SETI Ė Search for Extra-Terrestrial Intelligence
SETI, the branch of the astronomy that is dedicated to the search of extra-terrestrial intelligence, proposes 3 basic hypotheses:
The Drake Equation
The equation that expresses the chance of finding extra-terrestrial intelligence in our galaxy is called "equation of Drake" and it is formulated in the following way:
N = Ns x Fs x Fp x Ne x Fl x Fi x Fc x L, where:
The number of civilizations of our galaxy with which we could communicate is estimated to be between nearly zero and hundreds of millions.
There are 4 ways through which a contact with an extra-terrestrial civilization could be achieved:
The first hypothesis, whenever the idea is appreciated or not, hints the UFO phenomena. The hypothesis that the UFOs are alien transportation vehicles canít be dismissed outright. However, it doesnít seem that until today there were collected conclusive evidences that such aliens actually visit the Earth. Although there were reports about close encountersí with UFO-type objects for centuries, it was in 1947 that the modern exploration of this phenomenon began, when the interest about the issue increased a lot. The vast majority of the reports are about distant sights and it is explainable through conventional arguments. For that motive, only a small percentage of those phenomena offer a high enough quality, for the detail and consistence that characterize the reports and for the reliability of their sources.
Sight of an UFO (UFO BBS Graphics)
The second hypothesis is identified with the form of search that has prevailed on SETI during the last decades. The research has been essentially concentrated in the segment of radio waves, which travel at the speed of light.
If a civilization wants to transmit a high quantity of information then it shall use low frequencies (like the radio waves), in which each photon carries a weak quantity of energy. That way itís possible, using the same quantity of energy, to emit a higher number of photons, each one carrying one "bit" (unit) of information.
On the other hand, at the lowest frequencies, the sky is extremely bright, because every kind of objects Ė nebulae, radiogalaxies, quasars, pulsars, supernovae, etc Ė emits quite strongly at those frequencies. To search for a signal proceeding from an alien civilization would become so complicated as it is to try to take a picture of a star under the daylight.
During the last years there has been an increasing interest over the detection of alien signals emitted in optical frequencies (under the form of laser), with a wavelength much shorter than the radio signals. This trend is due to the fact that optical rays can be transmitted under very concentrated forms and because, when compared with the radio waves, they are far less exposed to dispersion and to the consequent degradation caused by the ionized hydrogen present in the interstellar space.
The main challenges faced by this kind of research are the vastness of the space that needs to be studied (given the huge quantity of stars capable of harbouring life) and the ignorance about the frequency at which the hypothetical aliens are emitting.
Looking for SETI laser signals in the visible and infrared segments of the spectrum, alternative to the search of radio signals (Dr. Stuart A Kingsley)
The third hypothesis could be materialized through the discovery of an artifact on an object of easy access for the humanity and subject to an almost inexistent activity across a period of many million years.
In bodies like Europa, Io or even Mars, any extraterrestrial trace could gave been easily erased or covered by the processes occurring there. On the other hand, Callisto presents the perfect conditions under which such an artifact would be kept almost untouched during several million or billion years.
Itís probable that an extraterrestrial object found on Callisto would unveil secrets about the civilization that would have left it there and, eventually, it would reveal entirely new knowledge about the cosmos and the reality.
In order to search for intelligent life through the direct exploration of the galaxy, it would be required propulsion systems far more advanced than those that humanity already managed to produce.
The Voyager 1, one of the fastest probes that humanity sent to the interstellar space, only will reach an equivalent distance to the closest starís in a time lapse of 63 570 years. It travels at only 7 / 100 000 times the speed of light. The system that has been used until today is the chemical propulsion, or in other words, the reaction between a combustible and an oxidizing agent.
One of the Voyager probes (Calvin J. Hamilton)
Other possible systems are the electrical propulsion (whose results havenít been encouraging) and the plasma propulsion (acceleration of ionized gas along a magnetic field).
A system that would allow the accomplishment of inter-stellar voyages would be the antimatter propulsion, which is very difficult to be materialized. This system is based in the fact that matter and antimatter annihilate themselves when they make contact with each other, producing an enormous quantity of energy (since that all the mass present in the matter and the antimatter is converted into energy). In order to reach a reasonable acceleration it would be necessary a considerable amount of antimatter, which would have to be stored. These two assumptions almost make impossible the materialization of the idea of such a propulsion system, because until today only tiny quantities of antimatter were successfully produced by humans and it hasnít been conceived any technique that allows the storing of antimatter avoiding the explosive phenomenon of the annihilation. A vehicle fuelled by antimatter would reach Alpha Centauri (the closest star) in 21 years and Epsilon Eridani (which along with Epsilon Indi and Tau Ceti is one the 3 solar-type stars that are closest to the Earth) in 53 years.
Along with the passive search for extra-terrestrial intelligence, there is also an active search, which is done constantly and most of the times is not voluntary. The radio and TV transmissions sent into space may reveal our existence to a distant civilization.
Another kind of active search is the sending of images or messages on board of probes like Voyager. The direct exploration of the galaxy by terrestrial vehicles, which was already mentioned, can also be faced as a form of active search (since it would reveal us to the aliens), but it was never tried and itís not foreseen that it will be in a close future.
Traffic in India: one of the images sent on board of the Voyager probes (United Nations)
And If Nobody Answers?
If no contact is achieved, this may mean that at least of the following statements shall be true: