aliens

Just about every popsci show I've seen in the last few years favors the idea of aliens being everywhere. I'm sick and tired of hearing "reputable scientists" utter the stereotypical sentencte: "The Universe is probably teeming with life". Just because a lot of extrasolar planets have been found lately and just because alcohol's been found in deep space (along with other organic chemicals such as aminoacids) and just because bacteria can live everywhere on Earth doesn't mean that life just appears everywhere there's water and heat. And then becomes intelligent. A few decades ago you didn't see this almost-consensus that aliens must exist with high probability. Now everyone's being disgustingly optimistic about "not being alone in the Universe".
There are a lot of problems with the whole extraterrestrial life issue.
First, you can't do statistics on a sample of one. Therefore, you can't scientifically predict anything about life elsewhere. If we were to find life on Mars or Jupiter's moons or wherever, we'd have a sample of 2, which would still be insufficient, but a lot better than 1. To be pedantic, we don't even have a clear, universal definition of life, or a clear method to identify it; we can only speculate that if it existed, it'd be similar to what we see on Earth because the physics and chemistry are the same everywhere.
Second, we are very limited in our capabilities to detect extraterrestrial life. Radio searches have so far yielded nothing and I personally doubt they ever will, because radio "leakeage" from aliens would be too weak to be detectable. The whole "our TV shows have already reached dozens of stars" is bullshit, the signals are too weak to be detectable. A 100 kilowatt radio source on Earth would shine just about 10^-24 watts of power on a generous alien antenna (one square kilometer) placed not very far away (10 light years), for the same reason that distant stars appear so dim to us. If that antenna were connected to an incredibly sensitive receiver, say cooled to a tenth of a degree above absolute zero, the electronic noise due to thermal motion would still be 1000 times stronger. They could probably barely receive Morse code, which takes a lot less bandwith than human speech. Ramp up the distance to 100 light years and the signal gets 100 more times weaker. So in order for the aliens to hear us, either they'd have to have incredibly advanced receiver technology, or our beam would have to be focused and directed towards them. We're faced with the same problem when trying to detect alien radio signals. We could probably detect a signal if it was being intentionally broadcast, but we haven't yet. This raises another problem: how many times, and for how long, did we transmit such signals into space? Not that many, not that long. Maybe the aliens are doing likewise.
What other means of detection could we use? Take artificial lighting for instance. City lights on Earth can be seen from space, and they have a very distinct spectral signature. Aliens would possibly also use artificial lighting at night, but light has the same problem as radio signals. Maybe if we had a telescope powerful enough to see planets around other stars, we'd be able to detect these hypothetical lights. At least we'd know what to look for, given that aliens would probably see in the same frequency range as we do, because their eyes or whatever they have would adapt to the spectrum of their star, which would be similar to ours. If we watched the planet from the right angle, we could possibly even see these lights turning on in the evening and off in the morning. The total power coming out of our lights is much bigger than the power from our radio transmitters, so things might be the same on their planet. Light is also easier to detect than decoding radio signals.
Another thing that comes to mind are nuclear explosions. Any sufficiently developed aliens would do them, even if just for testing. They would be strong enough to be detectable, but did we ever see any sudden flash of light near a star? Did we ever look for one? I have no idea.
Finally, contrary to what some may think, there's not a shred of physical evidence that aliens have ever visited us. That might be due to the difficulty of interstellar travel, but nonetheless it hasn't happened.
So yeah, even if aliens exist, they can't get to us and we can't get to them. Relativity, which is pretty much proven to model this world correctly, while not forbidding faster-than-light travel, predicts that such travel would result in grandfather paradoxes and such. We also can't talk to them, because information, however we send it, radio, lasers, X-rays, whatever, only travels so fast. Maybe in a "parallel reality" we could, but not in this one. So there. We are alone.