Jesus Zafra, Spain asked:
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?
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.
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. graham.d, Wed, 21st May 2008
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... chris, Thu, 22nd May 2008
4.37 years each way in fact (if Alpha centauri) graham.d, Fri, 23rd May 2008
"Do you want a cup of tea?"
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...
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.
A golden gramophone disc, (ask your parents), with "sounds of earth" was attached to the Viking space probes launched in 1977 ...
There a a couple of serious engineering issues, some of which others have alluded to: