The Fermi paradox is an argument that's often made in discussions about whether or not extraterrestrial life exists. It was originally put forth by physicist Enrico Fermi, and goes something like this: Because there are so many stars in the galaxy, many of which have planets, there should be such a large number of space-faring intelligent life forms that we should expect our planet to have been visited by now. But, as far as we know, we have not been visited. So, Fermi asked, "where is everybody?"
Some people argue that the answer to this discrepancy is that intelligent extraterrestrial life does not exist. While this is a possibility, I think jumping to this conclusion is premature, unjustified, and frankly demonstrates a lack of imagination.
I think an analogy will illustrate my point: Imagine that you encounter some tribe that lives deep within the jungle, and has been completely isolated from modern society. Given that they live smack dab in the middle of a continent, they know nothing about whales. You decide to take one of them to the beach with you, show him a good time. While the two of you are there, relaxing, you describe the concept of a beached whale to him. You say:
"Yeah, it'll sometimes happen: a whale that usually swims in the ocean will come ashore for some reason and get stuck in the sand."
The tribesman is skeptical, and says:
"Ok, so let's imagine that whales do exist, and that there are lots of them in the ocean. Why don't we see any beached whales right here? We're on the beach. We can look in that direction, in that direction, and we don't see any beached whales. Where are all the whales?"
What's wrong with his position? Well, it's not logical to use this one tiny stretch of shoreline as representative of all beaches worldwide. Furthermore, unless the two of you really hit it off, you're only going to spend, at most, a couple days at the beach. Just because a whale didn't beach at that specific location over the weekend while you were there doesn't mean it didn't happen last Thursday, and doesn't mean that it won't happen this upcoming Tuesday. The problem is that you have a limited area and timespan of observation.
The situation is very similar when we change the question to intelligent alien life, except the distinguished physicist Enrico Fermi and his acolytes take the place of our ignorant tribesman. Space is big. We can't even begin to wrap our minds around the size of our galaxy, let alone our universe. Even if there are thousands of intelligent alien lifeforms in the galaxy, there are hundreds of billions of stars, a sizable fraction of which have planets. And the distance between these planets is immense. Our galaxy is over 100,000 light years across. That means traveling at the speed of light, it would take 100,000 years to go from one end to the other. So exploring every planet in the galaxy is no small task. So it's possible that aliens do exist, and simply haven't explored earth yet, but might in the future. Just as even the most cosmopolitan tourist hasn't been on every stretch of beach in the world, perhaps these aliens have explored a bunch of other planets, but not ours yet. We are one of billions.
There's also the issue of time. Life has been around on this planet for about 4 billion years. The human species only came about something like 100,000 years ago, and human civilization is only a few thousand years old. We've been around for what amounts to a blink of an eye at the cosmic level. So it's possible that our planet has been visited dozens of times in the past, but just not recently. Maybe aliens came here during the Precambrian and took some samples of the primitive, microscopic life that was around. Maybe they came here during the Cambrian Period and checked out some trilobites. Maybe they came during the Jurassic Period and got to see the dinosaurs romping around.
Just because there's not convincing evidence that aliens have visited earth in only the past few thousand years gives us no reason whatsoever to conclude that intelligent alien life just flat out doesn't exist. In terms of the geological timeframe and the astronomical distances that we're dealing with, this would be like driving to just one beach, getting out of your car, looking around for just a few seconds, and then concluding that the beaching of whales is a myth.
If you just use your noggin and think about this, you can come up with countless explanations for why we seemingly haven't been visited by advanced extraterrestrial life. Maybe we have been visited in recent times, and the aliens are just really sneaky and haven't left any bread crumbs behind. If they're technologically advanced enough to travel across the vast emptiness of space, then they're probably also capable of being discreet. We tend to have this Hollywood-esque picture of alien visitation consisting of metallic saucers landing on the White House lawn or in front of the Capitol building, with tall, humanoid creatures emerging in front of an astonished crowd and saying "Take me to your leader!", but it obviously wouldn't have to happen this way.
Maybe they are here, but prefer to operate in secrecy, for whatever reason. Maybe they're just observing us go about our business, analogous to a wildlife photographer hiding in a camouflaged enclosure, allowing them to watch the animals act naturally without being aware of the observer. Maybe there's some sort of intergalactic code of ethics that prevents aliens from disclosing their existence to civilizations that haven't reached a certain threshold of advancement. Maybe they're just here to fuck with us, remaining hidden except for when they decide to abduct the occasional hillbilly and fiddle with his genitalia with some probes and strange instruments aboard their craft just so they can laugh at him attempting to explain his story to the people around him when he gets back. That sounds pretty hilarious if you ask me! Maybe they have a sense of humor!
Now let's be clear: I'm not saying that I think these possible explanations are probable or reflect reality. My point is: there's a huge number of potential explanations for why our planet apparently has not been visited by advanced alien life forms in recent times. There's even been a book published by Stephen Webb that I've wanted to get my hands on for quite some time that gives 50 possible explanations for the apparent lack of alien visitation. When you're talking about alien civilizations that could be millions of years more technologically and scientifically advanced than us, the possibilities really only end where your imagination ends. Everyone who's watching this, post a comment below with some of the more interesting or outlandish explanations you can think of for the apparent absence of alien visitors, and you'll see exactly what I mean.
The key point is: just because we don't have convincing evidence that aliens have visited earth in recent times doesn't mean that no alien life exists in the galaxy. As Carl Sagan said, "absence of evidence is not evidence of absence."