One issue that's become clearer since Fermi's time is just how not-empty space is. There's enough rocks and dust and rogue planets between the stars that any sh

One issue that's become clearer since Fermi's time is just how not-empty space is. There's enough rocks and dust and rogue planets between the stars that any ship traveling at even a small fraction of lightspeed could be instantly destroyed if a collision happened, even with a particle the size of a grain of sand. And it's dark so you cannot see it coming. You'd need very high frequency radar or more likely lidar to detect it and you wouldn't have much time to dodge it. (Not to mention that you have to design that lidar with relativistic effects in mind.) The density of this stuff is not high but it's not zero either.