From the polymath Freeman Dyson, in an essay called “Extraterrestrials,” which appears in his collection Disturbing the Universe (New York: Harper & Row, 1979, pp. 210-211):

“Given plenty of time, there are few limits to what a technological society can do. Take first the question of colonization. Interstellar distances look forbiddingly large to human colonists, since we think in terms of our short human lifetime. In one man’s lifetime we cannot go very far. But a long-lived society will not be limited by a human lifetime. If we assume only a modest speed of travel, say one hundredth of the speed of light, an entire galaxy can be colonized from end to end within ten million years. A speed of one percent of light velocity could be reached by a spaceship with nuclear propulsion, even using our present primitive technology. So the problem of colonization is a problem of biology and not of physics. The colonists may be long-lived creatures in whose sight a thousand years are but as yesterday, or they may have mastered the technique of putting themselves into cold storage for the duration of their voyage. In any case, interstellar distances are no barrier to a species which has millions of years at its disposal. If we assume, as seems to be probable, that advances in physical technology will allow ships to reach one half of light velocity, then inter-galactic distances are no barrier either. A society pressing colonization to the limits of the possible will be able to reach and exploit all the resources of a galaxy, and perhaps of many galaxies.”