SETI: peering into the future

Abstract

Part of the SETI Institute's 42-telescope Allen Telescope Array (ATA) in California. (SETI Institute) Part of the SETI Institute's 42-telescope Allen Telescope Array (ATA) in California. (SETI Institute) The first Search for Extra—Terrestrial Intelligence (SETI) project of the modern era was done by Frank Drake in the spring of 1960, using the Green Bank 26 m telescope. He was looking for narrow—band radio emission from two nearby stars, t Ceti and ε Eri, over a frequency range of 400 kHz near the H i line. Since then there have been six major and many minor searches, made both on specific targets and also over the entire sky. The searches have extended to the optical and infrared, and to search for artefacts in the solar system and beyond. There have also been more than a thousand papers in the scientific press. The searches have all come up negative. What does this mean? Can future searches extend in a significant way the present area of the “ETI phase space” that has been searched for the existence of extraterrestrial intelligences (ETIs)? This article will briefly describe some of the component parts of SETI and put forward the case that SETI does indeed have an exciting future. SETI has two main aims. There is the expanding exploration of that phase space, always with the possibility of “contact” and the leap forward in our understanding of life in the universe and in many other fields of science and culture that would result. But SETI also addresses the future of humankind, looking for other civilizations that have trodden this path before us. If we find them, then we will know there is a possible way forward. If a particular SETI search comes up with a negative result, then we know that our future may not include the path that that search would have revealed. SETI activity has other components. It involves studies of: how evolution leads from the origin of life to intelligence; the rise and nature of technological civilizations; the problems of communications with fundamentally different entities; the possibilities of interstellar travel. It provides a logical extension to the growing field of astrobiology. Like all high—tech work, it has spin—offs, such as the Berkeley BOINC system of grid computing, originally designed to deal with the flood of SETI data from the Arecibo telescope with the SETI@home project, and which is now used in many fields, including medicine, molecular biology and climatology. And SETI provides a powerful forum for engaging with the public on the nature of scientific studies, using a subject in which the public is already interested. We know very little for sure. We know from our own example that technological civilizations can arise and persist for thousands of years, and send both “leakage” and deliberate radio and optical signals of their existence out to the galaxy. We know that our civilization arose in the last 10% of the age of the Earth before the increase in the Sun's output will render the surface of the Earth uninhabitable. We know that no ETI has left evidence of its existence in any of the searches that have been made, on the Earth, in the solar system, or further afield. Our existence means that other civilizations could exist, but gives no indication of their probability. Our recent knowledge of extraterrestrial planets suggests that Earths hospitable to life are common. However, since we do not know how life started, we do not know if life is common, rare, or if the Earth is the single case. Within the next decades the study of the atmospheres of Earth—like planets may resolve this point. However, we next do not know the probability that once life has started whether it then evolves to a technological civilization. We cannot say that evolution is bound to produce intelligence, and we are unable to predict the nature of other technological civilizations, or how long such civilizations exist. Civilizations thousands, millions, or billions of years older than ours could be of a very different nature to our own. However, the late arrival of intelligence on Earth is most compatible with an average time for the arrival of intelligence being much longer than the lifetime of stars, and thus with us being alone. But this is only a probabilistic pointer, not a proof. The view shows the first LOFAR station to be built in the UK at STFC's Chilbolton Observatory, with the Low Band Array in the foreground. Unlike conventional radio telescopes, there are no moving parts, but steering of the telescope is done in software. When completed, LOFAR will consist of more than 5000 separate antennas spread in ‘stations’ all over Europe. The project is based in the Netherlands where the core of the array is located. (STFC/SEPnet) The view shows the first LOFAR station to be built in the UK at STFC's Chilbolton Observatory, with the Low Band Array in the foreground. Unlike conventional radio telescopes, there are no moving parts, but steering of the telescope is done in software. When completed, LOFAR will consist of more than 5000 separate antennas spread in ‘stations’ all over Europe. The project is based in the Netherlands where the core of the array is located. (STFC/SEPnet) The lack of evidence of ETI is known as the Fermi Paradox. Once a civilization gets to our stage, it would only be a short time before it could build Von Neuman probes — autonomous self—replicating space probes — whereby every planet in our galaxy could be visited within a few tens of millions of years. The simplest explanation for the fact that we do not see such probes here, and that none of our searches have found signs of ETI, is that we are alone. However, there is no lack of credible alternative explanations of how the existence and even widespread existence of ETI would be compatible with the negative search results. These searches have as yet only explored a very small fraction of ETI phase space. So although there is some indication that we are alone, all we can presently say is that it is possible that ETI is out there, but we cannot with any degree of certainty predict how often ETI arises, or what their natures or lifetimes would be. The most common way of looking for ETI is to look for narrow—band radio emission. Our civilization emits such radiation from the 1 Hz wide carrier beam of analogue TV stations through to the kilohertz wide emissions of such things as airport radars. ETI may also emit such leakage radiation, although present searches are only sensitive to much more powerful radiation than we presently emit. Narrow—band radio waves are also the cheapest and most efficient method of interstellar communication that we know of, and so may be ETIs' way of communication, and even of signalling their existence to us (“beacons”). The narrow—band signature can also be distinguished from natural sources, even rare natural narrow—band ones such as masers. The Harvard and Argentinian searches with 26 m telescopes covered the entire sky, and the Arecibo “piggy—back” survey covered some 25% of the sky. But these have integration times of only a minute or so. The SETI Institute among others has done many longer integrations on individual targets such as nearby stars. Searches have become more powerful as receivers, electronics and data handling and analysis software improve, as for example in the billion 1 Hz spectral resolution channels of the 42—telescope Allen Telescope Array (ATA). The most recent surveys are now a trillion times more capable than Drake's 1960 observations. Following the recent development of high—powered lasers, which in theory could be matched with telescopes to outshine the Sun in nanosecond pulses, searches have started to look for such ETI signals in the optical. Pointed observations at Berkeley and Lick and an all—sky survey at Harvard are now looking for such nanosecond pulses. Again these are distinguishable from natural sources. If an ETI were using one of our most powerful lasers and a 10 m telescope, these searches would pick them up from hundreds of light—years away. More exotic radiation sources, such as the neutrinos from supernova SN1987A, are also investigated for signs of an artificial nature. The most famous such search was in 1967 when the Cambridge pulsar discovery team checked that the pulses had no sign of orbital motion. Different searches have different aims, usually based on some sort of premise of the nature of ETIs. The most obvious choice is of nearby long—lived stars, where ETIs on planets have had time to evolve. Such searches range from Drake's observation of two such stars in 1960, to the million stars planned for ATA. Since stars can differ in ages by billions of years, and ETIs take an unknown time to emerge, a search of a million stars gives a chance of picking up an ETI radiating for a thousand years, which may be a reasonable estimate of the time until an ETI changes into a fundamentally different mode. Then there are the all—sky surveys and surveys of areas of the sky, such as the galactic centre, where no presumption is made of where ETI is — on or off planets, near or far. These necessarily have shorter integrations per pointing, so are sensitive to rarer but brighter sources. The extreme of this is surveys of other galaxies, looking for extremely bright sources, but sources so rare that there is not one in our own Milky Way. There are also specialized searches. A recent proposal is for a search on the ecliptic plane, where an ETI would have been aware for a long time, using the radial velocity and transit planet detection methods, that there is an Earth in orbit around the Sun. Perhaps this would prompt them to signal to us. Searches have also been done looking for artefacts of an ETI civilization. The most famous of these are Dyson spheres, where an ETI surrounds a star with solar panels, probably on many discrete mounts, to tap a significant fraction of the star's energy. The outsides of these panels will be cool, shining in the infrared. Each new infrared catalogue that comes out is scanned for objects of strange non—natural looking colours. There have been searches for strange colours in the asteroid belt objects which might indicate an artificial nature, and for objects in the unstable Earth—Moon L4 and L5 Lagrangian points. There are notoriously many “sightings” of UFOs, which all have either been explained or have not contained enough information to determine their natures. The most interesting ongoing scientific investigation is the Norwegian Hessdalen Valley Project where there have been repeated sightings. The main limit on these searches is funding. There are almost no public funds. Very little sustained work is done outside the US, and within the US the main work is done through private funding and the efforts of determined individuals at Berkeley and Harvard. The SETI Institute, which grew out of the NASA work of the 1970s and 80s, is privately funded and the Berkeley and Harvard projects are done from within radio astronomy and electronics groups with university funding and private support. Outside radio and optical searches there is almost no concerted academic work on the other areas of ETI phase space such as solar system searches or catalogue analysis. Theoretical work depends on the intermittent interest of individuals. There is a lack of resources to fund fresh blood. Over the past 50 years there have been hundreds of papers describing the capabilities of searches and suggesting new methods. There have been as many speculating about the existence, origins, lifetimes and natures of ETIs, about composing and decoding messages, the prospects for interstellar travel and many allied matters. There has been much cross—fertilization with other fields including biology, philosophy, spaceship propulsion, linguistics and planetary science. Is intelligence a convergent property, etc? Some pointers to this extensive body of literature are given in the “Further reading”. An important field for SETI is the evolution of intelligence. Once life is started, does it then always evolve to intelligence? Intelligence seems such a useful attribute that evolution would home in on it, but for two billion years bacteria reigned alone. Since then there have been millions of species on Earth, out of which only one, us, has evolved advanced technology. Were we inevitable? Is evolution convergent? And then there is the “Man from Mars” problem, as it is known in linguistic studies. Can there be ways of communication that are so fundamentally different from our own that the message may be incomprehensible? Concepts such as “signs” and “signifiers” may not be present. How would a communication system based on smells be coded into a radio message? A standing controversy is whether it is dangerous to send out signals. In fact any advanced ETI would probably know about us already from our various radio emissions of the past six decades, or from visible signs such as the existence of our cities over the past four thousand years. And because we do not know about the nature of any ETI, a signal might either provoke or forestall an attack by any ill—intentioned ETI. So there is no reason not to transmit. But in any case there is probably presently little point, as signalling for thousands of years would be needed to give the class of ETIs not much more advanced than us a reasonable chance to pick us up. (Only such ETIs would not necessarily know all about us already.) In studying the future of humankind, we already know that certain classes of ETI, those that our searches would have picked up, are not common. How much does that tell us about the long—term evolution of civilizations like our own? Will we become a civilization that SETI searches could detect? Will we survive the bottlenecks of the near future: global warming, nuclear war, biological terrorism, grey goo, a catastrophic meteorite strike, the rise of the machines? In the more distant future, will we establish self—sustaining colonies off the Earth that will lessen our vulnerability? In the very distant future, will we become a race that can persist for a million or a billion years? SETI provides an avenue, the only observational avenue presently available to us, for exploring these puzzling questions. An example of how SETI thinks about our own future is the “Great Filter”. Taking from the Fermi Paradox that advanced ETIs are not common, Hanson (1998) pointed out that in the progress from star formation to such ETIs there must be a limiting pinch point. If this is behind us, then we are one of the extremely rare cases to have got this far, and our future prospects are not limited. But if it is in front of us, then we will very probably be extinguished. Paradoxically, discovery of ETIs like us, but not too advanced, would be bad news, as then it must be easy to get as far as us, and the Great Filter must be in front, and quite close. If we detect an ETI, what would happen next? First of all, there is the getting out of the news, and present SETI searchers subscribe to the International Academy of Astronautics SETI Permanent Study Group's “Post—detection Protocol”, which basically says “be sure, have it confirmed, and then spread the news widely”. No signal should be sent back until international agreement has been reached. In practice, the experience of search groups is that, when investigating ambiguous signals, the news can leak out in an uncontrollable way. What happens next would depend on the origin and nature of the signal. A solar system detection would have its own possibilities and problems. The result of a radio or optical detection of a distant source would depend on its nature. A continuous narrow—band signal which simply says “I am artificial” would revolutionize the scientific field and trigger funds for a great search for more details. Does it show signs of orbital motion? Is it associated with a star? It would also trigger public and philosophical interest. It is generally thought that the public would be intensely interested, but would not overreact. However, if there were to be some sort of code seen in the signal, then as well as the scientifically fascinating cryptological and linguistic tasks of finding out what the message is, the public interest would be overwhelming. Coming from an advanced civilization, does the message tell us how to behave, explain about religions, contain a cure for cancer? Is there some sort of danger in the message? If we respond, how do we have a conversation that may involve time lags of centuries? Astronomers would be interacting with the community in ways that are difficult to envision. Given the negative results so far, is it worth going on? We do not know what ETIs are like, so we cannot say how large the phase space of possible ETIs is and thus we neither know if we are looking in the best way nor what our chances of success are. Many SETI searchers remain optimistic. The quotation from Cocconi and Morrison's 1959 foundational paper that “The probability of success is difficult to estimate; but if we never search, the chance of success is zero” has many supporters. However, without knowing the nature of ETIs we cannot estimate by how much we improve our chances by any particular SETI search. Within the next decade we should be able to rule out (or discover) leakage radiation similar to our own from nearby habitable Earths — but the chance of hitting the perhaps thousand—year window for such radiation for a planet millions or billions older or younger than us must be very small. The author's personal opinion is that although we cannot know what our chances are it would be a failure of nerve not to go on looking, as long as each new search does cover significant new phase space at a reasonably modest cost. Planned radio searches will get more powerful, from the privately funded ATA array partly dedicated to SETI searches, to the use of new telescopes such as the European LOw Frequency ARray (LOFAR), for which the author is PI on a SETI Pilot Programme, and the South African 64—dish MeerKAT array, which has recently announced that it wishes to “explore further the potential for SETI”. And there is the giant Square Kilometre Array on which funding are to advanced receivers, electronics and software we are for giant forward. A major present is in the electronics and and so an in with and telescope that to and all—sky with in the other of searches, such as the optical new solar system searches of new and into the next decade is with possibilities to extend the ETI phase space we are by the almost lack of public funding. When to the public about SETI and tell them that almost none of their astronomy to SETI are that such an interesting field is being If the panels of the astronomy funding were to to fund SETI at a of one of one of their SETI would be and much more powerful and searches could be would be an thought for us all — that we were the search and in this into the unknown the race is looking