It is complicated by the fact that this implies intelligent civilizations have a very finite window of opportunity in order to communicate. 500 Myr is a pretty short on cosmic timescale; so the odds that:
1) our civilization comes to exist in some 500Myr window
2) another civilization comes to exist in another 500Myr window
3) Signals sent from that other civilization arrive at the right time for ours to observe them
are considerably lower than just the odds that two civilizations develop that are advanced enough to communicate with each other.
Those windows are pretty big. These aren't life-scouring cosmic rays, just mass extinctions. Complex life is still there, and can develop a civilization in a couple million years.
The reshuffling of life could even increase the chances of a civilization occurring. And a smart species has pretty good odds of surviving a mass extinction.
I was actually basing this argument off of a similar one which gives much shorter windows (like ~1 Myr or something) as an upper bound on the amount of time a civilization can exit, which makes communication with other civilizations even more unlikely. This shorter bound comes from the very pessimistic notion that advanced civilizations will inevitably destroy themselves (e.g. nuclear holocaust). Obviously this one is a somewhat unfounded limit, but one could interpret the Fermi paradox as an empirical basis for the macabre hypothesis >:)
To survive a mass extinction from gamma-ray bursts, a smart species would build a self-sustaining subterranean city with 1 million humans in residence at any one time, perhaps rotating in and out on tours. I'm sure the Chinese will have one within 50 years even if no-one else does. And such subterranean cities are also likely how humans will be living on Mars for thousands of years before it's terraformed.
Well, my understanding could be wrong, but it seemed like the risk is to the ecosystem, not to human bodies directly. We personally don't need to be protected from the burst itself.
It's possible that gamma bursts are an example of a "great filter" - an event the stops life that Fermi Paradox and Drake equation can't really account for: http://en.m.wikipedia.org/wiki/Great_Filter
Probably not. Implicitly, the Fermi paradox looks at life in our own galaxy, which has apparently been quite lucky overall cosmologically. Many galaxies out there are hostile to life (GRBs are just one factor, I would argue that being a metal-poor galaxy with no active star formation is an even worse problem for life), but that doesn't have any bearing on life in our own galaxy - of which we have a sample size of one.
That said, even the friendly corners of the universe aren't a pony farm. It's more than likely for any given planet to find itself staring down the barrel of a stellar particle accelerator at some point during its time. Earth probably did, too, and we're still here. Whether it's a complete sterilization event depends on the amount of energy deposited. Depleting the ozone layer isn't enough. A planet would have to be pretty close to the event in order to get annihilated completely, so glancing blows with some limited impact on the ecosystem are probably more common in our cosmic neighborhood.
Unfortunately mixing is good enough that the increased UV will sterilize everything that does photosynthesis for quite awhile, including in the oceans. So yeah its enough.
One interesting part little discussed is that in typical physics fashion, there's many orders of magnitude from weakest (barely detectable, about once a day) to kills everything on the surface not just ozone disruption (Could happen?) and as a rough guess maybe a tenth the strength happens ten times as often.
So the disaster oriented contingent wonders about wiping out all non-ocean vent life on a planet, but its more likely that as a species we'll have way more "fun" with glancing blows or near misses leading to reduced crop yields.
Sometimes I wonder about unexplained civilization collapses and GRBs. All you need to is screw up crop yields to 99% of minimum and the mayans or whatever are all done. Don't need to kill everything green or kill everything living from radiation burns, just a glancing blow bad enough to tip an already tottering civilization over... Maybe there's enough spare capacity in europe / asia that nobody noticed or recorded a 1% hit but it was enough to do the Mayans in.
I realize that was a very imprecise statement on my part. Not enough to sterilize a planet is what I meant. It certainly is enough to cause a dent, even a huge dent, in the ecosystem.
> Sometimes I wonder about unexplained civilization collapses and GRBs
You wouldn't be the first, I think it's certainly probable these events did play a role in both prehistoric population changes as well as civilization collapses.
