They’ve been in existence for years now. And outside of a few minor blips on the radar screen (posters please feel free to fill in this part), SETI [the search for extraterrestrial intelligence] has yet to find anything. Why? And the way things are going, will they probably ever find anything “out there”?
Thank you in advance to all who reply 
There might not be anything to find, but even if there is, space is big, and SETI only looks at a small part of it.
Simple : the universe is huge, and so are the possible types of signals. Plus, we really aren’t trying that hard.
The universe is a hugely gigantic enormous place. We have to search in the right place *and * at the right time to detect signals.
We may not be looking for the right signals.
Intelligent civilizations may tend to destroy themselves leaving only a small window between mass communication and no signals at all.
There may be intelligent beings that transmit signals all over the place right now but communication is limited by the speed of light which becomes a real factor with these things.
We may have received signals but not recognized them yet.
The odds against finding signals even if they are fairly common are still against us. There are something like a trillion or more stars in the universe. If we could scan and analyze each solar system in one second, it would still take over 2 million years to search them all. Scanning 1% of them at this rate would still take 20,000 years and that is working at impossible speeds.
We really haven’t been looking all that long.
It’s the proverbial neeld in a haystack.
Except there are 100 million haystacks.
And the needle is invisible.
They’ve been around since the mid 60’s, but how long have they actually been listening? There is really nothing but a meager handful of stars within a 10 light year radius to be found. And with the possible exception of Vega, I don’t think any show signs of a planetary system. And Vega is very young, probably too young to have planets formed around it.
We need to find a planet that has settled into the “goldilocks point”, where everything is just right for a technological civilization to have evolved. Not only that, but the timeline has to be almost perfectly aligned with ours - if they are just a few decades behind us they won’t have the ability to communicate yet. And if they are a few centuries ahead, they may have already done themselves in.
How can you be so sure that they haven’t found something? We could very well have received and recorded, and failed to recognize a dozen signals from other intelligences. Don’t underestimate how alien aliens might be.
But mostly, if the ten trillion high tech civilizations that share the universe with us are all broadcasting away at high gain, the nearest one is still statistically likely to be more than a million light years away. So, if they haven’t been yelling at us for the entire last million years or so, we could easily have just missed them.
It’s a very large universe, and there is no call waiting.
“What have you done to that cat? It looks half dead!” ~ Mrs. Erwin Schrodinger ~
But can you imagine the sales potential!
Consider how far we’ve extended our influence.
Physically, I believe we have a couple of probes that have left the solar system.
Radio waves? No idea. Doubt it’s that far, though.
The most promising “blip on the radar” was the extremely powerful signal received on August 15, 1977, known technically as 6EQUJ5. Most people who know of it, know it by the simple, yet expressive name, Wow. Weighing in at a whopping 60 Jy over a 10 Hz bandspread, the signal has never been heard again.
Well, we’ve been fiddling with radio transmissions since the late 1800’s (mostly dits and dashes) and it wasn’t until 10 or 20 years later that AM radio was commercialy broadcasting at a whopping 10 watts.
Is there anything within 100 light years capable of receiving and understanding our amplitude modulated carrier waves? And can they respond, taking another 100 years for the trip back?
It’s just not very promising when you figure the odds.
Hah. SETI is plaugued by many uncertainties. Firstly, we assume that radio signals would be utilized. This is likely, though far from certain.
Let’s imagine for a moment that this extra-terrestrial life does indeed use radio: how on earth would we know what a signal from them would resemble? Just because our transmissions appear ordered does not mean that theirs would be as such.
Also, the Universe is quite large… it is very possible that we are missing transmissions from extra-terrestrial life.
The fact we have no evidence that indicates extra-terrestrial exists does not constitute evidence that such life does not exist.
See http://en.wikipedia.org/wiki/Drake_equation (with special emphasis on L, the expected lifetime of a radio-communicating civilization).
SETI is more or less doomed. Transmitting signals over a very large distance requires either very high power signals or very directional antennas or very low symbol rate. All of these imply that the entity sending the signals has somebody in mind to receive them. Unless those people that are supposed to receive the signals are near by there is no reason to expect that we will find anything.
WAG The extraterrestrial intelligentsia are so much smarter that we are that we don’t understand their messages.
The speed of light is ~1.08B km/h. Pluto is ~7.38B km from the sun (aphelion). If we neglect the distance from the earth to the sun (<.2B km), it takes a little over 7 hours for radio waves from earth to pass outside the orbit of Pluto. I think we can assume that radio and TV signals (weak though they may be) are well beyond the solar system by now…
In fact, our nearest star, Proxima Centari, is “only” 4.3 light years away. Theoretically, any Centarians with an antena could be watching the first few seasons of Survivor!
They would have to be. In order for a signal to carry useful information, it must be ordered, somehow. There are several methods for imposing order on a radio transmission, and we know all of them. There are, after all, only so many ways to modulate an EM signal. Even scrambled or encrypted transmissions have patterns which can be identified as carrying information, even if we cannot necessarily extract meaning from it.
Generally speaking, I’m an “agnostic” regarding the existence of ETs. I see no reason to assume they exist, and no reason to assume they don’t. So, simply, maybe there are no ETs.
Assuming that they do, I would actually find absolutely extraordinary that we would catch any signal (given the size of the universe and the short time we’ve been trying to listen). So, I’m absolutely not surprised by the lack of results, in the same way I wouldn’t be surprised not to win the lottery if I bought a ticket.
Finally, I’m quite convinced that anyway, we’re probably searching for smoke signals because we just invented fire, while the (hypothetical) ETs are sending messages all over the place with short-wawe radios. Basically, I’ve no reason to assume that they would be communicating the way we expect them to do.
Oh! And there’s also this possible answer I read once in some book and find intringuing, and that I would call the paranoid hypothesis :
Extraterrestrial civilizations stay silent because they know they should better be quiet. For some reason…
It could also be that civilizations only tend to use broadcast transmissions for a short while before moving to more efficient ways to communicate - microwave, lasers, fiber, cable… It’s possible that civilizations tend to ‘light up’ only briefly in the regular spectrum, then go dark as either their technology progresses to the point where they don’t need it anymore or they destroy themselves.
As far as ease of detecting a modulated signal, well, it depends on the technology. Spread-spectrum, digital with high compression, yada yada yada. And who knows what technologies other civilizations may have come up with.
What we need to do is build big honking interferometry arrays in space and just go looking. There’s no reason we can’t build a telescope array big enough to directly image planets around the nearest few thousand stars, and measure their atmospheres for things like signs of combustion, pollution, bio markers, etc.
If we can do a mapping survey of the atmospheres of the nearest 1000 stars, and we don’t detect any signs of life at all, that will be a pretty important data point.
Because they’ve already seen what we’re sending out! 