It did get me thinking though... what kind of message would you send? For this thought experiment, suppose we follow the suggestions and sweep out portions of the sky on the regular basic at common frequencies likely to be noticed. Add a "Contact"-like header of bursts of signals representing the prime numbers to get attention, and then follow with the message.
What would its form be? A binary encoding? What would the message be? Intention? Form? What clues could you include to help with the decoding?
We must also consider the possibility that the "aliens" in question may potentially be "reasoning" and "self aware" machines who have already struggled against and overcome their organic creators. Its quite hard to win the argument for "sanctity of life" with a self-aware machine. (In that light, the voyageur message, complete with return address, is a potentially grave error that future generations will dearly regret, with the silver lining that it was sent before we had the tech to nano-encode our genome on the far too revealing message ;) I think central to these thoughts is the consideration of the correlation between advancement in technology and (for a lack of a better word) enlightenment.
Then again, it's probably safe to assume any civilization capable of reaching us would not want to destroy or enslave us -- why would they need to? What power would they gain that they wouldn't already have? True that there is not necessarily a correlation between tech advancement and enlightenment, but what we're talking about here, traveling between stars, is a nearly unimaginable leap in technology. So the correlation is perhaps better posited as immense advancement in technology and lack of interest in destroying a primitive culture.
I'd bet that we will pick up the Pioneer and Voyager craft and put them in a museum someday. If we have to use light then I would go with primes too. But, I don't think it matters what we say. The real message is that there is a message. Once they found it they'd start listening to all the junk that we leak into space, and building instruments to listen better.
I think that makes little sense. How could aliens think that we have the same gestures? Or that we had knowledge of theirs? It is our gesture, that's the point. We are just saying "This is what we do. It's our greeting gesture." >The scientists say that another problem is in our assumption that aliens can properly perceive pictures or sounds with their own unique sensory organs. I'm sure they'll figure it out. That's the point. If we got a message from an intelligent extra-solar species, we'd put tremendous energy and time into examining it. We'd probably find more in it than that is there. As for what I'd put: I think the Voyager and Pioneer missions did well. They started with basic shared information, the excitation of electrons in hydrogen, and build up from there. I think the pulsar position map is a good idea too. http://en.wikipedia.org/wiki/Voyager_Golden_Record
http://en.wikipedia.org/wiki/Pioneer_plaque I don't think the intent of the message is important. We are just giving them a primer, so the more information they have, the more context they have to build a picture from. Let's given them enough to have fun. I'd send Wikipedia if I could. -In multiple languages. They would decode it in no time, with no help from us. We could do the same, and chances are, they are far more advanced. For a EM signal that we sent out, I'd send a repeating signal, that was a sequence of primes. Maybe have the primes alternate at two nearby frequencies, creating a binary signal that coded the message.