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. Finding these civilizations is another issue entirely — scientists said they would be thousands of light years away. Our current technology makes it nearly impossible to detect or communicate with possible alien life.
Scientists said that searching for extraterrestrial intelligent life could give us insight into how long our own civilization can survive. The more civilizations we find close to home, the better the chances for humans' long-term survival. Sophie Lewis is a social media producer and trending writer for CBS News, focusing on space and climate change. Alien life in our universe? Sophie Lewis. This prior, he says, could be another instance of selection bias—a notion influenced by the evolutionary pathway by which our own intelligence emerged.
That suggestion is exactly what Kipping attempted, estimating both the probability of abiogenesis and the emergence of intelligence. For a prior, he chose something called the Jeffreys prior, which was designed by another English statistician and astronomer, Harold Jeffreys.
It is said to be maximally uninformative. Turner and Spiegel had also tried to find an uninformative prior. In their analysis, the researchers employed three priors, one of which was the least informative, but they fell short of using Jeffreys prior, despite being aware of it. All four corners were equally likely before the Bayesian analysis began. Turner agrees that using the Jeffreys prior is a significant advance.
Combining the Jeffreys prior with the sparse evidence of the emergence and intelligence of life on Earth, Kipping obtained a posterior probability distribution, which allowed him to calculate new odds for the four corners.
And even if intelligence is not rare, the life-is-common scenario has a minimum odds ratio of 9 to 1. Those odds are not the kind that one would bet the house on, Kipping says. Not all Bayesian statisticians would agree. Turner, for one, interprets the results differently. Anyone interested in questions about the origin of life would be skeptical about claimed answers, given that any such analysis is beholden to geologic, geophysical, paleontological, archaeological and biological evidence for life on Earth—none of which is unequivocal about the time lines for abiogenesis and the appearance of intelligence.
If we did have rigorous definitions, problems persist. When did Earth become hospitable to life? All this sketchy evidence makes even Bayesian analysis difficult. One or two more data points, and suddenly, we know about, essentially, the universe in terms of its propensity for producing life or possibly intelligence. And that's rather powerful. Credit: Nick Higgins. Already a subscriber? Sign in. Thanks for reading Scientific American.
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