People have been talking about the extraterrestrials again. The former Canadian minister of defense is arguing for public hearings on "exopolitics" and a "Decade of Contact", delegating public monies to education regarding our unearthly bretheren. Meanwhile, a particle physicist at the US Fermi National Accelerator Laboratory is worried that alien signals received by SETI could contain viruses bent on taking over the world's computer networks.
The latter speculation is original thinking, I must admit. Thinking "outside the box" in this way is helpful in recognizing and addressing genuine future risks, even though I think this particular concern is off-base. It's also consoling that the mainstream media is willing to cover it, because some of the most truly serious risks to our well-being as a civilization will indeed sound "fringe" before they make headlines (nuclear weapons, chimera virii, others you haven't heard of).
But there are no aliens. Not around here, anyway. Why not? Because if there were, they'd already be here by now.
Radio has been in use for almost a hundred years. The Earth is surrounded by a sphere of intense electromagnetic activity almost 200 light years in diameter. It only gets more intense as time elapses. Short of bending space, there is no way we can ever take that information back. It's on its way out to the cosmos, in every direction at the speed of light.
You can't miss it. Natural phenomena, like supernovae and the cosmic microwave background radiation, have a characteristic signature that could never be confused with the orderly pulses of language and images. Information-theoretically, apples and oranges.
Radio is easy to invent, once you get to a certain stage as a civilization. You can't afford not to invent it. Harnessing electromagnetic waves to facilitate near-light-speed communication among the members of a civilization is as natural as constructing shelter or combating disease.
The Milky Way galaxy is about ten billion years old. Yet it's only a hundred thousand light years across. That's a ratio of a hundred thousand to one. If there were aliens about, we'd be bathed in radio signals continuously. Maybe they showed up so recently that their radio waves haven't hit us yet? Implausible. If life were to evolve in this galaxy, it would have done so already, and they'd be blasting us with their television dramas.
Perhaps alien civilizations have evolved to a medium of communication beyond electromagnetism? That could be the case, but then they'd be colonizing other worlds. Even moving at a tenth of the speed of light, saturating the galaxy with their presence would only take a mere million years, tops. But where are they?
Our solar system is appealing. We have a stable, mild star capable of providing billions of exawatts of free power to any alien race interested enough to set up shop here. There is no reason to pass us up. But our neighborhood is silent.
An advanced extraterrestrial civilization couldn't be missed. Life is constructed to flourish and reproduce. At no point will it collectively say, "we've had enough". Individual beings must explore, travel, and consume. Barring dictatorial control forbidding space travel, it's bound to happen. Not as a trickle, but a flood. Once a form of travel becomes technologically feasible, it becomes progressively easier until millions can do it.
The fact of the matter is simply that life is rare. Scientists believe there are a multitude of universes in existence, probably an infinite number. Presumably there are also an infinite number of intelligent civilizations. They are just separated by vast distances. The Self-Sampling Assumption compels us to treat ourselves as typical observers. If we're typical, then typical intelligent civilizations are separated by such vast distances that for most practical purposes they are alone.
The lack of alien presence is also evidence that FTL (faster-than-light) travel is impossible. Either that, or we are the only intelligent species with a civilization in the universe. (Or, faster-than-light travel exists, but is sufficiently weak that it only permits travel at a few times the speed of light - unlikely.)
The popular obsession with aliens and UFOs closely reflects the obsession with fairies in the early 1800s, and the fixation on angels and demons before that, and beliefs in the presence of spirits throughout history. We just want to believe it because the possibility is so exciting. This article from The Onion does a great job poking fun at this human tendency. Also see this Tech Central Station article ("Internet Killed the Alien Star") on how the Internet has helped us realize that alien visitation is make-believe.
If we want to witness bizarre new forms of life, or different types of intelligence, we'll just have to create it. You might say that creating it isn't the same thing as discovering, but this concern can be sidestepped by creating new forms of intelligence randomly, or constructing forms of intelligence that give rise to further forms in an unpredictable fashion. Both will happen, we just have to stick it out until the technology is here. You'll get your aliens soon, star-gazers!