox 1974). Figures 13-16 show some of the most ordered geometries uncovered by Mariner 9. These include sand dune fields; arrays of generally crater-associated linear surface patinas, almost certainly produced by aeolian transport; a striking oval which turns out to be the interior ramparts of a crater whose floor and exterior are still covered by winter frost; and a number of pyramidal hills of polygonal cross-section, possibly produced by

wavelength

FIGURE 18. Schematic illustration of the departure from a black body spectrum of the Earth's emission to space due to radio and television broadcasting.

aeolian erosion (Gipson & Ablordeppy I974). Likewise the innermost satellite of Mars, Phobos, which had been thought, on the basis of what we now know to be erroneous celestial mechanical data, to be a hollow artificial satellite of the planet (Shklovskii I966), proved to be an entirely natural object (figure 17) which, from its crater density, is very likely contemporaneous with the origin of the terrestrial planets (Pollack et al. I973). Thus the preliminary photographic reconnaissance of Mars has yielded entirely negative results on the possibility of intelligent life there, although no experiments bearing closely on microbial life - or even macro- bial non-technical life - have been performed. The Viking landings on Mars in 1976 may conceivably cast some light on this problem.

Perhaps the most significant indication of intelligent life on Earth - and certainly the one most readily discernible - occurs in the radio part of the spectrum. Except for minor departures due to atmospheric absorption and emission, the electromagnetic emission spectrum of the Earth closely follows that of a black body at temperatures between 210 and 290 K depending upon cloud cover. However in the radio part of the spectrum there is an immense departure from radiative equilibrium (figure 18): at some metre wavelengths the brightness temperature of




. . . ... .

FIGURE 14. Crater-associated array of streaks in Hesperia. Mariner 9 photograph DAS 6175228. Again a regular quasi-linear pattern due to windblown particulates.

FIGURE 15. Crater in south polar region of Mars with defrosted interior ramparts. Mariner 9 photograph DAS 2496905.

(Facing P. 146)



Sagqan Proc. R. Soc. Lond. B, volume 189, plate 18

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FIGURE 16. An array of pyramidal forms near Cerberus. Facets are probably of aeolian origin. See Gipson & Ablordeppey (I974). Mariner 9 photograph DAS 7794853.

FIGURE 17. Phobos. Mariner 9 photograph IPL 1008:082243.



The recoqnition of extraterrestrial intelliqence 147

the Earth is tens of millions of kelvins, and at such wavelengths the Earth is brighter than the Sun (for further discussion, see Kardashev 1973). This extreme disequilibrium situation is caused by radio, radar, and, especially, television transmission. Despite the enormous interstellar distances and the inverse square law attenuation of this transmission, such signals would be detectable by a civilization no more advanced than ours at a distance of some 10 light years.t Although

co -, signal ' rms locations I radiometer Isignal intensity

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FIGURE 19. Sum of two simulated radiometer records in which 18 signals of intelligent origin and of equal intensity have been inserted. After Drake (I973).

0 02 Ca40

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0 C

_0.1 --40 -20 0 20 40 reference delay-units

FIGURE 20. Cross-correlation function of the two records shown in figure 19. After Drake (I973).

auto-correlation techniques in the frequency and time domains can uniquely specify the artificiality of the signal, and enhance the range over which detection can be performed (see, for example, Drake I973, and figures 19 and 20), the call signals of radio and television transmission, the repetitious nature of much military and commercial traffic at these wavelengths, and the coding of military and astronomical radar pulses all lend themselves to such a detection with only a modest investment of effort on the part of the recipient civilization. This appears to be true despite the absence of any prior information on, for example, the number of lines per television picture.

The foregoing remarks apply only to the so-called eavesdropping mode, in which radio transmission from civilization A is detected and decoded by civilization B, while A intends such transmission to be used only for domestic purposes. Where there is a conscious effort to accomplish interstellar radio communication, the signal detectivity and range can both be improved enormously. A useful

t 1 light year 9.5 x 1015 m.




example can be drawn from the most advanced terrestrial radio technology. Suppose we were to use the 305 m semi-steerable Arecibo radio telescope (operated by Cornell University for the National Science Foundation) as a receiver, and

imagine an identical radio telescope on a planet of some other star used as a transmitter. With a 1 Hz bandpass and bit rate of about 1 bit per second, the two radio telescopes could be essentially anywhere in the Milky Way Galaxy and still communicate with each other. (With its new radar transmitter and narrow bandpasses the Arecibo telescope is 106 - 109 times brighter than the

Sun at the same frequency.) This implies that intentional communication amongr civilizations within, say, a 1000 light year radius of the Earth might

be accomplished even if the other civilization were no more advanced than we. Such radio contact is however not trivial, because there has been no prior agree- ment on which star, frequency, bandpass and time constant to employ. (The last two parameters are not independent by the Fourier integral theorem.) However thousand-channel receivers will shortly be in operation in terrestrial radio astronomy and at most a dozen 'natural' channels for interstellar radio communication have been proposed (Drake & Sagan 1973). We have only a very poor idea of which stars might be most likely to harbour technical civilizations and a 'brute force' systematic examination of all nearby stars beginning with the nearest seems

appropriate. Several hundred stars have been so examined at one or at most two frequencies to date, the most elaborate search, which is still underway, being that

of Zuckerman and Palmer at the National Radio Astronomy Observatory. How- ever even estimates which are considered optimistic imply that hundreds of

thousands of stars must be examined before the nearest technical civilization is

uncovered (Shklovskii & Sagan I967; Sagan 1973 a). Therefore we have performed only 0.1 % or less of the necessary effort to date.

Because terrestrial civilization has only recently achieved radio astronomical

capability, it is highly improbable that there are any other communicative civilizations in the Milky Way Galaxy so backward as we. It therefore makes

considerable sense for us to receive rather than to transmit messages. It also follows

that the transmitting civilization is likely to have technological and scientific

capabilities immensely in excess of our own. For example, it seems not improbable that such advanced civilizations would know which stars are most likely to

harbour technical civilizations and thereby greatly narrow the number of targets. Such civilizations likewise might be in communication with other advanced

civilizations and be thereby able to pool transmission strategies. The mere fact

that they have survived the invention of technology (as we have so far) suggests that such civilizations might be very long lived - perhaps long lived and mature

enough to transmit messages with no expectations, because of the finite propaga- tion velocity of light, of an answer for hundreds or thousands of years. It is precisely the recognition of the uniquely backward position of terrestrial technology which argues against the otherwise cogent contention of Bates (I974) that the

search for signals might take uncomfortably long periods of time. With existing



The recognition of extraterrestrial intelligence 149

technology, a dedicated search programme of some decades would represent a very significant attack on the problem.

There is no difficulty whatever in designing a radio message of obvious artificiality. Such a beacon or announcement signal need not contain much information. For example a set of pulses containing the first 30 prime numbers, repeated many times at different bit rates and radio frequencies, would be an unambiguous announcement signal. The actual message might be sent at much slower repetition rates. The announcement signal can be likened to the call letters of terrestrial radio or television transmission. In the case of a multi-frequency announcement signal, part of the announcement message might be information on the frequency and bandpass of the principal message - in partial analogy to the convention in short-wave radio broadcasting of employing several frequencies and announcing on each frequency which other frequencies are available.

Because there are many human languages which have not yet been decoded, it is sometimes thought that a radio message from another civilization, separated from us by hundreds of light years in space and immense periods of time, and enormous biological barriers, will have no chance whatever of being understood. However the Cretans and the Mayans were not attempting to communicate with us today; nor were they technological societies in the sense used in the present discussion. The mere existence of a radio channel already insures overlapping science and technology, and the linearity of radio transmission lends itself very well to easily broken codes. The problem is the converse of the usual military context where the security of the message is the objective of the encryptment; here we have the need for a new art which has been called anticryptography: the design of messages so simple that even civilization as backwards as ours will understand them.

One such message format is the repetition of a binary message which contains a number of bits equal to the product of two prime numbers, n1 and n2. (The zeros and ones of such a binary message can be encrypted in time, frequency or amplitude space.) This immediately lends itself to a raster format. Upon the receipt of such a signal it would be natural to arrange it into n1 rows of n2 columns and n2 rows of n1 columns. If one of these two presentations had a strong geo- metrical content - straight lines or circles for example - the message would likely have been decrypted successfully. Note that the principle here, as in an announcement or beacon message which is a series of prime numbers, is again significant departures from thermodynamic equilibrium. The first example of such a prime number image encrypting was provided by Drake (see figures 21 and 22). The method can be generalized: an m-dimensional message can be encrypted as message groups of the form m

fr, j=1

One of the dimensions can be time and the transmission of three-dimensional motion pictures by this method can readily be envisioned.

Ani alternative method has been proposed which employs simple algebraic




statements to define progressively more complex mathematical relations. Message statements eventually broaden to include scientific and social information (Freudenthal I962).

Shannon's theorem says that the maximum information communication rate equals the bandwidth times the logarithm to base 2 of the signal-to-noise ratio. A

11110000101001000011001000000010000010100

1 o o o o o 1 1 0 o 1 o 1 1 0 o 1 1 1 1 o o o o o 1 1 o o o o 1 1 0 1 o o O o 0 O 00100000100001000010001010100001000000000

0 0 0 0 00 0 0 0 0 1 0 0 0 1 o o o o o O oo oo0 1 0 1 1 o oo o o 0 00 o o O O

00000000000100000000000001011000100000110000

0 0 1, 1 1 0 1 o o O o 1o 1 1 0 o o o o 1 1 1 o o O o o o o 1 o o o o o O o o

10000000110010111110101111100010011111001

0 0 0 0 0 0 1 0 0 0 0 1 1 1 1 1 0 0 1 o O o 1 0 1 1 0 0 0 1 1 1 1 1 1 1 0 0 0 0 0

10000000010000011001 00101

00 1000 11 1100 101 11 1

FIGuRE 21. A 551 bit encrypting prototype interstellar message. After Drake; described in Shklovskii & Sagan (1966).

FiGuRE 22. Decrypting of the message in figure 21 into a 19 x 29 television raster. For further discussion, see Shklovskii & Sagan (i1966).



The recoqnition of extraterrestrial intelliqence 151

suspected communications chamnel could be checked to see how well it approaches the Shannon limit. Until recently, only pulse-coded modulation devices - that is the creations of human technology - were, so far as we knew, close to the maximum efficiency of the Shannon theorem. However pulsars apparently have a range of frequency channels of individual bandpasses of about 10 kHz, ranging through the entire electromagnetic spectrum of about 10 GHz. This is the equivalent of about 106 frequency channels. There are also about 100 amplitude channels, and about 10 time channels for fine structure. There are also some polarization channels. With of the order of 0.1 s/pulse, this is a communications rate of very roughly 1011 bits per second. Over the life of a pulsar, this is some 1024 bits - just the number which in Philip Morrison's view corresponds to the information content of our present civilization. A civilization about to be destroyed in a supernova explosion might very well want to communicate those 1024 bits. But now if we use Shannon's theorem with some fair signal-to-noise and the information transmission rate of 1011 bits per second, we find that the optimum bandwidth is about 10 GHz, the observed bandwidth. Thus pulsars are the only 'natural' phenomena in astronomy to approach the information transmission efficiency of Shannon's theorem. But this is not, of course, a compelling argument for their artificiality, and no message content has ever been suggested.

The foregoing discussion of interstellar radio communication has been concerned exclusively with radio communications modes. Alternative electromagnetic transmission by, for example, optical frequency lasers should employ the same principles on the detectivity of signals and the encrypting of message contents. There is however an entirely separate conceivable mode of interstellar communication which is not excluded by present physics, and that is interstellar spaceflight. This method is much more expensive, difficult and time-consuming than electromagnetic channels, but it cannot be excluded on such grounds. In recent folk scientific literature, three kinds of claims are being made: that there is a record in historical artefacts and legends of past visitations of the Earth by extraterrestrial civilizations; that there is evidence in the sightings of so-called unidenti- fied flying objects of present such visitations; and that, in the echo delay times of ionospheric soundings beginning in the 1920s, a pattern can be discerned which is explicable only in terms of an active radio repeater system on board an interstellar space vehicle in cislunar space, perhaps at one of the Lagrangian points. My personal view is that for none of these claims is there anything even approach- ing compelling evidence for interstellar visitation. Instead they seem to represent a curious potpourri of wishful thinking, psychological projective tests, the mis- apprehension of natural phenomena, and an ignorance of archaeology. I discuss some of these points elsewhere (Sagan & Page 1972; Sagan 1973 b). There is no reason to think that evidence of interstellar visits may not be forthcoming in the future. But by the usual standards of scientific evidence there is no reason to think that such evidence exists today.

One indication that interstellar spaceflight is not impossible is the fact that




human technology is already engaged in it. The Pioneer 10 spacecraft, which flew by Jupiter on 3 December 1973, and the Pioneer 11 spacecraft scheduled to pass Saturn in 1979, both have trajectories which, through momentum exchange with a major planet, will eject them from the Solar System. Their interstellar spaceflight is unpowered and the transit time over a few light years is about 105 years. It may be possible to do terminal trajectory adjustments which will direct these spacecraft to the vicinities of stars within 100 light years of the Earth. In such a case, the spacecraft might just conceivably be intercepted in about 106 years. If no such terminal manoeuvres are performed, simple collision physics calculations

0 ' 0 0 0X 1-0 /a.. NI

o ? ' * ; f E~ r o-E-

FIGURE 23. The plaque aboard the Pioneer 10 and 11 spacecraft.

show that the spacecraft will never, in 1010 years, enter any other solar system -

even if every star in the Milky Way Galaxy has a planetary system comparable to

our own. In this case, the spacecraft can be intercepted only by an interstellar

spacefaring society much more advanced than our own.

Despite the remoteness of these contingencies, we felt it appropriate that a

simple cosmic greeting card be aboard the two spacecraft. The message (Sagan, Sagan & Drake I972), attached to the antenna support struts of the two spacecraft, is engraved on a 15 cm x 22- cm gold-anodized aluminium plate, and is

shown in figure 23. Except for the two human beings who will be deeply mysterious, the remainder of the message is written in a scientific language which we think

will be at least moderately clear to any extraterrestrial technological civilization.



The remoanition of extraterrestrial intelliaence 153

It employs a physics and astronomy which such a civilization must share with us. A discussion of the message contents can be found in the last reference.

The Pioneer 10 message, even without the human beings, spacecraft and Solar System representations, was readily decrypted by academic astronomers and physicists in tests performed before any public announcement of the existence of the Pioneer 10 plaque. This is of course not an ideal test because of the common biology and culture of the designers and decrypters. A better test of the decrypt- ability of this plaque, mankind's first conscious effort to transmit a message to extraterrestrial civilizations, is unlikely to come soon. Attempts to detect interstellar radio messages transmitted in our direction may conceivably be successful on much shorter time scales.

I am grateful to F. D. Drake for several helpful discussions. This research was supported in part by N.A.S.A. Grant NGR 33-010-101.

REFERENCES (Sagan) Anders, E. & Fitch, F. W. I962 Science, N.Y. 138, 1392. Bates, D. I974 Nature, Lond. 248, 317-318. Drake, F. I973 In Sagan (I973a). Drake, F. & Sagan, C. I973 Nature, Lond. 245, 257-258. Freudenthal, H. I960 Lincos: design of a language for co8mic intercourse. Amsterdam:

North Holland. Gipson, M. & Ablordeppy, V. I974 Icaru8 22, 197-204. Haynes, V. I973 Science, N.Y. 181, 305. Kardashev, N. S. I973 In Sagan (I973a). Lederberg, J. I965 Nature, N.Y. 207, 9-13. Lippincott, E., Dayhoff, M., Eck, R. & Sagan, C. 1967 Astrophys. J. 147, 753-764. Morris, E. C., Mutch, T. A. & Holt, H. E. I972 Atlas of geologic features in the dry valleys

of South Victoria Land, Antarctica. Interagency Rept. Astrogeol. 52, Menlo Park, Cali- fornia, U.S. Geolog. Survey.

Pollack, J. B. et al. 1973 J. geophys. Res. 78, 4163-4196. Sagan, C. (ed.) I973 a Communication with extraterrestrial intelligence. Cambridge, Mass.:

M.I.T. Press. Sagan, C. I973b The cosmic connection. New York: Doubleday; and Londoli: Hodder &

Stoughton. Sagan, C. & Fox, P. I974 Icarus (submitted for publication). Sagan, C. & Page, T. (eds.) I972 UFO'8: a scientiftc debate. Ithaca, New York: Cornell

UJniversity Press. Sagan, C., Sagan, L. & Drake, F. 1972 Science, N.Y. 175, 881-884. Sagan, C. & Wallace, D. I'97I Icarus 15, 515-554. Shklovskii, I. S. I966 In Shklovskii & Sagan (1966). Shklovskii, I. S. & Sagan, C. 1966 Intelligent life in the Univer8e. San Francisco: Holden-

Day.

To Vol. I89. B.



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