Where are the other intelligent species?
My personal favorite, "the galaxy is filled with killer robots looking for signals". Seen on (of all places, a slashdot post) are my two favorites:
We could simply be the first -- the Elder Race, if you will. While life may have come into being on millions of planets across the galaxy, it could be that we're the only planet to have intelligent life or even complex life. Earth was very different as of only 2.5 billion years ago, dominated by a methane atmosphere with next to no oxygen. Since the rapid oxidation of oxygen is key to complex life on Earth, and since the Earth's high oxygen content in the atmosphere is dependent on the very complex chlorophyll molecule's existance, it may very well be possible that no world has gotten past the slowly dividing Archae state of evolution.
Also, if FTL travel is impossible, we could simply be too far away from everyone for them to have noticed or cared. A species living a scant 200 light years away would have no idea we exist still. Since there are stars billions of light years away, it's possible that no one will ever notice in the 4 billion years or so before our sun burns out. Even if they do, I doubt they'd be willing to set up a generation ship to take the billions of years to cross to what may be a dead civilization by the time their ship gets here. Even a closer civilization may already be trying to contact us, and we just haven't gotten the signal yet.
that aside, interesting.
Plus, I have to figure that once any species gets more than one egg basket (planet) to grow in, it reduces the risk of a failure mode in which everyone dies. For example, if humans spread to, say, five or six different systems, it's hard to envision what would kill everyone off. I mean, one system might nuke itself, but all of 'em?
That said, the way to figure that one out would be to see how many glowing dead planets there are out there. I don't know if you could see a freshly (or not so freshly) nuked planet from here. Who knows... maybe they're all over the place?
But, if they are, wouldn't the next species to come along notice and try to do something about it? i.e. "hey, the last five sentient species all nuked themselves to death... let's not do that."
WRT the resources, there's a lot of matter & energy out there, free for the taking for a starfaring species. For example, if the price of heavy metals started to go up enough, it would be worth sending ship[s] up to get metal rich asteriods and crashing 'em into the desert/coast.
Yeah, interesting stuff.