The Search for Extraterrestrial Intelligence. Astrobiology: Understanding Life in the Universe, First Edition. Charles S. Cockell. © 2016 John Wiley &

The Search for Extraterrestrial Intelligence

Astrobiology: Understanding Life in the Universe, First Edition. Charles S. Cockell. © 2016 John Wiley & Sons, Ltd. Published 2016 by John Wiley & Sons, Ltd.

The Search for Extraterrestrial Intelligence (SETI)

It was inevitable that as soon as people began to wonder about life beyond the Earth, they also began to question whether that life might be intelligent.

In hindsight, given that to date the search for such signals has been fruitless, it is easy to laugh at this hypothesis. But sixty years ago, who knew? Maybe the radio and optical frequencies of space were full of signals from frustrated aliens trying to make contact. Only by at least seeking such signals can the hypothesis be tested. The Search for Extraterrestrial Intelligence (SETI) can have Earth-based spin-offs. For example, complex algorithms developed to search for signals in large amounts of data have wider use in developing computer technology and communications technology, for example processing information in many narrow waveband channels. However, we need no excuses for this endeavour, as it addresses one of the fundamental questions of the human scientific enterprise: are we alone in this vast Universe?


SETI

The Search for Extraterrestrial Intelligence seeks to find out whether there are other communicating intelligences in the Universe. This image shows the Green Bank radio telescope, West Virginia which has been used in SETI programs.


SETI: The Drake Equation

How many intelligent communicative civilizations are there in the Universe? We have no data to answer this question, but we can try to break down the question in such a way as to find out exactly what we need to know to be able to answer it. The first attempt to do this was made by Frank Drake in 1961. His famous mathematical relationship is a simple expression designed to specify the number of such civilisations in the galaxy (although it can be applied to scales beyond the galaxy). The equation is shown below: N = R* . fp. ne . fl. fi. fc. L where the terms are:N = the number of civilizations in our galaxy with which communication might be possible R* = the average rate of star formation in our galaxyfp = the fraction of stars that have planetsne = the average number of planets that can potentially support life per starfl = the fraction of planets that could support life and that develop lifefi = the fraction of planets with life that go on to develop a civilizationfc = the fraction of civilizations that develop a technology that produces detectable signs of their existenceL = the length of time for which such civilizations release these signals into space.


SETI: Methods

However the first serious attempt to pick up a signal using radiowaves from intelligences was Project Ozma in 1960, pioneered by Frank Drake and run from the Greenbank Telescope. About 150 hours of total observing time were spent studying the Sun-like stars Tau Ceti and Epsilon Eridani. At nearly the same time a paper published by Cocconi and Morrison in 1959 advocated the possibility of finding alien signals. During the 1960s, Soviet scientists also launched a variety of projects. Since that time, other efforts have been devised, including SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations), a project that was initiated by the University of Berkeley, California. None of these projects found signals.


SETI: Methods

Frank Drake, a pioneer in SETI searches and originator of Project Ozma.


SETI: Methods

Today most SETI projects either use spare time on radio-telescopes or, like SERENDIP, they piggy-back – collecting data whilst other more conventional radio-astronomy is being performed. Clever approaches to data analysis have included SETI@Home and SETILive, in which personal computers in people’s private homes are linked across the internet to collectively analyse the huge quantity of information gathered by radio-telescopes.

New telescopes offer other possibilities for SETI. The Square Kilometer Array (SKA) is proposed as a large number of dishes in South Africa and Australia with an effective total radio dish size of a square kilometre. The facility might be used for SETI, searching within 300 light years for radio leakage similar to that given off by our civilisation.

A question that faced the SETI community from the early days is at which frequency should they search?


SETI: Methods

A favoured search location has been at 1420 MHz (21 cm wavelength). This was the wavelength used in Project Ozma. Hydrogen atoms have one proton and one electron. Crudely, in analogy to two planets, their spins can be in the same direction or opposed. When they flip from one state to another a photon is emitted at 1420 MHz. Hydrogen is common in the Universe, so we suppose that alien astronomers might want to look in that region. Near to this frequency is the frequency (1640 MHz) associated with hydroxyl radical (OH) emission. Hydrogen combined with OH produces water (H2O). Just as animals collect at the water hole in the desert, so in analogy we might expect that communicating civilizations might collect at this cosmic radiowave ‘water hole’. The atmosphere is relatively transparent to these frequencies, making them quite detectable. Although space telescopes eliminate the theoretical significance of this advantage, they are expensive. This wavelength has been used in many SETI searches, but it is by no means the only sensible place to search. One permutation is to search at frequencies obtained by dividing or multiplying the hydrogen frequency by constants such π, since π is a universal number that could be known by other intelligences.


SETI: Methods

The hydrogen and hydroxyl frequencies provide one location in the electromagnetic spectrum (the ‘water hole’) where attempts have been made to search for alien signals. This region is also relatively transparent for observation on Earth. The orange denotes the ‘noise’ which can be considered here to be the absorption caused by the atmosphere. Axes are logarithmic scales.


SETI: Methods

More recent searches have turned to ‘optical SETI’, attempting to pick up signals that could have been transmitted in laser pulses in our direction. Harvard University initiated such a programme. Human civilisation has built immensely powerful lasers that can achieve a brightness about five thousand times greater than the Sun. Perhaps other civilizations are using such technologies to send messages. A problem that optical SETI faces is that there is a great deal of noise in the optical region of the spectrum. More recent ideas have included infra-red SETI, which is less prone to dust absorption in the interstellar medium.


Communication with Extraterrestrial Intelligence

There have been two notable attempts to send physical materials as messages. The Pioneer 10 and 11 spacecraft, sent to Jupiter and Saturn, both took with them a gold anodized aluminium plaque 23 cm wide and 15 cm high. These craft are now greater than 80 AU from the Earth.



A second attempt to send a material message was made in 1977, when two 30 cm diameter gold-plated copper records were attached to the Voyager 1 and 2 probes. On both records is an ultra-pure sample of uranium-238 to allow for radiometric dating.

The 116 images on the record are encoded in analogue form and composed of 512 vertical lines.

The images included mathematical and physical quantities, the Solar System and its planets, the structure of DNA and information on human anatomy. Pictures of landscapes, plants, and insects and other animals were included. Images of humanity show people going about their lives and there are images of food and architecture. The record contained a variety of sounds, such as those made by wind, surf, thunder and the sounds of animals including whales and birds. The musical selection included Mozart, Bach, Beethoven, Stravinsky, Chuck Berry and other musical selections from around the world.



The Voyager record.



The most distinctive attempt to transmit an electromagnetic message was the Arecibo message. On November 16, 1974, a 1000 kW message was sent to the globular star cluster M13, 23,000 light years away and transmitted at 2380 MHz. The message was sent for a duration of three minutes and was made of 1679 binary digits. This number was chosen because it is the unique product of the two prime numbers 23 and 73. Binary notation is used as the simplest way of communicating information. When the message is split in this way it provides a set of information about us and our civilisation.



The key components of the message in order from the top are: 1) The numbers one to ten, 2) The atomic numbers of the elements hydrogen, carbon, nitrogen, oxygen, and phosphorus, which make up DNA, 3) The formulas for the sugars and bases in the nucleotides of DNA, 4) The number of nucleotides in DNA, and a graphic of the double helix structure of DNA, 5) The height of an average human, a graphic figure of a human, and the human population of Earth, 6) A graphic of the Solar System indicating which of the planets the message is coming from, 7) A graphic of the Arecibo radio telescope and the diameter of the transmitting antenna dish.


The Fermi Paradox

The search for signals from other civilizations has not yet yielded a positive response. It is easy to simply dismiss this result, but it does raise a very perplexing problem that was recognised by physicist Enrico Fermi (1901-1954). We live in a Universe of maybe over 100 billion galaxies. Our own galaxy has 200 billion stars. There are many stars, both in our own galaxy and elsewhere, that are much older than our Sun.

So from a purely qualitative consideration of these statistics, one might suppose that, assuming the origin of life has occurred elsewhere at a reasonable frequency, there is a high likelihood that not only would there be other civilizations, but that some of them could be very considerably older than ours. In less than half a million years we have come from the Great Rift Valley in Africa to building spaceships. If some of these civilizations were a few million years older still, surely they would be travelling the galaxy? Why, then, is alien contact not an everyday and rather mundane event?


The Fermi Paradox

There have been a very large number to responses to the so-called Fermi Paradox.

Civilizations are too far apart in space. No other, or very few, civilizations have arisen. Intelligent life destroys itself. Life is periodically destroyed by natural events. It is the nature of intelligent life to destroy other civilizations. They exist, but we see no evidence of them. They are in the local area, but observing us rather than attempting to make

contact. They are too busy online. They are here.

We could go on with a few more of these ideas, but the intellectual return gets incrementally less. The main point to understand is that there are a variety of responses to the Fermi Paradox. Some of them, unfortunately, are not amenable to experimental testing, making those interesting ideas, but little more. Nevertheless, the overarching discussion is not in vain. It remains the reality that we see no evidence for aliens and we need to find the definitive reason for that fact.


Classifying civilizations

Let us say that regardless of the Fermi Paradox, civilizations are out there. How might they differ from us? There have been a number of attempts to classify civilizations. The Kardashev scale is a method of measuring a civilization's level of technological advancement, based on the amount of energy it can use. The scale has three categories that are designated Type I, II, and III civilizations. The scale was first proposed in 1964 by the Russian astronomer, Nikolai Kardashev.

A Type I civilization uses all available resources on its home planet with an energy accessible to it of about 1012 Watts. Human society is still within the bracket of an emerging Type I civilization. A Type II civilization harnesses all the energy of its star, giving it the capacity to harness energies equivalent to the luminosity of our Sun, about 1026 Watts. A Type III civilization has managed to grapple with all the available energy in its galaxy – about 1037 Watts. It would achieve this capacity by building Dyson spheres around many stars in a galaxy or more dramatically, it would be able to tap into the energy released from supermassive black holes at the center of most galaxies.


Policy implications

Even the remote possibility of contact with an alien intelligence raises a whole variety of questions. One such question is: Who should represent the Earth in the event of communication?

Who should coordinate response to alien contact? The United Nations or other human representatives?


What have we learned?

Various methods have been used to search for extraterrestrial intelligence, including radio and optical SETI.

Our attempts to communicate have been more limited. They are split into two types – electromagnetic messages and physical messages.

There remains the problem of The Fermi Paradox which seeks to understand why we are not visited if there are other (older) civilisations.

Whatever we think about extraterrestrial intelligence and the probability of its existence it may still be worth considering protocols for detection, just in case….

So far we have no signal from an extraterrestrial intelligence.

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The Search for Extraterrestrial Intelligence. Astrobiology: Understanding Life in the Universe, First Edition. Charles S. Cockell. © 2016 John Wiley &