We had a similar discussion this time last year. What I posted then seems relevant to this thread:
The biggest problem we are going to face in expanding to other planets, and even sustaining our current population, is energy. Our planet spent 500 million years storing sunlight as oil and natural gas, and we’ve burned through half the known reserves in a little over 100 years. And that was the easy half. What we have left is remote, hard to extract, and of lower quality than what we have used so far. Our window of opportunity to expand beyond this planet is closing rapidly, and it will take concerted international cooperation to make it happen. Given the current state of world affairs, resource wars later this century are much more likely than international cooperation to send a privileged few off planet.
Could something similar have happened on other planets? Possible. It is also possible other intelligent species built sustainable societies early, realizing that replacing their resources is the only path to long-term survival. Unless they have energy sources we cannot imagine they would be limited to colonizing their local solar system(s).
We also face the problem of global warming disrupting the world economy and turning our attention away from space exploration and toward saving what we can of our civilization. If we avoid resource wars as oil depletion forces us into a lower-power, sustainable society, we may decide staying alive on this planet is the only choice we have. If global warming reaches a tipping point and accelerates we may find our species too busy trying to survive to worry about space travel. With billions of people starving, coastlines receding, and weather extremes increasing, feeding people and avoiding (or surviving) wars will be far more urgent than building space colonies.
Building an off-planet biosphere will take tremendous amounts of energy. Does anyone here really believe people will give up their suburban lifestyles, complete with plasma televisions and affordable air conditioning, so a few hundred or few thousand people out of billions can move into an orbiting biosphere? Moving people to Mars and building sustainable biospheres there will take even more energy and money out of our economy. Are we going to quit feeding starving people in third-world countries so a few people can live on Mars? I doubt it.
How much more energy would it take to build a spacecraft capable of taking generations of people to another solar system? Assuming, of course, we could even identify a viable solar system from Earth. Given our current understanding of physics it would take us hundreds of years to send a probe to another solar system so it could look around and send a message back regarding the feasibility of colonizing a planet around another star. How many such probes would we need to send before we found a suitable planet?
I’m not at all surprised we haven’t seen evidence of an alien civilization. There are many, many ways for a civilization to fail, even one as supposedly advanced as ours. Even a successful civilization is highly unlikely to expand beyond its own solar system. The distances are too vast and the return on investment too low.
There may other advanced civilizations in our galaxy, but they have not contacted us. That means one of two things: Either we are the most advanced civilization in this galaxy, or the distances and energy investments interstellar travel requires are too great to overcome.
Consider how fragile is our civilization.
We have finite resources, and we’ve already burned through half the easiest to exploit and most energy intense of them (oil). When we run out of oil (whether that is 50 years or 500 years from now is irrelevant in the long-term) we’ll have to make do with lower energy sources to power society. How many people can our civilization support without oil? Certainly not six-billion-plus.
Whether human-induced or natural, global warming is real and the results will be catastrophic to our civilization. People in poor countries (Darfur, for one) are already suffering greatly due to global warming. Meanwhile, we continue pumping poisons into our atmosphere, and blithely ignoring the consequences. How many civilizations in our galaxy have poisoned themselves to the point they cannot reach the stars?
All the paleontological records we have on this planet show a violent history. From dinosaurs to humans, the best killers survive to pass on their genetic material. This suggests evolution favors aggressive species. I realize, of course, we have only one planet as an example, but the only evidence we can see points toward advanced species having a disturbing tendency to kill. This does not bode well for long-term survival of any species that discovers relativity and quantum physics, then develops nuclear weapons to protect themselves from aggressors. How many species in our galaxy have blown themselves to extinction through warfare?
The universe is out to kill us. There have been at least five mass extinction events on Earth (Source). From what we know of solar system formation these events should occur everywhere. How many advanced, or potentially advanced, civilizations have been wiped out by marauding comets and meteorites in our galaxy?
Our ancestors almost went extinct, recently. Only 70,000 years ago we were down to fewer than 3,000 homo sapiens on this planet. One bad winter could have wiped us out. How many intelligent species in our galaxy suffered that bad winter?
So, does all this mean we are doomed? Of course it does. The only question is “When”? It could be 20 years from now or 20 million years from now, but sooner or later our species will die out. We are not as special as we think we are, because when you get right down to it the universe doesn’t care one way or the other.