You've gotta keep relativity in mind here. If a species can develop a means of simply traveling at roughly 1g acceleration indefinitely, they will become -in te

You've gotta keep relativity in mind here. If a species can develop a means of simply traveling at roughly 1g acceleration indefinitely, they will become -in terms of time- practically immortal. A single human can travel literally billions of light years in a single lifetime by continuing to accelerate at 1g for years at a time. Here [1] is a calculator to determine how long it would take to travel a given distance at a given acceleration. A common misunderstanding is the difference between observed mass increases and perceived. Your mass does not increase as you approach the speed of light relative to something else. But your observed mass does. Anyhow, the point here is that once a species reaches this level of technology, if it's possible, they will start to move beyond anything like the borders of time as we understand them. In my opinion, the most simple and probable explanation is technology (we don't know what's out there), but that's closely followed by the terrifying great filter being complete and unadulterated pleasure. As lives become easier and more pleasurable, the inherent drive towards procreation becomes less and less of a factor. If that reaches below a certain point, that's lights out for a species. Create a guaranteed means of subsistence on a planet, perhaps entirely automated, and now create a technology that becomes more inherently enjoyable and rewarding than procreation itself. Congrats you evil villain, you've destroyed your species in a far more effective way than any weapon ever could. Society goes out not with a bang, but also not with a whimper; they go out with a moan of pleasure. [1] - http://convertalot.com/relativistic_star_ship_calculator.html http://convertalot.com/relativistic_star_ship_calculator.htm...