Short answer ... there is a high probability they do. But why haven't we detected any? "Maybe it's because they are using technology far beyond our abilities." ~ Seth Shostak @ SETI Maybe not. The excuses (such as the one above) offered by SETI researchers (and parroted by believers) for our lack of finding artificial signals emanating from the cosmos are, upon unbiased analysis, flimsy at best. All one has to do is look at the Fermi Paradox, the Drake Equation, and many other popular proposals objectively to see the underlying problems associated with all of them. For example, Fermi assumed FTL technology was possible, and therefore intelligent species older than ours must have achieved it somewhere along the line. With all due respect to Kaku and others, this is hardly a given. http://en.wikipedia.org/wiki/Fermi_paradox And of course we know the Drake Equation was never an actual hypothesis or theory supported by any evidence ... rather, it was little more than mathematically organized wishful thinking on the part of Drake, dreamed up to encourage funding. Successfully, I might add. http://en.wikipedia.org/wiki/Drake_equation No, I think the answer is stupidly simple. There is another ... better ... reason as to why we have not detected any confirmed artificial signals; a not only possible solution, but plausible one seemingly ignored by researchers and masses alike. "Human" may be ... by far ... the best, the most advanced life form the universe can produce. The only life form that will ever achieve the capability to make a radio, let alone leave the planet. This just in ... http://news.nationalgeographic.com/news/billions-of-earthlike-planets-found-in-milky-way/ Even with the potential of billions of earth-like planets in the MWG alone, we still haven't detected a signal ... I suggested on the "Big Bang Evidence For God" thread that "Life is an extremely rare by-product of the natural laws of space. Intelligent, technologically advanced life is even rarer." I insisted (despite Alex's protests) we had "literally millions of samples" and "billions of locations" to support this 'hypothesis'. Let's start with the millions of samples. How many species on earth have achieved technology capable of creating radio? One. Can I be so sure of this? Could there have been a technologically advanced species equal to or greater than ours earlier in our history? I often wondered about this as a child in the 50s and 60s. But soon it became apparent this was not a viable possibility. Certainly any advancing species would have had to eventually develop radio. After all, there are only so many known ways to achieve rapid communication, and they would have had to deal with the same laws of physics, chemistry, etc that we have. What is the likelihood of a species being curious (or driven) enough to achieve radio and space travel and not leaving a shred of evidence behind? And what are the chances they jumped from fire to FTL? Possible, I suppose. But not plausible. And by the same line of reasoning, we can also effectively rule out other species on other worlds jumping from 'nothing' to FTL technology. They would have invented (and used) radio at some point. No, if there had been a species before us on earth that achieved radio, we would have found the evidence in space, or on the moon, on Mars ... somewhere. It stretches credulity to the extreme to suggest they could have (or found it necessary to) destroy every last little bit of space junk that would have been an inevitable by-product of radio communication and space exploration. We were the first. Millions of samples ... Sharks. Turtles. Birds. Reptiles. How long have they existed? There is good evidence sharks (in a plethora of different forms) may have existed for as many as 420 million years. How close are they to achieving radio? After all this time? If they had another 2 or 3 billion years, would they ever build a radio? Possibly. But they wouldn't be sharks anymore. Turtles? Same story. Hundreds of millions of years and they are still turtles. Birds? Will they ever make a radio? No. There have been millions of species on this planet. A planet arguably very well suited for life to flourish and evolve. Yet, even after (in numerous cases) hundreds of millions of years, none have come anywhere near radio. And, in their present forms, never will. Isn't this obvious? How about hominids? Once again, there have been numerous species that have existed for millions of years. How much longer before chimps develop radio? Gorillas? They never will ... as long as they remain in their present forms. What about the humanoid species? Neanderthals? Homo erectus? http://en.wikipedia.org/wiki/Human Evidence from molecular biology The closest living relatives of humans are gorillas and chimpanzees. With the sequencing of both the human and chimpanzee genome, current estimates of similarity between human and chimpanzee DNA sequences range between 95% and 99%. By using the technique called a molecular clock which estimates the time required for the number of divergent mutations to accumulate between two lineages, the approximate date for the split between lineages can be calculated. The gibbons (Hylobatidae) and orangutans (genus Pongo) were the first groups to split from the line leading to the humans, then gorillas (genus Gorilla) followed by the chimpanzees and bonobos (genus Pan). The splitting date between human and chimpanzee lineages is placed around 4-8 million years ago during the late Miocene epoch. Evidence from the fossil record There is little fossil evidence for the divergence of the gorilla, chimpanzee and hominin lineages. The earliest fossils that have been proposed as members of the hominin lineage are Sahelanthropus tchadensis dating from 7 million years ago, and Orrorin tugenensis dating from 5.7 million years ago and Ardipithecus kadabba dating to 5.6 million years ago. Each of these have been argued to be a bipedal ancestor of later hominins, but in each cases the claims have been contested. It is also possible that either of these species are ancestors of another branch of African apes, or that they represent a shared ancestor between hominins and other apes. The question of the relation between these early fossil species and the hominin lineage is still to be resolved. From these early species the Australopithecines arose around 4 million years ago diverged into robust (also called Paranthropus) and gracile branches, one of which (possibly A. garhi) went on to become ancestors of the genus Homo. The earliest members of the genus Homo are Homo habilis which evolved around 2.3 million years ago. Homo habilis is the first species for which we have positive evidence of use of stone tools. The brains of these early hominins were about the same size as that of a chimpanzee, and their main adaptation was bipedalism as an adaptation to terrestrial living. During the next million years a process of encephalization began, and with the arrival of Homo erectus in the fossil record, cranial capacity had doubled. Homo erectus were the first of the hominina to leave Africa, and these species spread through Africa, Asia, and Europe between 1.3 to 1.8 million years ago. One population of H. erectus, also sometimes classified as a separate species Homo ergaster, stayed in Africa and evolved into Homo sapiens. It is believed that these species were the first to use fire and complex tools. The earliest transitional fossils between H. ergaster/erectus and archaic H. sapiens are from Africa such as Homo rhodesiensis, but seemingly transitional forms are also found at Dmanisi, Georgia. These descendants of African H. erectus spread through Eurasia from ca. 500,000 years ago evolving into H. antecessor, H. heidelbergensis and H. neanderthalensis. The earliest fossils of anatomically modern humans are from the Middle Paleolithic, about 200,000 years ago such as the Omo remains of Ethiopia, later fossils from Skhul in Israel and Southern Europe begin around 90,000 years ago. Homo habilis made stone tools. They had exactly the same materials to work with that we have now. They had essentially the same pressures and challenges we have now. Extremes of weather, food, predators, etc. And how close did they get to radio after a million years or so? Modern homo sapiens emerged a mere 200,000 years ago. And only 50,000 years ago did they begin to exhibit modern behavioral patterns. They went from fire and stone tools to space in 50,000 years. 50,000. All other species (physical configurations) had millions, if not hundreds of millions of years to accomplish radio. None came close. What can we infer (if not conclude) from this? First and foremost, it takes an extremely specialized life form to create a radio. If the life form lacks any of homo sapiens specializations, radio is not going to happen. Again, millions of 'failures' ... only one success. The life form must have sufficient brain power to observe and comprehend the world around it, and just as importantly, learn how to manipulate it in extremely complex ways. The life form would need to develop mathematics. Physics, chemistry, electro-magnetism, etc. So what about those 'other' planets? Well, we can eliminate all exoplanets similar to the others in our solar system. Clearly, it is going to be rather difficult to have evolved complex, intelligent life forms on a waterless world, such as Mars. Or on a gas giant. Or on a planet that's 700 above zero Celsius. Or 700 below. You can say all you want about earth's extremophiles, but none of them are going to ever be able to manipulate the environment ... or create mathematics. Both of which are prerequisites for radio. How about 'earth-like' planets? I don't think earth-like is sufficient. Yes, our evolution (both the planet and us) was in many ways serendipitous, and were it not for a big rock, given the previous history of earth (equally serendipitous) we would probably not be here. But it comes back to how specialized we are. We are, among millions of species, the only ones uniquely qualified to build radios. Even the smallest variations appear to preclude such an achievement. Given the serendipitous nature of our existence, while clearly possible, it (millions of intelligent species) remains exceedingly unlikely as a common feature of the universe in such a short (13.7 billion years) time. Happily, there are many chances for this to have happened in an observable universe containing anywhere from 200 to possibly 500 billion (or more) galaxies ... each with as many as a trillion stars (on average). Especially if those galaxies (again, on average) contain several billions of planets very similar to earth. When Frank Drake began his search for extraterrestrials in 1960, he had high hopes for detecting an artificial signal. Indeed, when SETI began, even using the relatively primitive tools of the day, the expectation was that we would discover 'alien' life forms within a mere 5 years. I was around then, and I was very excited about it. If for no other reason than to prove Christian dogma/theology was bullshit. Over 50 years later and no signal confirmed to be artificial. Our technology ... our ability to detect, ability to analyze the data ... has improved on the order of several magnitudes during that time. And nothing. We have 'observed' literally billions of locations since 1960. NO SIGNAL. If there are (or were) multitudes of intelligent, advanced life forms in the observable universe, it is absurd to suggest (as Shostak has) that they could be using technology beyond our ability to detect. Or as others have suggested, that they are all (or mostly) xenophobes. Or that they use magic, or telepathy. Or they simply aren't interested. Or they all killed themselves off. Billions upon billions of intelligent (and varied) species in the universe, and no evidence? Absurd. Just us? Equally absurd. By the way, what would be the point of creating a universe of 200 to 500 billion galaxies, each with billions or trillions of planetary systems ... all just to support one species? Rather inefficient for an omnipotent god, don't you think? Wouldn't one galaxy be sufficient? Or less? Oh wait. That's what Christians were so sure of: Earth was all by itself, except for those little lights in the 'firmament'. Whatever ... In any case, we do have millions of samples, and billions of locations. Yes, there is in all likelihood other intelligent, radio-capable life in the universe. I don't remember which scientists suggested this, but I must agree with them (in part): It might take an entire galaxy (or more) to produce just one radio-capable species. I would go even farther and suggest it might take a million or more galaxies to produce just one radio-capable species. But we know it happened at least once. And even if it takes a billion galaxies, that still means there are (or were) around 200 to 500 radio-capable species in the observable universe. And in all likelihood, they are identical to us.