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Originally Posted by pan6467
In the end it comes down to belief. I believe because I feel there's "evidence" in the Bible, from NASA, from films, from drawings throughout history, and because no one can prove that UFOs don't exist. Others disbelieve because the "evidence" is sketchy and one can interpret the "evidence" any way they wish, thus I may see "grays" someone may see shadows and unclear markings, I may read in the Bible, an interview, etc and interpret things one way, someone else may interpret another (go figure).
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The problem with this stance is that it's virtually impossible to prove a negative. I can't prove that extra-terrestrial life doesn't exist, nor can I prove that we haven't been visited by greys. The question for me, then, needs to be one of probability. And, well... let's talk astronomy.
Recently, a NASA probe made headlines. New Horizons completed a fly-by of Jupiter back in February en route to the Kuiper belt and Pluto-Charon. Using Jupiter's gravity for a boost, New Horizons was able to increase it's speed to just over 36 000 mph, making it the fastest spacecraft we've ever launched. For reference, if we were to express this speed as a mach number it would be roughly mach 47.
At that speed, New Horizons ought to reach the outer edges of our solar system around July of 2015, almost 8 years from now and approximately 9.5 years after launch.
The star nearest our own is Proxima Centauri. Proxima Centauri is a red dwarf star with absolutely no ability to support life as we understand it and no known planets; it's about 4.2 light years away. If we were to direct New Horizons, the fastest spacecraft we've ever launched, towards Proxima Centauri, it would get there in approximately 681 million years.
But maybe the greys are more advanced. I can dig that. Our fastest speed is roughly 0.000005% the speed of light. If we assume that our extra-solar amigos are advanced beyond anything we can begin to dream of and can accelerate their vehicles to, say, 1% the speed of light (and somehow ignore the relativistic effects) then it would only take them 42 years to get here from Proxima Centauri. Well, maybe they're really long lived; only, we've already established that life as we know it is incapable of existing at Proxima Centauri, since the star itself isn't hot enough and doesn't have any known planets, so they'd have to get there from somewhere else. Maybe they're from the Centauri system (which consists of Alpha Centauri, Beta Centauri and Proxima Centauri); if so, that'd be an amazing stroke of fortune. If not, then we'd have to look elsewhere. There are a handful of stars 20 light years or less from our own; if we assume that our alien friends can handle a 200 year trip without any real ill effects, then they are candidates. Except that there are only one or two of them that are capable of supporting life as we understand it (depending on how loose our criteria are). Outside of that, the distances and attendant times get very big, very quickly. And then, we have to ask the question of why would they trundle along in deep space for even 40 years to come and visit us who, speaking in galactic terms, have only just stuck our toe out on our own doorstep?
Therein lies the problem with the idea of extra-terrestrials coming to visit us, as I see it. People are quick to apply the balm of 'advanced technology' without really understanding what a cognitive leap it is. Space is big. Really big. Our galaxy is 100 000 light years across and is one of billions in the Universe. The nearest star is 4.2 light years away, which sounds like a small number. It looks a lot less small when we express it in miles and find that it comes to just over 24 000 000 000 000 (twenty-four trillion). We can then couple this with a few little informational nuggets from physics. For one, we know from the thoery of relativity that physics are the same all over for everyone. So our aliens need to obey the rules just like everyone else. We can then combine another piece of information provided by the same theory, which is that the closer we get to the speed of light, the more energy it takes to accelerate further (remember e=mc^2?); and thus, we know that even hitting 1% of the speed of light for anything more than a single particle would take more energy than we've generated in all of human history. Them's some pretty advanced aliens and they're going to an awful lot of trouble for us.
Mind, through all this I'm not saying it's impossible. I just don't see it as terribly likely and it's going to take more than a few blurry drawings and vague quotations to convince me, even if said quotations come from a host of former US Presidents.