Search for extraterrestrial intelligence using satellite communication pdf

Howard, A. Dyson, F. Marshak, R. Brin, G. The 'great silence': the controversy concerning extraterrestrial intelligent life. Morrison, P. Ponnamperuma, C. Houghton-Mifflin, Boston, Efficient interstellar rocketry. Bracewell, R. Sullivan, W. III, Brown, S. Eavesdropping: the radio signature of the Earth. Glaser, P. Power from the Sun: its future Science , — Wilson, T. Tools of Radio Astronomy: Problems and Solutions p.

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If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate. Advanced search. Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily. Skip to main content Thank you for visiting nature. Abstract As far as we know, humanity is alone in the Universe: there is no definite evidence for the existence of extraterrestrial life, let alone extraterrestrial civilizations ETCs capable of communicating or travelling over interstellar distances.

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Figure 1: Noise from the Earth's atmosphere and the minimum noise from our Galaxy, versus wavelength. Figure 2: How increases in effective radiated power ERP of a transmitting radio dish are related to wavelength and the size and distance of the target. Figure 3: An example of a possible message from an ETC. References 1 Hart, M.

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Rights and permissions Reprints and Permissions. About this article Cite this article Wilson, T. Copy to clipboard. Thin sections of sedimentary rocks between 2. These inclusions have been identified by Elso S. Barghoorn of Harvard University and J. Bacteria and blue-green algae are evolved organisms and must themselves be the beneficiaries of a long evolutionary history.

There are no rocks on the earth or on the moon, however, that are more than four billion years old; before that time the surface of both bodies is believed to have melted in the final stages of their accretion.. Thus the time available for the origin of life seems to have been short: a few hundred million years at the most. Since life originated on the earth in a span much shorter than the present age of the earth, we have additional evidence that the origin of life has a high probability, at least on planets with an abundant supply of hydrogen-rich gases, liquid water and sources of energy.

Since those conditions are common throughout the universe, life may also be common. Until we have discovered at least one example of extraterrestrial life, however, that conclusion cannot be considered secure. Such an investigation is one of the objectives of the Viking mission, which is scheduled to land a vehicle on the surface of Mars in the summer of , a vehicle that will conduct the first rigorous search for life on another planet. The Viking lander carries three separate experiments on the metabolism of hypothetical Martian microorganisms, one experiment on the organic chemistry of the Martian surface material and a camera system that might just conceivably detect macroscopic organisms if they exist.

Intelligence and technology have developed on the earth about halfway through the stable period in the lifetime of the sun. There are obvious selective advantages to intelligence and technology, at least up to the present evolutionary stage when technology also brings the threats of ecological catastrophes, the exhaustion of natural resources and nuclear war.

Barring such disasters, the physical environment of the earth will remain stable for many more billions of years. It is possible that the number of individual steps required for the evolution of intelligence and technology is so large and improbable that not all inhabited planets evolve technical civilizations It is also possible-some would say likely-that civilizations tend to destroy themselves at about our level of technological development.

On the other hand, if there are billion suitable planets in our galaxy, if the origin of life is highly probable, if there are billions of years of evolution available on each such planet and if even a small fraction of technical civilizations pass safely through the early stages of technological adolescence, the number of technological civilizations in the galaxy today might be very large.

It is obviously a highly uncertain exercise to attempt to estimate the number of such civilizations. The opinions of those who have considered the problem differ significantly.

Our best guess is that there are a million civilizations in our galaxy at or beyond the earth's present level of technological development. If they are distributed randomly through space, the distance between us and the nearest civilization should be about light-years. Hence any information conveyed between the nearest civilization and our own will take a minimum of years for a one-way trip and years for a question and a response.

Electromagnetic radiation is the fastest and also by far the cheapest method of establishing such contact. In terms of the foreseeable technological developments on the earth, the cost per photon and the amount of absorption of radiation by interstellar gas and dust, radio waves seem to be the most efficient and economical method of interstellar communication.

Interstellar space vehicles cannot be excluded a priori, but in all cases they would be a slower, more expensive and more difficult means of communication. Since we have achieved the capability for interstellar radio communication only in the past few decades, there is virtually no chance that any civilization we come in contact with will be as backward as we are. There also seems to be no possibility of dialogue except between very long-lived and patient civilizations.

In view of these circumstances, which should be common to and deducible by all the civilizations in our galaxy, it seems to us quite possible that one-way radio messages are being beamed at the earth at this moment by radio transmitters on planets in orbit around other stars. To intercept such signals we must guess or deduce the frequency at which the signal is being sent, the width of the frequency band, the type of modulation and the star transmitting the message.

Although the correct guesses are not easy to make, they are not as hard as they might seem. Most of the astronomical radio spectrum is quite noisy. There are contributions from interstellar matter, from the three-degree-Kelvin background radiation left over from the early history of the universe, from noise that is fundamentally associated with the operation of any detector and from the absorption of radiation by the earth's atmosphere.

This last source of noise can be avoided by placing a radio telescope in space. The other sources we must live with and so must any other civilization.. There is, however, a pronounced minimum in the radio-noise spectrum. Lying at the minimum or near it are several natural frequencies that should be discernible by all scientifically advanced societies. They are the resonant frequencies emitted by the more abundant molecules and free radicals m interstellar space. Perhaps the most obvious of these resonances is the frequency of 1, megahertz millions of cycles per second.

That frequency is emitted when the spinning electron in an atom of hydrogen spontaneously flips over so that its direction of spin is opposite to that of the proton comprising the nucleus of the hydrogen atom. The frequency of the spin-flip transition of hydrogen at 1, megahertz was first suggested as a channel for interstellar communication in by Philip Morrison and Giuseppe Cocconi. Such a channel may be too noisy for communication precisely because hydrogen, the most abundant interstellar gas, absorbs and emits radiation at that frequency.

The number of other plausible and available communication channels is not large, so that determining the right one should not be too difficult. We cannot use a similar logic to guess the bandwidth that might be used in interstellar communication.

The narrower the bandwidth is, the farther a signal can be transmitted before it becomes too weak for detection.. On the other hand, the narrower the bandwidth is, the less information the signal can carry. A compromise is therefore required between the desire to send a signal the maximum distance and the desire to communicate the maximum amount of information.

Perhaps simple signals with narrow bandwidths are sent to enhance the probability of the signals' being received. Perhaps information-rich signals with broad bandwidths are sent in order to achieve rapid and extensive communication. The broad-bandwidth signals would be intended for those enlightened civilizations that have in vested major resources in large receiving systems.

When we actually search for signals it is not necessary to guess the exact bandwidth, only to guess the minimum bandwidth. It is possible to communicate on many adjacent narrow bands al once. Each such channel can be studies individually, and the data from several adjacent channels can be combined to yield the equivalent of a wider channel without any loss of information or sensitivity. The procedure is relatively easy with the aid of a computer; it is in fact routinely employed in studies of pulsars.

In any event we should observe the maximum number of channels because of the possibility that the transmitting civilization is not broadcasting on one of the "natural" frequencies such as 1, megahertz. We do not, of course, know now which star we should listen to. The most conservative approach is to turn our receivers to stars that are rather similar to the sun, beginning with the nearest. Two nearby stars, Epsilon Eridani and Tau Ceti, both about 12 light-years away, were the candidates for Project Ozma, the first search with a radio telescope for extraterrestrial intelligence, conducted by one of us Drake in Project Ozma, named after the ruler of Oz in L.

Frank Baum's children's stories, was "on the air" for four weeks at 1, megahertz. The results were negative. Since then there have been a number of other studies. In spite of some false alarms to the contrary, none has seen successful.

The lack of success is lot unexpected. If there are a million technical civilizations m a galaxy of some billion stars, we must turn our receivers to , stars before we have a fair statistical chance of detecting a single extraterrestrial message.

So or we have listened to only a few more than stars. In other words, we have mounted only. Our present technology is entirely adequate for both transmitting and receiving messages across immense interstellar distances.

For example, if the ,foot radio telescope at the Arecibo observatory in Puerto Rico were to transmit information at the rate of one it binary digit per second with a bandwidth of one hertz, the signal could be received by an identical radio telescope anywhere in the galaxy. By the same token, the Arecibo telescope could detect a similar signal transmitted from a distance hundreds of times greater than our estimate of light-years to the nearest extraterrestrial civilization..

A search of hundreds of thousands of stars in the hope of detecting one message would require remarkable dedication and would probably take several decades. It seems unlikely that any existing major radio telescope would be given over to such an intensive program to the exclusion of its usual work. The construction of one radio telescope or more that would be devoted perhaps half-time to the search seems to be the only practical method of seeking out extraterrestrial intelligence in a serious way.

The cost would be some tens of millions of dollars. So far we have been discussing the reception of messages that a civilization would intentionally transmit to the earth. An alternative possibility is that we might try to "eavesdrop" on the radio traffic an extraterrestrial civilization employs for its own purposes.

Such radio traffic could be readily apparent On the earth, for example, a new radar system employed with the telescope at the Arecibo Observatory for planetary studies emits a narrow-bandwidth signal that, if it were detected from another star, would be between a million and 10 billion times brighter than the sun at the same frequency.

In addition, because of radio and television transmission, the earth is extremely bright at wavelengths of about a meter.

If the planets of other civilizations have a radio brightness comparable to the earth's from television transmission alone, they should be detectable. Because of the complexity of the signals and the fact that they are not beamed specifically at the earth, however, the receiver we would need in order to eavesdrop would have to be much more elaborate and sensitive than any radio-telescope system we now possess.

One such system has been devised in a preliminary way by Bernard M. The system, known as Cyclops, would consist of an enormous radio telescope connected to a complex computer system. The computer system would be designed particularly to search through the data from the telescope for signals bearing the mark of intelligence, to combine numerous adjacent channels in order to construct signals of various effective bandwidths and to present the results of the automatic analyses for all conceivable forms of interstellar radio communication in a way that would be intelligible to the project scientists.

To construct a radio telescope of enormous aperture as a single antenna would be prohibitively expensive. The Cyclops system would instead capitalize on our ability to connect many individual antennas to act in unison.

The Very Large Array consists of 27 antennas, each 82 feet in diameter, arranged in a Y-shaped pattern whose three arms are each 10 miles long. The Cyclops system would be much larger.

Its current design calls for 1, antennas each meters in diameter, all electronically connected to one another and to the computer system. The array would be as compact as possible but would cover perhaps 25 square miles.

The effective signal-collecting area of the system would be hundreds of times the area of any existing radio telescope, and it would be capable of detecting even relatively weak signals such as television transmissions from civilizations several hundred light-years away.

Moreover, it would be the instrument par excellence for receiving signals specifically directed at the earth. One of the greatest virtues of the Cyclops system is that no technological advances would be required m order to build it. The necessary electronic and computer techniques re already well developed. We would need only to build a vast number of items we already build well. The Cyclops system not only would have enormous power for searching for extraterrestrial intelligence but also would be In extraordinary tool for radar studies If the bodies in the solar system, for traditional radio astronomy outside the solar system and for the tracking of pace vehicles to distances beyond the each of present receivers.

Moreover, the argument in favor of eavesdropping is not completely persuasive. Half a century ago, before radio transmissions were commonplace, the earth was quiet at radio wavelengths. Half a century from now, because of the development of cable television and communication satellites that relay signals in a narrow beam, the earth lay again be quiet.

Thus perhaps for only a century out of billions of years do planets such as the earth appear remarkably bright at radio wavelengths. The odds of our discovering a civilization during that short period in its history lay not be good enough to justify the construction of a system such as Cyclops. It may well be that throughout the universe beings usually detect evidence of extraterrestrial intelligence with more traditional radio telescopes.

It nonetheless seems clear that our own dances of finding extraterrestrial intelligence will improve if we consciously attempt to find it. How could we be sure that a particular radio signal was deliberately sent by an intelligent being? It is easy to design a message that is unambiguously artificial. The first 30 prime numbers, for example, would be difficult to ascribe to some natural astrophysical phenomenon. A simple message of this kind might be a beacon or announcement signal.

A subsequent informative message could have many forms and could consist of an enormous number of bits. One method of transmitting information, beginning simply and progressing to more elaborate concepts, is pictures.