[thedoc (OP)]

What kind of antenna (for instance, size) should be used to communicate with aliens in other stars, like Alfa Centauri?  Knowing the possibility of life somewhere, in a star like our Sun, what kind of device should we use?  Do we have any possibility of communication with our nearest stars?
Asked by Jesus Zafra, Spain

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[thedoc (OP)]

Because light travels at a fixed speed it takes a long time for a signal to reach us. The basic problem with communicating with aliens is that the nearest stars are 3-10 light years away. If you said hello it would take three years for that message to get to the alien star and three years to come back. The other problem of course is aliens will not necessarily have communication of the same kind as us. What we would have to do is send signals that are numbers like 2, 4, 6, 8. That's the content of the communication. The second part is how do we communicate? Scientifically I think there are two really approaches that are currently available to us on Earth. One is the standard technique which is to use radio signals and we listen with the biggest telescopes on the Earth. The Arecebo telescope, which is a 300m diameter has been used. Radio astronomers are currently working on the design of a radio telescope which will be ten times larger. Those telescopes will be mainly used to listen for extra terrestrial life. An alternative approach is to use lasers. Lasers have the advantage that they're highly directional. You could point high-powered laser at a specific star. One thing about technology is that we're in a period where technology's improving all the time. If you want to communicate with aliens and you think it's going to take many years to start this conversation it is possibly better to wait maybe 10, 20, 30 years until our technology improves significantly.

[graham.d]

Alpha Centauri is 4.37 light years away so the conversation may be a bit stilted, but I get the idea. The concept of an antenna means that there is an assumption of radio communications. Really as big as possible set of dishes (I mean biggish dishes and an array so as to have as narrow a beam as possible). I am afraid that as much as I like the idea of communications with aliens I rather think the concept is a waste of time. This is because aliens that are much in advance of us will communicate with us if they wish to. If they do not wish to, we are unlikely to detect their communications. It is unlikely that we could detect are own spread spectrum communications used today (say) 30 years ago. We probably would think it noise. So think what difference 100s or 1000s of years could make. If they are not as advanced as us we clearly could not communicate. It is very very unlikely we would hit upon a civilisation at the same stage of development as ourselves. Shame really.

[chris]

The messages are going to take a long time going back and forth though aren't they? It takes 15 minutes just to get a signal to the rovers on Mars...

[graham.d]

4.37 years each way in fact (if Alpha centauri)

[DoctorBeaver]

"Do you want a cup of tea?"

8.74 years later comes the reply "Yes please"

"It's gone cold now"

 []

[Pumblechook]

Broadcast type signals fizzle out quite quickly. It is myth that aliens could be tuning in to our early broadcasts. They might just about (with a really massive antenna) detect the presence of a TV signal but no picture or sound...   

From a website... ""Detection of broadband signals from Earth such as AM radio, FM radio, and television picture and sound would be extremely difficult even at a fraction of a light-year distant from the Sun. For example, a TV picture having 5 MHz of bandwidth and 5 MWatts of power could not be detected beyond the solar system even with a radio telescope with 100 times the sensitivity of the 305 meter diameter Arecibo telescope"""

Narrow band communications type signals are detectable at much lower signal levels but even then transmitter powers and antennas would have to be very large.  Bigger antennas have increasingly narrow angles or reception and transmission and the chances of pointing in the correct 'random' direction are millions to one. 

The space agencies have found that signals travelling over several millions of km arrive somewhat garbled because of very slight differences in the speed of propagation at different frequencies.  Bits of the signal arrive before othres.  The problem will be much worse over  light year distances.

[lyner]

For a start you would need a humungus antenna on a relay station out in orbit. Hundreds of km diameter with hundreds of kW transmitter power.
The signal would have to be a combination of crude "I'M HERE" type signals and very sophisticated coding and modulation. Anyone likely to be capable of or interested in what we could have to say would be quite capable of making sense of our signals.
Whether we could receive, recognise or understand theirs would be another matter.

[RD]

A golden gramophone disc, (ask your parents), with "sounds of earth" was attached to the Viking space probes launched in 1977 ...
http://en.wikipedia.org/wiki/Voyager_Golden_Record

A reply could take some time, particularly as a gramophone player was not included, only the stylus, (ask your parents again).

[techmind]

One thing about technology is that we're in a period where technology's improving all the time. If you want to communicate with aliens and you think it's going to take many years to start this conversation it is possibly better to wait maybe 10, 20, 30 years until our technology improves significantly.

On the contrary, the communications systems (analog, AM/FM etc) which we used up to 20 years ago were quite primitive and therefore fairly easy to analyse or reverse-engineer by anyone (or any alien) with a modicum of experience in the field. The specification sheets for analog TV signals runs into about 6 sides of A4 (and they're quite sparse).

Conversely the increasingly "clever" or "advanced" digital technologies would be hopelessly impossible to decipher by aliens. The specifications (eg for CODFM digital TV broadcasting and MPEG coding) run into hundreds of pages, and by virtue of their "efficiency" (lack of redundancy) the signals are are almost indistinguishable from noise.



I agree that we'd probably want to use fairly high frequencies (microwaves or even optical, ie light) to get directivity - and beam it to specific targets. Otherwise the signal power would soon be too weak to be viable.
I guess some form of heirarchicial modulation or coding should be used, ie baseline components which are very basic and obviously-structured and easy to decode (but necessarily also rather low information-content) and then higher components which are increasingly sophisticated (carry more data, but more complicated coding). The information coded by the more basic layers may contain clues as to how to decode the more advanced coding.

[jeremy k]

There a a couple of serious engineering issues, some of which others have alluded to:

• transmitters/recievers probably need to be space based and very large arrays. The moon might be a good pick.
• what error correction and encoding to use? Usually simple looped repetition of a prime scaled rectangular bitmap is assumed, which limits the total message size. Modulation is usually assumed to be simple duo tones, like old modems.
• frequency - what frequency to use. SETI uses the neutral Hydrogen line, but there is no particular good reason to assume this is a good choice! You also have doppler effects - orbiting planets have relative motion! (this is actually the basis of one of the SETI detection algorithms - look for the doppler of a moving transmitter). Atmospheres eat and aweful lot of radio frequencies ...
• beam width vs power - the wider the beam, the more power you need (by the cube of the beam width). Megawatts of radio energy are not simple to produce, check out the power RF valves that TV stations use for an example. Think several water cooled garbage can sized light bulbs running at KVolts. Lasers/Masers/Grasers might be attractive, but even they diverge a lot over several light years.
• tracking - solar systems move, and so do planets. At 4+ light years with reasonable beam widths, you need a pretty good tracking algorithm to "lead" the beam. The best way to track might be to use a large "phased array" of very simple antennas

My best guess (for what its worth) would be a massive laser/maser in space with solar power, or a forest of simple antenna on the moon.