|active and passive methods of searching
In these we face another unknowns: the only thing we know for sure is, that we currently don't know all laws of physics. This implies for example, that we can't judge accurately, if and in the case of possibility how an information exchange with an effective speed faster than light (Tachyons?) or even travelling at this level (wormholes?) are possible. The behaviour of far-developed civilizations can largely depend on such yet unknown physics, if there are chances at all.
Therefore we have to resort to our todays knowledge; only in this framework we can do realistic actions and straightforward, but less probable predictions.
investigating subintelligent life
This sounds easy, but has several major drawbacks: direct spacecraft exploration is bound for a long time to our planetary system, with very few targets offering (slight) chances at all (I already mentioned Mars and the Jupiter satellite Europa as the now most likely places). Mars may have harbored life only in the past, to find its traces, it would require massive spacecraft investigations, likely also with expansive manned missions. Despite it's possible, this will need decades... Current life on Mars is less likely, but would be more easy to find. Europa poses big problems: far away, and due to its heavy radiation environment caused by Jupiters giant magnetic field, it can be scrutinized only by robotic explorers and to dig or otherwise break through the thick ice crust is no easy task, especially in respect of sending back the results!
Out of our solar system only indirect methods are feasible: for example an extremely difficult atmospheric analysis with not yet invented spectral analyzing tools, which have to be in space or at least on our moon. Direct imaging of surfaces is far out of reach, this is even in our system limited to a certain size of features. And even there space probes get much better detail resolutions.
But even with such probes, it's not trivially, to find out, if life exists there. These problems were already illustrated in the second half of the 70s, when the Viking missions on Mars produced results, which were not easy to interpret correctly.
principle ways of contacting alien beings
The above announcement requires an introducing remark: if faster-than-light information exchange or even space-travel should be possible, of course no advanced civilization would waste time and resources with sending radio transmissions or use other methods, which are available to us today. The currently known physics seems to make this unlikely, but not only due to its incompleteness, but also to certain strange objects it can't be excluded for sure - see above.
The main communication way, if we are right with our conclusions from our current knowledge, should be by radio waves. The reason is twofold: first it has the highest available speed (but this holds true also for visible light), and second it is cost-effective. That means, that the energy consumption is low compared with the transmitted information (caused by the long wavelength and low frequency), contrary to visible light. A third, also important point, is the ability of most radio waves, to penetrate the interstellar medium nearly without absorption or other energy loss (dispersion), a prerequisite to long distance communications. But I have to state, that most recent even LASER (Light Amplification by Stimulated Emission of Radiation, does everybody know this abbreviation?) techniques are also discussed. But for now I will concentrate on the classic radio technique.
This is also a twofold possibility: we can send messages into space (famous was a decades ago done message send in the direction of M34, a globular star cluster in the constellation Hercules, in a distance of about 10 kpc or about 30,000 light years) or - most often chosen by the active people of the topic - try to detect any radio signals from others. But we are already sending without such an intent messages into space: Radar waves (RAdio Detection And Ranging, probably more familiar than Lasers true meaning) and usual radio and TV frequencies are emitted constantly on a large scale since decades. With very sensitive equipment this could be registered by very near civilizations (at most a few dozen lightyears away, because they are no longer than about half a century emitted!) due to the above estimates. Vice Versa we are also able, to detect such emissions from other civilizations. But the requirements for sensitivity are very high.
Communication for its own purpose is thought to chose frequencies near the always in the universe present hydrogen one. But even in the vicinity of that frequency multiple, even millions of adequately chosen channels in terms of bandwidth are possible. Such a frequency was chosen for the above mentioned message, sent by the giant 300 m radio telescope in Arecibo. To detect such messages from others, today multi canal detectors with high sensitivity are employed. More about this topic you find at the last page of this set of pages: Join SETI!
The major drawback even with the speed of light, which is the same for radio waves, is the time needed for the radio waves to travel through the vast space between civilizations. Even next neighborship (about 10 pc or 10 lightyears, only the order matters) requires information exchange times of several decades! For distances of hundreds or thousands of lightyears this is already questioning, if a two way communication can be performed by two civilizations for a sufficient time, that it makes a real exchange.
How long at all civilizations are able or willing to contact others by radion waves? Another unsolved riddle, lurking in our galaxy and others...
Final remark: it is nice to equip space probes like Pioneer and Voyager
with informations about us. But the chance, that any civilization will
pick up such a probe from another one, even in a state, which enables deciphering
the informations, is depressingly low and only of historical interest (millions
of years are the least, what is required)... The size of the galaxy is
far too often grossly underestimated.
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