During such task solving as: the transmission of radio messages from the Earth towards supposed locations of extraterrestrial intelligence (METI, acronym for English «Messaging to extraterrestrial intelligence»), it should be defined:
- Where to signal.
- When to signal.
- The wavelength of the signal.
- What polarization should be used.
- The power of the transmitted radio emission.
- What modulation should be used.
Let us try to give short answers.
Where to transmit interstellar radio messages?
An outstanding discovery was made in 1995: Swiss astronomer Michel G.E. Mayor and his graduate student Didier Queloz discovered first exoplanet at the ordinary (not neutron) star 51 in Pegasus constellation. It became clear, that such planets — as ordinary and widely spread celestial objects as stars and galaxies. There are about 100 billions of stars in our Galaxy, 1% of them — are the stars of solar type. And it’s suggested to choose the addressees of our interstellar radio messages from the stars among this remarkable billion at first. The present list of requirements for stars-candidates is of the next form:
- Belonging to the Main sequence;
- Constant luminosity;
- The age in the range from 4 to 7 billions of years;
- Single stars with spectral classes similar to Solar are preferable;
- The position on the sky should be near preferable areas — not far from the plane of the ecliptic, from the directions to remarkable astronomical objects, to the center or anticenter of the Galaxy.
In order that other civilizations could find us, for example, in the process of usual astronomical observations, It is desirable for us too to be observed from there, at the background of remarkable astronomical objects.
If the star-candidate has its own planet or planetary system, it’s desirable that the orbits of these planets had low eccentricity, for such planetary systems are more longevous, there is no great temperature difference, which prevents life origination.
It’s better to choose stars inside the «life belt» — the «greenhouse» area of our Galaxy, where the conditions for life origination and continuous evolution are most favourable because of stellar motion speed and spiral arms coincidence.
With the lapse of time, as far as our knowledge of outer space accumulates, the appearance of other criteria and even other addressees than stars is possible. And it’s quite naturally, but today the criteria coincide with listed here or they are very close to them.
When to transmit the radio message to the chosen star?
The questions of time synchronisation of our radio transmission and somebody's search (or of somebody's radio transmission and our search, what isn’t less actual) are very important. The competent synchronization allows to increase the possibility of radio contact establishment into several dozens times. One of such methods— is the associating the moments of transmission and search with famous universal events, observable everywhere in or Galaxy, for example, with the moment of maximum light of any nova or supernova explosion. Today, the modern large telescopes succeed in registration of supernova outburst moments in neighbor galaxies, which could be used for emission processes and search time synchronisation.
At what wavelength the message should be sent?
The bandwidth of radio signal transmission coincides with the bandwidth, which had been established earlier for SETI, — from 20 to 1 cm. The maximum distance of radio communication achieves exactly in this range. The energy potential of space radio line defined as product of the transmitter power and the transmitting and receiving antennas amplification coefficients, divided by noise temperature of the receiving system. With the modern level of Earth technology development such ratio works maximum exactly in centimeter range. It is possible that with the lapse of time, as the space communication develops, the best energy potential will be reached by the systems of infrared or optical range, and then our conception of optimal wavelength will change. The exact value of the wavelength can be one of “magical”, for example 6,72 cm = 21 cm/?, defined as the ratio of two universal constants — physical (radio line of interstellar hydrogen) and mathematical.
What polarization should be used?
The parameters of radiated fluctuation polarization are called to show its artificial origin. In addition, the direction of circular polarization rotation or linear polarization plane orientation can be changed discretely or uninterruptedly, by inserting one or another intellectual message (polarization modulation).
In the fascinating science-fiction novel by American astrophysicist Carl Edward Sagan «Contact» the radio message, received by the earthmen from Vega, contained digital message which have been transmitting by intermittent polarization switching. Such methods are not widely spread yet here, but as it seems, Sagan’s hypothesis deserves serious attention of them, who deals with transmission and search of intellectual signals in the Universe.
What is the power of transmitted radio emission?
The calculation gives next values of information rate for three of the most powerful transmission systems, existing nowadays (the numbers in brackets are the diameter of transmitting antenna, mean power and wave length correspondingly):
- Radiolocating telescope in Arecibo, Puerto Rico (le300 m; 1000 kW; 12,5 cm) — 1000 bit/sec;
- planetary radiolocator in Goldstone, California (70 m; 480 kW; 3,5 cm) — 550 bit/sec;
- planetary radiolocator near Eupatoria, Crimea (70 m; 150 kW; 6,0 cm) — 60 bit/sec.
It had been accepted in calculations, that the distance of our message transmission makes up 70 light years and that the extraterrestrial civilization receiving system has an antenna with the effective area of 1 million square meters (the project of such radioastronomical antenna is working out now, and it could be constructed in a decade ahead).
What modulation should be used?
The continuous search of intellectual signals from the other civilizations, which goes already over 40 years, in overwhelming majority, uses the surprisingly alike algorithm of detection. The receiving emission goes through digital spectral analysis, the quantity of analysis channels at that reaches hundreds of millions or even some billions. For example, «Phoenix» project of American institute SETI uses digital spectrum analyzer for 2 billions of channels with the width of 1 Hz, what allows to analyse the band of 2MHz in real time, and of 2 GHz in record mode! Supposing this algorithm to be optimal, we make a conclusion, that modulation should have clear spectral language, permitting to detect our signals emitted with minimal loss with the help of parallel spectrum analyzers exactly. Such modulation is known; it is called frequency modulation and used on the Earth widely.
What the optimal structure and encryption methods should be?
In accordance with three types of single-valued frequency function — constant, continuous, discrete — radio message has three-level structure and uses three languages: «nature language», «emotion language», «logic language». The table uses «voiceprint» term, indicating two-dimensional visualization of signal spectral composition in X-Y: X axis — frequency, Y axis — time.
Spectral structure of interstellar radio
messages |
It’s suitable here to make an analogy with the triune structure of our thinking, where three components discern — intuitive, emotional and logical. The first part of radio message is constructing by radio engineers and represents coherent probing oscillation, for example elemental monochromatic or with periodical linear frequency modulation. A variable Doppler correction can be inserted in its frequency, so that we could be observed from the side at the constant frequency. With the presence of intuition another civilization could understand the original probing signal. The second part is creating by the people of art and represents analogue frequency variations, reflecting our emotional world and our artistic images. The elemental example — classical music melodies.
The voiceprints of three 40-second music fragments, played on theremin — electric musical instrument, which generates signals with minimum overtones level. From left to right: the final of Beethoven’s 9-th symphony, «Swan» by Camille Saint-Saens, and «Summer» by George Gershwin |
The third part — discrete frequency-shift modulation, digital data stream, the reflection of our logical constructions: algorithms, theories, accumulated knowledge about ourselves and the world around us.
The first page of radio message, sent to the nearest stars by the participants of «Space call» project in summer, 1999 with the help of radio telescope at Ukraine. The page consists of digits only, what should make it more easier for decoding |