Communication StrategiesASSUMPTIONS | WHY RADIO? | FREQUENCIES | OPTICAL METHODS | LANGUAGE | CONTENT | PIONEER/VOYAGER HistoryOnce there is an acceptance of the "plurality of inhabited worlds", then it becomes a natural desire to make contact with alien races. The speculations on life in our own solar system earlier this century led eminent scientists to propose a series of signalling attempts to attract the inhabitants of Mars and Venus. Sir James Jeans suggested that powerful searchlights should be aimed at Mars during its close approach of 1941 that would flash out prime numbers. There were also plans to mark out a gigantic 3,4,5 triangle in the Sahara desert to inform Martians of our geometric pre-dispositions. These schemes might seem naïve by today’s standards, but they were serious attempts to clarify an hypothesis. At its most optimistic, the Drake Equation gives us the hypothesis that there should exist some 106 civilisations at our level or beyond within the Milky Way. How, then, should we begin a dialogue? AssumptionsOne very useful assumption to make is the Copernican Principle. This states that we occupy no privileged position in the Universe, and that we should expect to find that our locality is more typical of the Universe in general, rather than unique. This may not be the case where intelligent life is concerned, but as far as the principles of physics go it ought to hold well. In fact the greatest assumption that is made in astronomy is that the laws of physics are the same in all parts of the Universe. This is borne out by experiment since the starlight we analyse from even the most distant parts of space be actly as it does here in the terrestrial laboratory, and the processes that created it can be duplicated (mostly) on Earth. In fact if the laws of physics were even slightly different, their large-scale effects would be dramatic enough to detect easily. So, with this in mind, what should we ask of an ideal information carrier? Whatever carrier we choose to use, ideally it should have the following properties:
The first and third properties are more important to our technology than some more advanced race, but the second and fourth points should be desirable whatever the source. To clarify the effect of the level of technology on the type of signal we might expect to receive. Kardashev (1964) proposed that civilisations be classified into 3 categories according to their energy usage:
Clearly to have the luminous output of an entire galaxy at your disposal will not really constrain either the content or reach of the information you wish to transmit. Contrariwise, we must seriously consider deploying the limited energies we can muster to maximum effect and adopt a strategy that will make the most of our current technological capability. We have two options in establishing communication with any possible ETI. If we assume they are at a level beyond our own and actively engaging in their own search for intelligent life, then we should listen carefully. Of course if every civilisation is thinking the same way and engaging in passive detection, then nothing much will happen! The alternative is to advertise our own presence to the universe by pursuing an active strategy. Why Radio?James Clerk Maxwell synthesised the electric and magnetic forces in his elegant electromagnetic equations at the end of the C19th. These equations predict that an oscillating dipole will produce electromagnetic waves that can travel freely through space at the speed of light - radio waves. It was Marconi who first harnessed this effect, and both he and Tesla created controversy by claiming detection of intelligent signals of extraterrestrial origin. The discovery of radio waves will be mirrored throughout the intelligent universe. Using this technology to communicate on the host planet will lead to the recognition of radio sources in the sky and the development of radioastronomy. The overriding condition for detectability is that the signal should be greater in amplitude than the background radio "noise" produced by the parent star, in our case the Sun. The Sun’s output is large and spreads across a broad frequency band, but expands spherically from the source, and so obeys an inverse square law of intensity fall off. If we can collimate the radio emission into a tight beam, then it will lose proportionally less of its intensity with distance, and will appear brighter than the Sun in a specific frequency band. We can calculate the expected range of an emitter based on Earth using contemporary technology. Assuming the Sun radiates as it does during a "quiet" phase in its 11 year cycle of activity and considering just emission at 10cm wavelength (3 GHz), the Rayleigh-Jeans approximation for a black body gives the power output per unit frequency as Ws = 4pRs2(2pkTs/l2) Where Rs is the radius of the Sun, k is Boltzmann’s constant, Ts is the "brightness temperature" of the quiet Sun at this wavelength (50 000K) and l is the wavelength. This gives a value of some 2.6 kW/Hz. To create a directional beam using a paraboloidal antenna, the beam spread can be reduced by using the largest diameter (D) of antenna possible, since there is the approximate relation that the angular diameter of the cone of radio waves produced is given by a = l/D. The gain of any transmitter is defined as the ratio of power received in the direction the dish is pointing compared to the smaller amounts of power received off axis (due to secondary lobes). We define this as follows: G = 4pA/l2 Where A is roughly the area of the transmitter dish. So at 10cm wavelength a modest (100m) antenna will give a gain of some 107 allowing it to produce ten million times more power in the direction it was pointing than the Sun at this wavelength. This would mean that a signal would only have to have a power of 10-4 W to equal that of the Sun. Today’s technology is capable of producing megawatts of radio power in very narrow frequency bands, so it is a relatively easy matter to outshine our Sun. But how far away could these waves be detected against the background noise emanating from other sources in the galaxy? The ability to discriminate is given by the antenna temperature TAof the receiver: TA = (p2/16k)(W/r2)(D12D22/l2) Where W is the power per unit bandwidth, r is the distance between source and receiver, D1 the diameter of the transmitting dish, D2 that of the receiving dish. So in order to extract the signal from the background noise, it must have a value > TB (the brightness temperature of the sky at that wavelength, which is of the order of 10K at 10cm). For two 100m dishes one transmitting at 100 W/Hz, then the transmission range is ~10ly. The Arecibo dish at Puerto Rico is the world’s largest semi-steerable radio telescope and has a diameter of 305m. Transmitting at a power of 1MW/Hz to another similar installation would give a range of some 100 000 ly. – this is comparable with the diameter of our galaxy! So in theory, even at our modest level of technology, we can send and receive signals from anywhere in the Milky Way at radio frequencies. This is very encouraging. There is a relationship between the range r and integration time (t) required to distinguish the signal from the background noise, namely that r a t0.25 Going back to Kardashev’s civilisation types, then a Type I civilisation (us) could transmit the contents of a 100 000 average sized books across 100 ly. In a few days. A Type II civilisation could do this across the galaxy in only 100s, whereas an extragalactic Type III civilisation could transmit this information over 10 billion ly. in only 3s! We have inadvertently been sending signals out in all directions ever since the start of powerful radio and TV broadcasts, but these weaken considerably by the time they reach the edge of our solar system. There is a chance that the stronger of these signals could be picked up at greater distance, and in fact the televised 1936 Olympic Games has travelled out a distance of 62 ly (an idea at the heart of Carl Sagan’s novel Contact). What Frequencies?We have learned from our recent foray into this subject (only 50 years so far!) that there are preferred frequencies for transmission and reception due to the following factors:
Higher frequencies (>10 GHz) get absorbed by our atmosphere, whereas low frequencies (<0.5 GHz) are swamped by galactic noise. There is a radio "window" covering a frequency interval of 1 - 10 GHz with a minimum absorption around 2 GHz, which presumably would be shared with any other planetary based technical civilisation. Cocconi and Morrison proposed a galactic frequency standard of 21cm or 1.423 GHz - the magic "hydrogen line", since any galactic astronomers would be surveying this dominant emission band as they mapped out the clouds of hydrogen in the disc of the Milky Way. There is a problem with this in that the frequency could be shifted by the Doppler effect due to:
In order to be received at the hydrogen line frequency, any targeted signal would have to be continuously shifted in emission frequency to account for all these factors. Conversely, the reception of a fixed frequency from another planet might necessitate a search over quite a wide frequency band to catch the Doppler shifted signal. One could assume that any technological civilisation would correct their emission for this, but it might not be the case. Integer multiples or fractions of the hydrogen line might be the preferred choice since this implies intelligence, we could even broadcast at non-integer multiples of this (e.g. p x hydrogen, Sagan & Schlovskii) The hydroxyl radical (OH) is the second most abundant feature detected in the interstellar medium. It has a group of four lines in the band between 18 and 21 cm (1.667 and 1.420 GHz). This has come to be known as the "water hole" and some astronomers believe this could be an interstellar meeting place. For communication outside the galaxy we have to add in the effect of cosmological expansion, which increases with distance according to Hubble’s Law. For this reason, extragalactic searches in the hydrogen line would be very time consuming. Drake & Sagan (1973) and Gott (1982) proposed a "cosmological frequency standard" based on the cosmic microwave background radiation discovered by Penzias and Wilson in 1965. This afterglow from the Big Bang is uniform and of black body character, peaking at a frequency of 56.8 GHz. This corresponds to a temperature of 2.726K and the mean fractional variation with direction in the sky is extremely low (~10-3). Recent results from COBE have identified the peak emission frequency to one part in 105 which means a search in this band would only have to cover a bandwidth of some 400 MHz centred on 56 GHz. The great advantage of choosing this frequency is that any signals sent to another galaxy would be stretched by exactly the right amount to be detected as the local background frequency, which would have reduced correspondingly in the time it takes the signal to arrive. again, multiple of fractional variations of the standard could be employed, which would overcome detection problems (28.4 GHz is detectable from the ground, 56.8 GHz requires spaceborne detectors as a result of strong absorption at 60 GHz due to atmospheric oxygen.) There are now good arguments for a multiplicity of frequencies throughout the entire SETI window which stretches from 1 to 10 GHz. Interstellar dispersion of radio signals indicates that it is pointless to transmit with a bandwidth of less than 0.1 Hz. This then gives a possible total of 100 billion frequencies! Optical MethodsAre we restricted to the radio part of the electromagnetic spectrum?
With the advent of highly directional monochromatic sources (lasers),
is it possible to send an optical message out to the nearest stars that
would seem brighter than our Sun? Calculations based on a similar premise
to those above would indicate that even with high powered (10kW) sources,
the beam dispersion would be so great as to render the apparent magnitude
of such a source at a distance of 10 ly.to be +21, compared to the Sun's
apparent magnitude of +2.2. In short the signal would be some 30 million
times fainter than the Sun – not too encouraging. It would be possible,
using a large telescope, to pick this signal out from the background glare
due to its monochromaticy, especially if the frequency was chosen to match
one of the dark Frauenhofer absorption bands visible in the Sun. Detectors
would need to be equipped with high resolution spectroscopes scanning
for "artificial lines" in the spectrum of a star.
Most of the benefits of operating in the visible spectrum are the result
of light having a smaller wavelength and higher frequency than microwave
radiation. For instance, compared to the large antennae needed for radio
wave emission, lasers are extremely small and light. In addition, due
to the higher frequency of light waves (43,000GHz for red light vs. 1.4GHz
for microwaves) allows extremely fast data transfer. The rapid pace of development in optoelectronics has been driven by information technology here on Earth, and as we come to depend increasingly on optical telecommunications networks, the next generation of computers will exploit the speed and bandwidth advantages of optical components. Optical links between the Moon, Mars and Earth will be feasible, and the future will be photonic. Whether this rapid pace of change will throw up optical generators that will operate at frequencies and powers that allow interstellar communications is not known, but optical searches are being conducted by amateurs in the belief that these barriers have been overcome by more technological races. Until recently the SETI community has not invested too much effort in optical detection, but there have been several initiatives on both the professional and amateur fronts in the last few years, most notably Columbus University in Ohio with their COSETI programme. What Language?Joseph Fourier, the famous French physicist has summed up the position of the SETI community to this question in the following statement: "There cannot be a language more universal and more simple, more free from errors and obscurities ... more worthy to express the invariable relations of natural things [than mathematics]. It interprets [all phenomena] by the same language, as if to attest the unity and simplicity of the plan of the universe, and to make still more evident that unchangeable order which presides over all natural causes" The incredible intricacy and nuance of natural language has enriched
human experience by allowing effective communication of complex subjective
ideas. This same subtlety is also guilty of causing terrible misunderstanding,
as well as being the plasticene of politics. We must be clear that an
objective and accurate language must be employed in our communication
strategy, and what we lose in the richness, we will gain in the understanding.
Mathematics has been described as the only pure science in the sense that
it is abstracted from the physical world, and according to the Platonic
ideal, the truths of mathematics are out there existing as independent
"things". Having ten digits, our number systems are predominantly to the base ten (although not every culture has adopted this). This accident is unlikely to be duplicated across the galaxy, so we must use the simplest possible counting system - that based on a two state or binary output. Computers use this, where a switch can be either on (binary 1) or off (binary 0). There are many different ways of generating a binary code e.g. high voltage, low voltage; light on, light off; long pulse of sound, short pulse of sound (Morse code); radio wave on, radio wave off. In searching for signals we should be expecting some simple coding of this form as a beacon alert. Possibly there will be richer layers of complexity overlaid upon this which can be decoded using a suitable key (provided by the binary code). To send the entire contents of the Encyclopaedia Britannica using the Arecibo telescope would take a few weeks if it were encoded as a binary message. What do we Say?Our natural bias is towards our own species, and so it seems logical to us to tell the galaxy about us and our relationship to the planet we live on. We must be prepared for the reaction of indifference or disdain that we harbour for ideas from other cultures. There may also be hostility and less benign intent. Any information that we send out must be diplomatic in the sense that it should not create a climate of fear or hostility on reception, nor should it reveal too much of advantage to any potential colonists. True these are very human reactions that may not have their counterparts "out there", but an attempt to concentrate on the higher human achievements - our philosophy, art, science and cultural diversity will advertise our species at its best. It will also invite a similar response from the receiver. On November 16th 1974 a radio signal was transmitted from
the Arecibo telescope to the globular cluster M13 in Hercules
- a distance of 26 000 ly. This was chosen for its high concentration
of old red stars. The signal contained 1679 bits of information. 1679
= 73 x 23 or the product of two prime numbers, the intimation being that
this message should be arranged as a 73 x 23 array of 1s and 0s - a "picture".
Pioneer and Voyager - our first "message in a bottle"The Voyager spacecraft will be the third and fourth human artefacts to escape entirely from the solar system. Pioneers 10 and 11, which preceded Voyager in travelling beyond the gravitational clutch of the Sun, both carried small metal plaques showing where they came from in relation to established pulsar signals. NASA placed a more ambitious message aboard Voyager 1 and 2 - a kind of time capsule, intended to communicate a story of our world to extraterrestrials.The Voyager message is carried on a 12 inch gold-plated copper disk containing sounds and images selected to portray the diversity of life and culture on Earth. The contents of the record were chosen by a committee chaired by Carl Sagan of Cornell University. Dr. Sagan and his associates assembled 115 images and a variety of natural sounds, such as those made by surf, wind and thunder, birds, whales, and other animals. To this they added musical selections form different cultures and eras, and spoken greetings from people in fifty-five languages (including Welsh), and printed messages from President Carter and UN Secretary General Waldheim. Each record is encased in a protective aluminium jacket, together with a cartridge and a needle. Instructions, in symbolic language, explain the origin of the spacecraft and indicate how the record is to be played. The 115 images are encoded in analogue form. The remainder of the record is in audio, designed to be played at 16 2/3 revolutions per second. It contains the spoken greetings, beginning with Akkadian, which was spoken in Sumer about six thousand years ago, and ending with Wu, a modern Chinese dialect. Following the section on the sounds of Earth, there is an eclectic 90-minute selection of music, including both Eastern and Western classics and a variety of ethnic music. Once the Voyager spacecraft leave the solar system (since 1990, both are beyond the orbit of Pluto), they will find themselves in empty space. It will be forty thousand years before they make a close approach to any other planetary system. As Carl Sagan has noted, "The spacecraft will be encountered and the record played only if there are advanced spacefaring civilisations in interstellar space. But the launching of this bottle into the cosmic ocean says something very hopeful about life on this planet. |