There’s a lot of misunderstanding about risk factors in all of human life, and so it’s not surprising that similar misunderstanding should occur with regard to potential contact with an alien technological civilization. If it’s any consolation to those who’ve just added Alien Invasion to their long list of things to be afraid of, we can relax.
Here’s why: Space is really, really big (see The Hitchhiker’s Guide to the Galaxy). In addition, as we’re limited to one sample planet, we have absolutely no idea how common Eukaryotic life is in the universe. Prokaryotic life is probably reasonable common, but anyone who’s glanced at the topics of bioenergetics and cellular biology knows that Eukaryotic life may be much, much rarer. And without multi-celled life, any technologically advanced civilization can’t get started. We also know from our own example that tool-making creatures are exceedingly rare even when you have plenty of Eukaryotic life forms upon which evolution can work its wonders. In the 3.5 billion years of life on our own planet, this rarity has occurred precisely once. And we’re in all likelihood going to exterminate ourselves within the next 200 years because a tiny percentage of clever people makes amazing things and then the vast majority of our dull-witted species uses those clever things to destroy everything around us.
So the probability of a technologically advanced alien civilization arising at the same time as our own, close enough to have any remote chance of physical contact, is essentially zero. Even with MagicDrive, our favorite cowboys-in-space trick to move around faster than 300,000 kps (the speed of light), it’s vanishingly unlikely that anyone is “out there” to notice our fleeting existence.
And guess what? Even if, hundreds or thousands of years from now, some other improbably rare alien civilization does pick up a signal from Earth, we’ll long since have obliterated ourselves along with most of the other life on our fragile but unappreciated little planet. Unless these putative aliens have a taste for charred bric-a-brac, it’s unlikely they’d bother to make the trip.