One of the problems faced by people who would dearly love to find unequivocal proof of extraterrestrial life is: space is big.
No, not big. Really fucking huge. Here's an analogy that may help. Let's start out with saying the Earth has been shrunk to the size of the period at the end of this sentence. The Sun would be the size of a pool ball, and would be located about six meters away. The farthest decent-sized object in the Solar System we know of -- Pluto (yes, yes, I know it's not a planet, I don't want to discuss it) -- would be a dust speck 230 meters away, a bit more than twice the length of an American football field. The nearest star to the Sun, Proxima Centauri, would be another pool ball 1,570 kilometers away, roughly the distance between where I sit now (in upstate New York) and Jacksonville, Florida.
And in between us and it is a whole lot of bugger-all.
Just seeing any kind of detail in objects that far away is tremendously difficult, and that's even considering the amazing strides we've taken in telescope design. Not only is there the distance involved, but there's dust and debris in between us and everywhere else, blurring the image further. There could be friendly aliens on one of the planets orbiting Proxima Centauri leaping about and waving their six arms and holding up signs saying, "HERE WE ARE!", and we wouldn't see them.
And that's the nearest star.
Things become even worse when you consider actually going there. Voyager 1, currently the most distant human-made object from Earth, is traveling outward at a little over sixty thousand kilometers per hour. A decent clip, right? Well, even so, it would take ten thousand years to reach Proxima Centauri, if it were heading that way.
Which it's not.
To me, this is the strongest argument against UFOs having an extraterrestrial origin. Every indication we have is that the laws of the Special and General Theories of Relativity, which prohibit faster-than-light travel, are enforced in every jurisdiction. It's hard to imagine space-faring aliens crossing all this distance to come see us (only to abduct some cows and leave a crop circle in Farmer Bob's wheat field, then leaving). We may well not be the only intelligent life in the cosmos, but the likelihood of having a face-to-face (or face-to-whatever-they've-got) visit is slim to none.
Even having a nice chat with them from a distance is gonna be tricky, not to mention boring. Once again, using Proxima Centauri (at 4.2 light years distant) as an example, if we were to beam a focused radio wave signal toward it containing some kind of encoded message, the best-case scenario of what it'd be like in Earth's SETI Command Central would go something like this:
Us (into microphone): Hey, Proxima Centaurians, how are y'all doing?
[8.4 year silence]
PCs (voice from speaker): We're doing fine. The weather's been nice, although we could use some rain. How are you?
Us (into microphone): Same old, same old. You know how it goes.
[8.4 year silence]
PCs (voice from speaker): Don't we ever. It's the same everywhere in the universe, amirite? LOL
So anything approaching scintillating repartee would be kind of out of the question.
Another complication is that intelligent life doesn't mean intelligent life we can communicate with. Consider the fact that until the invention of the radio telescope (1937), there could have been extraterrestrials positively screaming at us, and we'd have had no way to know. And it's no better with messages going the other way. Prior to our own radio signals, the Earth itself would have appeared completely silent; there would have been little in the way of indication that there was anything alive down here, despite the fact that the Earth had already hosted life for three billion years.
As an aside, it's an interesting question as to whether we're going silent again, given the increasing efficiency of signal transmission -- our "radio bubble" is getting weaker, not (heaven knows) because we've got less to say, but because less of the signal is leaking out into space. This might not be a bad thing, although it's probably already too late. Recall in the brilliant send-up of the original Star Trek, Galaxy Quest, that the aliens (the Thermians from the Klaaaaaatu Nebula) thought our early television signals were documentaries:
Lieutenant Madison: They're not all "historical documents." Surely you don't think that Gilligan's Island...
Captain Mathazar (sadly): Oh, those poor people.
Enter the Compact Color Biofinder. This amazing device, developed at the University of Hawaii - Manoa, uses an interesting feature of many organic compounds -- fluorescence. Fluorescence occurs when light at one frequency is absorbed by a molecule, resulting in the electrons in its atoms bouncing to higher energy levels; when those electrons fall back into the ground state, they emit light at certain characteristic frequencies. (An example you may, unfortunately, know about; if you shine an ultraviolet light on cat piss, it fluoresces green, which will allow you to find where you need to clean up if Mr. Fluffums decides not to use his litter box.)
Because the fluorescence spectrums of different types of organic compounds are pretty well known, this allows you to analyze the light coming from an object that contains organic residues and determine what those residues are made of. The concept, of course, is hardly new; it's the basic idea of spectroscopy, which has been around for two hundred years. But the Compact Color Biofinder has refined the process to unbelievable levels. It was able to detect and identify traces of the biological compounds in a fifty-million-year-old fish fossil from which you'd think every organic trace would have disappeared long ago."The Biofinder is the first system of its kind," said Anupam Misra, who led the team that developed the new device. "At present, there is no other equipment that can detect minute amounts of bio-residue on a rock during the daytime. Additional strengths of the Biofinder are that it works from a distance of several meters, takes video and can quickly scan a large area... If the Biofinder were mounted on a rover on Mars or another planet, we would be able to rapidly scan large areas quickly to detect evidence of past life, even if the organism was small, not easy to see with our eyes, and dead for many millions of years. We anticipate that fluorescence imaging will be critical in future NASA missions to detect organics and the existence of life on other planetary bodies."