Oh, doubtless, I'm just kinda replying to all the various theories en masse before they get repeated for the n-teenth time in the thread, force of habit, this whole subject matter is as you might guess, kinda dead horse and heavy interest simultaneously to me.



Well, an option like that is a sound explanation, on the surface, until one considers that it would be pretty common sense with such a FTL comm system to leave satellite monitors around pretty much every star just on general principal, and that they'd have some common sense antennae and sensors on them, and that presumably they'd have FTL comm ability too, and someone would think to include a little program with the parameter 'relay any non-natural emissions to high command'. I mean even assuming they utterly indifferent to new emerging life, which would seem improbable, they'd probably want to have early warning systems about sudden star fluctuations, supernova, or say ships that had problems and needed help and might need to signal for it in some natural fashion, like strobing their engine in galactic morse code so the local sensor buoy could say 'oh, someone needs help'. It would just make too much sense, especially with FTL comms, to have such a net, and if they had FTL comms but no FTL travel then they'd definitely want it whereas if the had both then it's so easy to deploy and maintain that they'd just tend to do it anyway I'd think.



Well, we're not really 'on the fringe' that's kind of out of date, we're also kind of a mish-mash of at least seven other galaxies we've eaten plus a lot of the older stars are pretty far out from the core. Also, I think you need to be pretty much in the same system as a supernova to get a full blown planet kill, the release on a supernova is generally around 10^44 Joules, parallel to the energy our sun releases over its entire lifetime, however that is spreading of an obscenely large area to get anywhere, with a light year being about 10^16 meters, 1 Ly away you're surface is some 4πx10^32 m² and that means a given square meter is absorbing, at 1 Light year, around 10^11 Joules in a fairly short interval of time... a sunny day on earth you get about 1000 W/m² so 10^8 seconds worth of that, a few years worth of light, definitely crispy critter range though you might have surviving life on the other side of the planet especially in the sea... and stars do pack that tight in a lot places, but 10 Ly out that's down to 10^6 seconds worth of juice, a couple weeks worth, and I'd want to run the numbers to see how much actual immediate temperature rise that would cause, on Earth that would mean we'd absorb roughly 2.5x10^21 Joules of energy all at once, I'm sure it would blind anyone who saw it but a 1 megaton nuke packs 4x10^15 Joules of energy so this would parallel around a million nuclear warheads going off. Sounds utterly obliterating but I'd actually bet people in bunkers or even basement might live, and in terms of heat absorbtion, but it would be enough to heat up 10^18 liters of water by a degree fahreheit, for instance, which my on the fly math says would be enough to heat up the first ten feet of the oceans by a degree, it might be hugely cataclysmic but I can't see it wiping out all life on the planet and its entirely possible someone sleeping in an igloo might emerge an hour later unscathed. So they're definitely not going to leave a place unscathed, I wouldn't want to be within 100 Light years of one, but they don't blow up planets not already around them and probably don't sterilize anything but very near neighbors if that. Usually one talks about the effect absorbing that much gamma would have on a planets ozone layer more than anything else but the models I can think of usually say we would survive if 30+Ly away. You mentioned this but I always like to cold run the numbers to make it more solid. Another thing to remember is that early life, while it might develop near the core as that's where supernova would be most frequent and thus compile the heavy material for planets soonest, would probably tend to get zapped by Supernovae more... however supernova aren't really all that common, around one a century for the whole galaxy in any sort of recent time frame (since Earth formed). Now, I remember hearing a theory that life on Earht and elsewhere may all have been limited to stupid-simple for a long time as supernova were more prevalent and would come by and obliterate anything complex every few thousand-hundreds of thousands years while simple microbes would just respawn back to normal levels in a few years but I don't recall any data indicating supernova were ever that prevalent in terms of density and I can't remember who came up with it, so I don't take that one too seriously. The density thing does matter though, if supernova are occurring every century or so, and exterminating all life in a 100 Ly bubble around them (which as indicated is being generous about it's destructive potential) then in a billion year period one can expect around 10 million such events, charring up around a million cubic light years each, or about 10^13 cu. ly get roasted, pretty huge but the main stellar disc's volume itself is around 100,000 Ly across by 1000 ly thick, on an order of 10^13 cu Ly itself.

I wouldn't call that even odds since I've been rounding an awful lot, bad thing when squaring and cubing values, and because stars to tend to huddle up... they also tend to huddle by type. Also, if you drop that boom radius to 35-30 ly your char volume drops to around 10^11, or less than 1% over a billion years. An astrophysicists could give you better values but I don't think anyone has tried for a comprehensive model. Dreaded Anomaly might know for that matter, he seems more up to date and interested in such things, I'm kind of yanking this out of semi-thin-air. I pick a billion year because that's longer than life's really been on the surface, I don't think the super-simple deep oceans soup stuff from a billion years ago would be seriously impacted by anything but a very close nova, I gather multi-celullar life didn't even pick start till about 600 My ago, and the rate at which unicellular life multiplies I think you'd need to damn near kill it all to keep it from being fully recovered in a couple years. But it is what brought that theory to mind, that life may simply not have been able to get complex anywhere until about the time we did because the only places with enough heavy material to form planets were all getting zapped by supernova sufficiently frequently to prevent complex life emerging... again I can't even remember who's theory that is though and I have no idea if it included modern data or is even scientific and not just a crackpot concept that I heard at some point and got stuck in the back of my head. My SWAG is that for billions of years now planets have been able to exist with relatively low odds individually of being hit by cataclysmic supernova and thus wouldn't be a reasonable Fermi Paradox solution.