Space isn't really big at all when you think in geological rather than individual scales. It probably only takes a couple of thousand years post-Apollo to colon

Space isn't really big at all when you think in geological rather than individual scales. It probably only takes a couple of thousand years post-Apollo to colonize dozens of stars. The real reason we don't see aliens is probably contemporaneity. Although in that context I have another remark. The Drake formula, often brought up in these discussions has a major omission: it doesn't account for multiple intelligent species over the total lifespan of a planet. I don't see why, a couple of million years after humanity's extinction, another intelligent species could arise and go through the whole space-flight thing again. A planet like Earth could have a typical cycle: 0.0 GA: formation 3.0 GA: bacteria 5.0 GA: intelligent species 1 5.1 GA: intelligent species 2 5.2 GA: intelligent species 2 5.3 GA: intelligent species 3 ... 8.0 GA: oceans boil away, parent star going manic 9.0 GA: destruction due to parent star going kabloom