[ExI] Fermi Paradox and GRB bursts

[Anders Sandberg]

The problem with the GRB explanation is that it is too noisy. GRBs produce mass extinction within a few kilo-parsec, but they are really directional - if you are outside the beam the exposure is way lower. If GRBs were spherical in effect, then they could cover volume well, but most jet models fall off as theta^-2 or worse - nearly all energy is along the jet, within a few degrees. And the jet is typically pointed out of the galactic plane. So when you try to model this, in order to ensure that every inner system get whacked with a mass extinction say ever 100 million years you need a very high rate of GRBs in order to make it work. That is tough to balance with observed rates, which are on the order of one every million years. Even if you have a lot of GRBs, there are going to be unaffected systems fairly nearby too: the chance of some stars being lucky over several galactic rotations is pretty high. So the overall probability distribution of GRB impact ends up with a huge variance - it won't work as an effective Fermi paradox answer. 

Anders Sandberg, Future of Humanity Institute Philosophy Faculty of Oxford University
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