Here's a post I made about that subject quite some time ago. Originally Posted By: OldFolks 12/19/05As for life elsewhere in the universe, I would think it improbable that there aren't at least billions of advanced civilizations out there. Especially considering that there are estimated to be some 50 billion galaxies in the visible universe with an estimated 70 sextillion "visible" stars. To give 70 sextillion a point of reference, that is 10 times the estimated number of grains of sand in all the desserts and on all the beaches on Earth.As for how advanced life elsewhere would be, depends on seemingly endless list of variables. The most important of these variable is the avalibilty of time to develop. The universe, as we know it, is estimated to be between 12 and 14 billion years old. While at least one planet has been found that is estimated to be around 10 billion years old it is the exception, it is huge, gaseous, and more likely a failed star than a true planet. Life as we know it, most likely, requires a terrestrial planet which puts a much greater restriction on time to develop. All heavy elements are forged in the depth of stars and then broadcast into space in giant supernova. Even given that large stars live short lives, it is estimated that there were not enough heavy elements, yet forged, to make non-gaseous objects anything other than rare, until around 8.5 to 9.5 billion years ago. This means that, most likely, any intelligence out there is only 2 or 3 billion years older than us (the age of our solar system). Neglecting all limiting factors, within a solar system, to uninterrupted development, the question then becomes, how does intelligence evolve. Does intelligence grow exponentially, does it grow in spurts with long plateaus in between, does it take one great leap forward then only have low steady growth thereafter. Lastly assuming intelligence grows exponentially, can intelligence ever overcome distance, even for communication, without regard for travel. The speed of light is horribly slow given the scale of space and therefor makes communication between civilizations impractical. In a universe where it will take 1000 years to ask the question and another 1000 years to get the answer(assuming there is somebody reasonably close), with at least one of the parties (us) living only around 100 years, can that communication be, in any real terms, productive and advance both or one of the civilizations. Travel become nearly out of the question, in terms of physics as we understand them. While opening a wormhole is mathematically possible, intelligence still has some major hurtles to cope with. For instance, how do you come up with the practically infinite power needed to keep a wormhole open for longer than a millisecond and big enough to allow passage to anything bigger than a particle, how do you make sure the wormhole dumps you where you wanted to go, once your there how do you get back. There are many other mathematical methods for traversing space but all have their limitations. Chief among those limitations, are, that the results achieved are the results of the laws of physics and not necessarily the results the traveler desires.Now having listed why interaction between civilized societies is at best highly unlikely, the caveat must be applied that all these arguments are based only on physics as we understand in our visible universe. Furthermore our understanding of the true nature of physics is lacking at best and what we observe in physics, with regard to gravitation, electromagnetism and quantum mechanics, is often contradictory and only seems to further obfuscate the finale unifying theory and there-in a complete understanding of the cosmos and all of its variables. Thus leaving open the possibility that, being that, there are many things we do not understand and have yet to discover in just the realm of physics, it is entirely possible that some galactic civilization has conquered all these hurtles and is able to pop in and out of our realm of understanding.The long and short of this is that, while science can say galactic communication between civilizations is unlikely based on our understanding of the universe, science must also add, that given our incomplete understanding of all of the factors and the variables of physics any answer we give is incomplete and therefor invalid, as far as being a matter of fact. So, all in all, all we know is that we don't know enough to give any definitive answer on whether any alien is capable of visiting us. In ending, the door for alien visitation is neither anymore closed or anymore open than it was at the beginning of the exploration of this question.
