The problem with the notion of “coloniz[ing] a second planet,” stems from the disconnect between what the general public imagines extrasolar planets to be like (based upon decades of science fiction showing humans in spandex onesies finding habitable “M-class” planets all over the place) and what planetologists and ecologists actually know about the evolution of our planet; specifically, that our planet is habitable (for us) because of a very long history of development and modification of the biosphere, which includes every aspect of the planet which supports life, from the just-so atmosphere (thick enough to protect us from charged particle and high energy cosmic radiation but transparent enough to allow just the right amount of electromagnetic radiation to keep the surface of the planet in a delicate thermal balance), to the biome of interconnected species, to the hydrosphere which continually purifies and delivers water to where it is needed, to the deep organic lithosphere which creates a rich organic substrate that supports all planet life. Our planet would not have supported most modern surface animal life even as much as 600 million years ago, and the odds of finding a world that will have developed a biosphere analogous to our own, with just the right balance of atmospheric oxygen, not too much carbon dioxide, et cetera is wildly unlikely. Nor is “terraforming” a planet–converting it to closely resemble the surface environment of the Earth–very likely given the massively interconnected and interdependent systems required to sustain it. By the time we might have the hypothetical energy and skill to terraform a planet human civilization (or more likely, whatever has replaced it) will have long since adapted to living in space and making more efficient use of materials and mass than to thinly array itself across the unprotected surface of a planet with the vast wealth of resources wholly inaccessible below the crust.
The general public also has very unrealistic notions about space travel and exploration. The expectation of being able to achieve even modest fractions of the speed of light is belied by the rocket equation (which rat avatar has referenced) which makes it prohibitive to even achieve, much less carry sufficient propellant to decelerate from, speeds of even 0.01c. Even using the temperatures developed in nuclear fusion would not give sufficient specific impulse to be able to travel between stars within a human lifetime. (See [POST=18236632]Is there ANY realistic mode of interstellar travel?[/POST] for more discussion, which I won’t repeat here.) And if you did have a source of energy (e.g. antimatter) capable of exceeding those temperatures to reach much higher specific impulse, you’d find yourself facing an even more fundamental problem; the thermodynamics of rejecting the waste heat from such an energetic transfer would rapidly heat your spacecraft to incandescence. Even positing some kind of “sleeper ark” ship which is vastly beyond any capability to keep a complex mechanical system running for thousands of years or being able to suspend and reanimate mammals, actually exploring interstellar space would take massive resources and effort just to go a few hundred light years in the span of millions of years, which is not only magnitudes longer than any human society has continuously operated, but actually longer than humanity has actually existed.
As for the Fermi paradox, the assumption is that some kind of alien civilization will expand and send out detectable signals, either deliberately toward us, or incidentally as they communicate between one another, or else construct cosmic megastructures which would be visible to our observations, all within a timeframe of our own evolution as a technological species capable of making detailed observation of stars in our local interstellar neighborhood, is highly restrictive. This is actually a large set of different assumptions, from the idea that we would be able to detect alien communications, to the ability for an interstellar civilization to maintain some continuity over eons without diverging, to the idea that a species would be technically, socially, and evolutionarily static over such a period instead of evolving beyond technology and science as we know it. Civilizations could have come and gone hundreds of millions of years before modern humans even existed on Earth, and there could literally be millions of worlds inhabited by intelligent technological civilizations in our galaxy alone, and yet so distantly separated that we would never detect them and vice versa even assuming plausible advances in observation capability.
The idea of aliens constructing a Dyson sphere, for instance, assumes that an alien society would desire to collect the maximum amount of energy from their star; however, that might make as much sense as people today constructing some kind of boiler next to a bonfire in order to generate electrical power; an advanced technological society may be able to tap into sources of energy that vastly dwarf that created by the gravitationally driven fusion of stars, e.g. from direct control of intranuclear forces, or to some kind of quantum base state, or something even beyond our wildest speculations from known physics. It may make no sense for an advanced species to put the energy into building the structure and machinery for a swarm of solar-powered satellites if they can tap into much greater and more accessible energy. Similarly, an advanced species with a vast interstellar civilization will likely not bother communicating using radio frequency signals (which are inherently limited in distance and absorbed by the interstellar medium) if they can directly control and produce modulated gravitational waves, which are essentially unlimited in distance.
The so-called Fermi paradox is really just an expression of what we do not know about how an alien civilization could develop and what technologies they might have that are well beyond what we can imagine. It is not any reason to believe that extraterrestrial life is rare. Nor should we have any expectation that even if we were to detect intelligent life, that we would be able to communicate on anything but a very primitive level given the fundamental differences in cognitive evolution.
Stranger