Calling it irrelevant just because you can't travel there is short-sighted. We could still learn a lot just by watching the broadcasts of another civilization, even if they're already long gone.
Even if they are broadcasting it, we're probably listening to it with the wrong technology. Even if we happen to be lucky of listening with the "right" technology. We probably wouldn't be able to tell it from random noise.
I find it extremely unlikely humans will ever be able to watch alien broadcasts during our existence in the universe.
That being said, while I suspect that meaningful, structured information will be, by definition, structured and therefore obvious to spot in a broadcast, the real problem is that these transmissions are likely to be (a) compressed and/or (b) encrypted.
While compression formats may reveal a few hints that there is some structured information present, encrypted data will be all but impossible to distinguish from random transmissions. We may be picking up transmissions from alien life but be unaware simply because the data is encrypted.
I have no idea what proportion of our broadcast signals are encrypted today, but I suspect it's a fairly significant and growing proportion (privacy and intellectual property concerns being the primary reasons).
Serious question: How is it presumably simple to decode images and audio, but difficult to understand the audio? Is it because one can assume image representation would tend towards mathematical efficiency, but spoken languages do not?
>While compression formats may reveal a few hints that there is some structured information present, encrypted data will be all but impossible to distinguish from random transmissions. We may be picking up transmissions from alien life but be unaware simply because the data is encrypted.
I would think that encrypted transmissions would be easily distinguishable from background white noise, no? My thinking is, you have the general random noise of space, and then you would have something obviously different, is that totally not the case?
This is just my intuition and speculation but I would guess somebody could formalise some support in terms of Kolmogorov complexity (http://en.wikipedia.org/wiki/Kolmogorov_complexity): there are only so many ways to represent an image or audio, and over time an intelligent race will discover the least complex method and use it. Therefore, alien races of equivalent levels of technical advancement in a given field should be able to decode raw (uncompressed and unencrypted) transmissions of common types of information (images, audio).
Any additional complexity on top of this method will probably be quite structured, e.g. with television signals we have portions of the signal that are blank because during that time the CRT of old televisions would be retracing and not displaying data. (Actually, I believe these "dead" portions of the video signal are sometimes used to send additional structured data, e.g. Teletext, which would make the structure of the signal even more apparent.)
I suspect it'll be much easier to look at alien video transmissions and figure out what they're about than trying to listen do the same with audio transmissions because video would provide objective context (assuming it's not just a video of talking alien heads, or whatever the equivalent of a "head" for them would be).
With respect to encrypted transmissions, if the entire protocol is encrypted from start to finish, and you're using the right cipher* then no, you can't distinguish encrypted data from random noise. This is the principle behind TrueCrypt's hidden volumes (http://www.truecrypt.org/docs/?s=hidden-volume).
* What's the right cipher? So far no nobody has found a distinguisher for AES, and that's in common use, so that's probably good enough to make an alien race blind to us; i.e. we could be picking up signals from a race as sophisticated as c. 1998 humans (http://en.wikipedia.org/wiki/Advanced_Encryption_Standard) without realising it.
But, would there not be a noticeable difference between random noise in space, and that same space PLUS someone broadcasting random noise? (And would it depend on whether or not they cared about being heard, even if they still wished their transmission to be encrypted).
It just seems to me that if there is a particular point that is continuously broadcasting randomness, would it not be noticeable? Or is what you are saying that that is essentially what stars are doing is broadcasting randomness, across a broad spectrum, and we can't distinguish between a star and an intelligent race....they are literally indistinguishable.
That being said, from a physics perspective perhaps there's some way to say "these bits of random noise are unusually powerful, and based on our multiple noise detectors over a large area we've managed to estimate the path these signals travelled and there's only a planet on the other end with nothing else appearing in in the way (over the course of time it took for the signal to arrive) to explain them as mere background noise."
For example if you electrically sniff an ethernet cable over which only SSL traffic using AES is being sent, the first thing you will notice is a concentration of energy near 200Mhz. Then you may see bursts of a Manchester-encoded signal; further study reveals the regular structure of Ethernet headers and trailers, along with IP, TCP and SSL headers. Only within that will you hit a brick wall at the AES data itself.
Even if the payload is encrypted it should be possible in most cases to detect a modulated transmission.
Have you read ithkuil's comment? http://news.ycombinator.com/item?id=4188160
Even today we use some technology which is not really necessary to solve a given problem, just because it uses off the shelf components which are cheaper even if more complicated (e.g. a embedded cpu instead of dedicated and simpler electronics)