It's okay to believe in aliens. That in and of itself is a perfectly reasonable position. After all, there are estimated to be somewhere on the order of 10,000,000,000,000,000,000,000 (ten sextillion, or 10 to the 22nd power) planets in the universe. About 25% of the planets in our solar system alone are in the neighborhood of being able to support life, which means, if our solar system is typical (which it most definitely may not be), there are two and a half sextillion planets that can support life. Even if it only ever happens on one in a trillion (and estimates by reputable scientists have gone more likely than that even) of possible planets, then we'll still have life in some form on 2,500,000 planets. If it's only one in a sextillion, then there'll still be at least two planets in the entire universe, including this one, with life. So aliens, in the strictest sense, are really quite likely. The problem, though, is believing that aliens have discovered, reached, and interacted with us here on Earth. In this installment, I'll be exploring the many reasons why it's far more likely that believers in alien contact are far more likely to be deluded than right.
Let's start by listing off the propositions that would have to be true in order for alien encounters to have occurred.
1. Intelligent life. Sentience may be very common, a natural thing to occur at least once on every life-bearing planet. On the other hand, it may be extremely rare, making us for all intents and purposes unique. Alien encounters require that it happened on our chosen planet.
2. Scientific intelligence. The nature of the human brain has given us some big hurdles to scientific development. Religiosity during the Dark Ages managed to set back Roman achievements about 1500 years. What if our natural tendency towards superstition and tribal warfare were even more pronounced? Might we never have discovered the scientific method and started on the road to present technology? At the very least religious instincts would shave off a certain percentage of planets that would have otherwise made it further.
3. Faster-than-light travel. It doesn't matter how; what matters is that aliens are capable of moving faster than the 186,000 mile per second hard speed limit we know in our physics. It's extremely helpful if they have to cross the several light years between the nearest planets. It's absolutely necessary if they want to cross our 100,000 light year diameter Milky Way, even more so if they're visiting from the 2,500,000 light year distant Andromeda. With sufficient cryogenics, it's possible to get living aliens across thousands of light years without faster-than-light travel, but it would necessarily have to be a very important mission. After all, it would practically claim the lives of the crew: by the time they got back home, everything about home would be fundamentally different. Language and culture would be unrecognizable, and everybody they ever knew would be dead. So, unless we're proposing very nearby aliens (which would also imply alien civilizations at intervals every dozen light years or so), our aliens need faster-than-light travel. We have no idea whether it's possible to even do so. We have to assume it would take a good while beyond our current level to get there, if we ever will. So our aliens have to be a long way ahead of us. We've already discovered at least one planet that's 9,000,000,000 years older than ours (albeit not a terrestrial one), so I'll give this a plausible.
4. They've found us. It's tough to locate exoplanets. In the two decades we've been seriously looking, we've discovered some 20 a year (on average, though it's been rapidly curving upwards). Without getting pretty close, you need damn sharp instruments to spot planets. If there are a hundred billion planets in our galaxy alone, an alien civilization would need to check each one rather closely (at least closely enough to detect signs of life and intelligence). Our radio transmissions are at present filling a radius of about 100 light years, which is a tiny portion of our galaxy. Our aliens would have to listen from a point at least that close to identify us by our transmissions. This would at least imply that they've been searching for a very long time.
5. They care that we exist. From the aliens' perspective, what makes us interesting enough to investigate? If we're within their radius of travel and we've reached point #2 on this list, statistically speaking there would also be planets within that radius that have not: intelligent civilizations that don't practice modern science, or merely unintelligent animal life. Unless aliens are staggeringly different from us in many fundamental ways, it seems like they could learn enough about us by looking at the biology of non-sentient organisms, unobtrusively observing our cultural transmissions, and looking back at their own history. Why would they even have to go to the trouble to interact with humans?
6. They aren't interested in helping us. Highly advanced alien civilizations have a lot to offer to help out our primitive species. Medical technology to cure diseases. Agricultural technology to solve the world hunger problem. Energy technology to ease our struggle for resources. These things alone might be enough to foster world peace. The aliens could do it. They choose not to. Perhaps they're following some prime directive that prevents them from interfering in our business. That, however, causes dissonance with the next point.
7. They're only slightly concerned with secrecy. However they communicate across space, it would have to be undetectable to the SETI researchers -- which is an impressive feat, considering it's the sole mission of SETI to, well, Search for ExtraTerrestrial Intelligence. And they obviously put some effort into not leaving their ships around in places where lots of people will see them clearly, or take clear photos or videos of them. But they're obviously not very good at this secrecy thing. Their ships are often briefly spotted, and they leave lots and lots of eyewitnesses. The ship sightings are just carelessness. The eyewitnesses are slightly more difficult to deal with, but not at all hard if you think about it, as we'll see in the next paragraph.
8. They're only slightly concerned with our rights. This bit deals specifically with alien abductions, which aren't necessary to believe in alien contact but are frequently included in reports of it. So we have aliens who value our rights low enough to abduct us and perform invasive experiments on us. I've thought of three possible ways for them to get away with this without leaving eyewitnesses. The first is to have memory modification technology. For the sake of argument, we can say that that might be impossible. The second is to simply destroy the experimental subjects when they're no longer needed. It's possible that they value our rights too highly to do this. The third is to keep the abducted in captivity, so that they can live a pampered and relatively happy life -- possibly even with other abductees -- without going back to tell everybody; for minimal impact on our society, they could select only people who have no family or friends, and don't otherwise make a big difference in the world. These people's lives would likely be improved by being kept in an alien zoo. But it could be said that doing this impinges too much on our autonomy. So our aliens here have a very specific code of ethics: you can mess with individuals for a short period of time, even as much as performing embarrassing experiments on them, but you can't kill them or keep them. There are no such animals that we humans consider in such terms, so it's hard to imagine why that would make sense.
9. They're at least vaguely human-shaped. The most common form that these aliens take is that of the Greys, the famous image of big-headed bipeds. Now, this model for alien life is either remarkably naive or remarkably arrogant. Naive if we think that our body plan is the only viable one that can lead to intelligence: the smallest bit of imagination allows us to imagine vastly different general body plans, from the variegated creature designs of Spore to Douglas Adams' Hooloovoo, a hyperintelligent shade of the color blue. Arrogant if we presume that ours is the most likely body plan to be successful. Even a casual study will show how bizarrely different animals on Earth can be, and every single one of them is related to us. Just think how unimaginably, well, alien these aliens are likely to be if they evolved from a completely different origin in a completely different context.
I find that points #6, #7, and #8 are the most compelling reasons why alien contact is so unlikely. There's a very specific level of both secrecy and ethics required to produce a level of evidence that is only credible to some people and not accepted by mainstream scientists. It seriously smacks of special pleading, that the aliens are not willing to communicate openly with us but are bad enough at keeping their presence secret that almost everybody has heard fringe reports of contact with them. What's more likely: an alien civilization has reached us and has that specific level of policies that would cause this controversy, or some people dreamed or hallucinated a close encounter and some other credulous folks believed them? The best evidence for alien contact is photos and videos of lights and blurry objects in the sky, alongside eyewitness testimony. Photos and even videos are incredibly subject to tampering, as we can see all around us (ever watch a movie with compellingly realistic CG special effects, for instance?). And, as famous astrophysicist and skeptic Neil deGrasse Tyson tells us, eyewitness testimony makes for terrible evidence, as we're constantly subject to "brain failures", his apt renaming of optical illusions. Eyewitness testimony is only reliable if it's even more incredible that the evidence is false than that it's true.
So, odds are there are alien life forms somewhere. But there is no cause to believe that they've actually made it here; there won't be until either they make open contact in broad daylight in a populated area, or we at least manage to retrieve some obviously alien artifacts from them.