Right now the search for extraterrestrial intelligence involves scanning the skies for radio signals and looking for patterns that might indicate intelligent life. So far the search has been fruitless.
However, given technological trends, it seems to me that this approach might be doomed to failure. Consider:
The electromagnetic spectrum is a finite transmission medium.
Any technologically advanced civilization that uses the electromagnetic spectrum for communication will, over time, attempt to pack more and more data into that finite medium.
That means they will use more and more aggessive data compression techniques. The more perfectly compressed data is, the more it represents white noise. (Any deviation from white noise represents some form of structure – which, of course, is a candidate for further compression.)
Look how quickly our own civilization has leapfrogged from AM radio (which is a bandwidth hog) to the highly compressed transmissions of digital television. In a thousand years will every terrestrial transmission be so tightly compressed that it sounds like nothing but static to outside listeners?
It’s easy to imagine that every technological society goes through a similar phase of “radio bandwidth inefficiency”. In between the invention of radio and the advent of total data compression there might be only a window a few centuries wide where stray transmissions would contain recognizable patterns. Unless we’re extraordinarily lucky, our chances of finding another civilization during that fleeting window are vanishingly small.
If you’re sending signals that are compressed to the point where they are indistinguishable from white noise, you must do so in a portion of the EM spectrum where there is no natural noise, or at a power level much greater than than any background noise, or send the same signal repeatedly so that it can be separated from the background noise.
Well, there’s the chance of detecting radar transmissions, and deliberate attempts at contact. So I wouldn’t call it pointless; just less likely to succeed. On the bright side, it means that the “Great Silence” isn’t quite as grim a thing as once thought; maybe we just can’t hear them or misinterpet their signals as static, instead of aliens being nonexistant or dead.
On the other hand, while highly compressed data radio transmissions might look like static, we’d still detect it, so a planet with such highly compressed data transmisions would still be a bright radio star. Perhaps we should seek out radio stars that seem like random static, and either look for subtle patterns, or other signs of life.
Oh, goody! A chance to advance my crackpot agenda!
I think we would be better off looking for unnatural spectra, an idea stolen from Larry Niven. If people travel through space, they will use propulsion systems that are likely to give off exotic radiation between points in space, where its all dark, and stuff.
There are theoretical limits to compression though. Any statistical deviance in a signal from white noise over sufficient time can imply at least some information. Perhaps it’s Little Green Men? What was the Wow! signal.
No. Anyone (or Thing!) who cares to listen or is capable of doing so will be able to find structure in your signal. Decoding is of course another matter.
I think you misunderstand the data compression thing, but I can well imagine civilization-wide broadcasting being a short-lived thing.
Data compression is a different beast than the particular modulation scheme you use. You can have very well compressed data sent over a system that uses a strong carrier that seti will detect. Or you can use some spread spectum modulation on uncompressed data that does not have a strong consistent carrier that seti will not detect.
You overall point however is a good one. As our expertise with communication systems increases the trend has to devote less power toward systems with strong central carriers and more toward systems with complected reference signals that I don’t think will be detected by seti’s somewhat simple method of looking for strong narrow band transmissions. I would be interested in knowing how well seti will detect the newer digital TV signals.
There’s no real hope, at our current levels of technology, of us just happening to intercept stray communications between the aliens, unless they’re a lot noisier than we are. We’d have to assume that first of all, they’re using much stronger radio signals than we are, and secondly, that they’re just flinging those radio waves willy-nilly, with no attempt at beaming them to their intended recipients. So if we’re going to detect anything, it’s probably going to be an intentional message. And if they are sending out intentional messages to us, they’re not going to be compressing them to the point of unrecognizability. If they have a lot to say to us, they might send out two messages, first an uncompressed one to teach us their compression scheme, and then a longer follow-up using that compression scheme (the interstellar equivalent of a self-extracting ZIP archive), but we’d still be able to intercept and recognize the uncompressed “key” message.
I thought the whole assumption with SETI was that an alien civilization might be intentionally sending out signals as a beacon to other life out there. And that picking up stray radio signals from them was hopeless, compression or not.
The thing about SETI is that the problem isn’t just the amount of space, but the amount of time. An incredibly advanced civilization fairly close by might have sent out a signal … two million years ago.
Suppose we have an alien civilization that gets to the technology level of us in 1930 or so-this planet is sending out long-wave AM broacasts like crazy. At our current level of technology, from how far way, could we pick this stuff up? Or would AM signals dissipate so quickly, that we would never collect any?
It depends on how much power they are using, how big and how directional an antenna we are using. It also depends on how long we integrate for. A very low power sign wave in the presence of large amounts of noise can be detected if you correlate it with a reference for long enough. We are however transitioning away from broadcasting those sorts of signals. More modern communication systems tend to have short bursts of reference signals that I am not sure that seti will be able to detect.
If not intentionally, inadvertantly. I think it was Carl “Billions” Sagan who pointed out that if anyone had been scanning our particular section of Nothingness, they might well have noticed that some years back, this rather nondescript yellow star system suddenly became very lively in the radio-magnetic spectrum.
Depending on where you arbitrarily draw the line…lets say, 1930, for no particular reason… then there is a sphere of possible awareness. In this instance, 2007 - 1930 means a sphere of awareness roughly 77 light years across.
Just rough probability suggests that our future Insect Overlords are not within that sphere, but the advertisement has gone out.
What about radar installations, like the ones the US and Russians have near the North Pole? IIRC, an Arecibo-sized detector in a nearby star system might pick them up.
If we’re anything to go by, everyone wants to receive radio communications from extraterrestrials, but nobody is all that bothered about making a long term commitment to send them.
How many G star are in our neighborhood, much less scattered about this galaxy? Many of them could have life-hosting planets and we wouldn’t know. Dismissing Alpha Centauri (b?), the nearest is something like 12ly away, far beyond the probability of detecting random transmissions. The nearest solar twin is … um, is 20 ly? (Wiki and Google have destroyed my memory) Sorry, closer to 27 with several out at 50, at least one of which has planets, IIRC.
The odds that any of those currently have intelligent life with powerful radio transmitters is vanishingly small I suspect, but as we’re just starting to look in earnest I’m not ready to concede that we are alone in the galaxy, much less the universe.
And I read a book once*, so you can be sure I know what I’m talking about!
Well, it was more of a pamphlet than a book. Actually, it was a commercial for a TV show, but the show was about stars - rock stars, I think.