Invisible Aliens: The youth of the current galactic culture of the Associated Worlds does seem to call for some sort of Fermi Paradox explanation, especially considering the relative dearth of elder races, and the archaeological evidence of older species still, including much, much older species.
If you happen to find one, the Senate’s Standing Committee on Excruciatingly Long-Range Planning and the Imperial State Security Fifth Directorate Existential Risk Group would very much like to hear from you…
(i.e., until and unless I think of something adequately awesome, Shrug of God.)
The Gamma Ray Burster explanation is, IMO, both persuasive and terrifying, as the real world explanation. In universe, I would assume that GRBs are not a big deal to advanced spacefaring civilizations, roughly equivalent in “percent damage done to your civilization” as very heavy earthquake or unluckily aimed heavy hurricane is to ours.
Looking at the raw numbers, the problems of sheer intensity and spread compared to virtually anything else you might run into (up to and including relatively close supernovae) still make GRBs pretty much brown trousers time for even said advanced spacefaring civilizations. At least the ones that haven’t yet spread through enough of the galaxy (given what large chunks of the galaxy a GRB can effectively sterilize) that simply getting out of the way isn’t an option, heh.
The way I look at it, their key advantages for dealing with GRBs are two: first, FTL – which even though it doesn’t let you do much evasion, wormhole construction being c-horizon limited, at least gives you some forewarning. When the Periphery starts going silent, you’ve got at least that range’s worth of time to do something about the situation.
(Those civilizations on the Periphery which have thought about this may rather resent their role as meat-shielding for the inner Worlds, but, hey, that’s why expansion continues. So someone _else_ can be in the “hot” seat.)
And second, autoindustrialism, in which it’s at least theoretically possible to turn the von Neumann machines loose on the problem of building vast quantities, hundreds of star systems worth, of extremely heavy, multi-layer, high-grade radiation shielding and deploying it to cover everything that can’t be moved, evacuated, or shut down for the duration. (Like, say, most planets.)
So, y’know, while it’s still an existential threat, it’s a manageable existential threat, which is a comforting thought to those optimistic gentlesophs in the two above groups who subscribe to this explanation and less of one to those pessimists who think that, it being manageable, they should assume that all those previous civilizations that aren’t there should have been able to manage it.
But a big deal? That, it is, because the social and economic disruptions implicit in having to divert efforts to do all that – including the assorted moving, evacuating, and closing down – plus the slow but inevitable consequences for people who aren’t so advanced and thus can’t and can’t readily all move out of the way either, with ensuing megadeaths/gigadeaths and refugee movements, etc., etc., would make a head-on GRB within the next few centuries of the Worlds’ loosely-defined “present time” look like WWII, the Great Depression, the Dust Bowl, and a new New Madrid quake all stacked on top of each other, in scale terms.
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