Are We Alone?
Is Earth being visited by aliens, either now or in the past?
Plenty of people believe that aliens visit the Earth, either to draw circles in our cornfields then leave, or to show our ancient people how to stack rocks into pyramids, and then leave. One of the arguments they like to use is that it's completely absurd and arrogant to claim that in this whole enormous galaxy, we're the only planet on which life developed.
In fact, this is a false dichotomy. There's a third possibility: That there is plenty of life out there, but that nobody has been able to visit us yet. This is, in fact, the view held by the vast majority of astronomers and exobiologists.
We know from the chemical makeup of stars in our neighborhood that the same elements in our own solar system are also available in many others; and we know from observations (especially recent observations using the Kepler orbiting telescope) that planets abound throughout the galaxy. We've already found hundreds, and that's just from searching in one tiny, tiny region. It's practically a mathematical and statistical certainty that life exists elsewhere.
But we also know from physics that the problems of interstellar travel are incredibly difficult to surmount. But this often prompts the UFO people to point out how absurd and arrogant it is to claim that just because we can't solve a problem, aliens couldn't solve it either.
This is true. It's a perfectly valid argument. It's just not very convincing. Few futurists agree that the best way to contact an alien world is to vandalize some crops or stack some rocks, then leave.
Moreover, it would be much easier to send unmanned probes than manned UFOs, as it eliminates many of the problems relating to life support, time, and supplies.
But what's even a million times easier and cheaper than that is to send radio signals, and not bother to travel at all. If the aliens chose what is exponentially the most difficult way to visit us, would they really have used that opportunity to do something so stupid? If they have done so, they didn't leave us any evidence, which means they blew it. Hopefully, a civilization advanced enough to visit us is advanced enough to have put a little more thought into it.
So are we alone? Almost certainly not; but there's a good chance that almost all interstellar civilizations will, for the few thousand years that each exists, remain forever marooned in their own remote solar systems.
References & Further Reading
Gowdy, R. "SETI: Search for ExtraTerrestrial Intelligence." Astronomy: A General Education Course. Virginia Commonwealth University, 18 May 2008. Web. 10 Aug. 2011. <http://www.courses.vcu.edu/PHY-rhg/astron/html/mod/019/s5.html>
Lemarchand, G., Lomberg, J. "SETI and Aesthetics." JonLomberg.com. Jon Lomberg, 19 Jun. 2005. Web. 9 Aug. 2011. <http://www.jonlomberg.com/articles/seti_and_aesthetics.html>
Matson, J. "Alien Census: Can We Estimate How Much Life Is Out There?" Scientific American. 10 Feb. 2009, Volume 301, Number 2.
Plait, P. Death from the Skies! New York: Penguin Group, 2008. 7-32, 67-101.
Press, W., Teukolsky, S., Vetterling, W., Flannery, B. Nature. Cambridge: Cambridge University Press, 1986.
Shklovskii, I., Sagan, C. Intelligent Life in the Universe. San Francisco: Holden Day, 1966.