I have been reading lately about serious inquiries into the possibility of extraterrestrial visitors to our planet, so I thought I would say a bit about how unlikely I think that is.
There are, it seems to me, three possibilities: the first is that some other group of sentient creatures picked up radio waves coming from earth, and came by to take a look; the second is that some sentient creatures were just wondering about in our galaxy and stumbled on this earth at a time when they could pick up radio waves and infer that somebody was here; the third is that some sentient creatures sent out search vehicles throughout the galaxy which picked up our radio waves, sent word back home, and brought folks looking to see who we were. I am leaving entirely out of consideration the possibility that we are being visited by creatures from other galaxies – they are so far from us that that seems to me entirely beyond the realm of possibility.
Radio waves travel at the speed of light and they were discovered in the 1880s here on earth. Let us suppose that the most advanced creatures we can imagine are capable of interstellar travel at a rate half the speed of light (I am not sure that is even theoretically possible but what the hell.)
The first and third possibilities mentioned above would mean that radio waves left earth at the speed of light – let us suppose, in the 1880s – and creatures came back to take a look. A little simple calculation tells us that those creatures would have to live no more than 40 light years away from us – 40 years for radio waves to reach them and 80 years for them to suit up and travel back.
Well, our galaxy is the Milky Way. It is roughly 80,000 light years in diameter, which means that it has a surface area (it is, so to speak, flat more or less) of roughly 20 billion square light years. The circular area around the home planet of these extraterrestrials within which they could receive messages from us and come back to see who was there is roughly 5000 square light years. So unless thesse creatures happen to live in a circular area around us that is one four millionth the area covered by the Milky Way, possibilities one and three are ruled out. The possibility that these sentient creatures had just gone off on a 50,000 year jaunt and happened by just when we were sending out radio waves is so small I cannot even estimate it.
I rest my case.