[ExI] Fermi Paradox and GRB bursts

[Dan]

> On Thursday, October 2, 2014 2:12 PM, Keith Henson <hkeithhenson at gmail.com> wrote:
> 
> I agree with Anders and add that GRB are *short,*
> a few seconds at the most.  So ~half of the planet
> surface is shielded by the whole planet's mass.

> Even if a burst fried everything on one side, the

> other side would be untouched and life would
> rapidly spread back intothe damaged half.

Depends on how fast the planet is spinning, but, yah. Still, that can be a pretty big disruption and how "rapidly" land can be repopulated and the path to technological civilization retrode is a big unknown. If Earth is typical, one can imagine a GRB now knocking out humans, probably making most land vertebrates extinct, and collapse of ocean ecosystems might threaten marine mammals too... How long would it take to get something like humans -- in terms of intelligence, civilization, and technology? If Earth's typical and the path and tempo of evolution is roughly the same maybe in the 0.1 Gyear range or longer, no?

> Asteroid impacts are a much bigger problem


Well, one can adjust any threat to make it bigger than another. The really big life destroying impactors are likely to be rare the longer a planetary system is around because the bigger ones get absorbed early on. But estimating the rate of large impacts across the galaxy or across the known universe, to my untutored intellect, seems to be harder than estimating the number of GRBs. Not that this problem is insoluble, but I'm wondering if anyone has a good handle on whether Earth is typical here. (If it is, then it might move a step closer -- still needing assumptions that the path (no radical shortcuts from microbes to technology) and rate (no breakneck innovations early on) of evolution toward technological civilization is roughly the same as Earth's and there are no big surprises.)

Or maybe I'm wrong. Has anyone come up with good fairly precise estimates on large impactors over solar system life times other than ours? (Even ours seems to hold surprises. And the early exo-planet discoveries revolutionized thinking about solar system development. I'm not up to speed on all of this, but I feel it should give one pause.;)

Regards,

Dan
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