Gamma ray bursts as an explanation for the Fermi Paradox?

[Junghalli]

Since this board has a number of clever and scientifically knowledgeable people, I'd like to ask your opinion on James Annis's explanation of the Fermi Paradox. His idea is that gamma ray bursters were more common in the early galaxy than they are today, and so hindered the development of complex life. Ergo the universe has only recently (in cosmic time) moved into an era in which it is friendly to intelligent life. Here's a link to his paper:

An Astrophysical Explanation For The Great Silence

I ask your opinion of this because I'm thinking of incorporating this into my own hard SF universe as a partial explanation of the Fermi Paradox there.

One fly in the ointment I noticed was I've found a lot of articles about findings suggesting that GRBs aren't common in metal-rich galaxies like the Milky Way. Though I imagine the early Milky Way would have been less metal-rich than the present Milky Way, which should fit nicely with GRB frequency diminishing with time. The best estimate I've been able to find is that they would already have been rare by the time the sun formed. But I guess that would still work well if we assumed before that GRBs were surpressing the development of complex biospheres, so older planets than Earth didn't get much of a head start. It would probably also help if intelligent life took longer to evolve on most worlds than it did on Earth (I can think of at least one good reason that might be - not to mention in this universe I assume most biosphere worlds never develop intelligent life). Plus I still lean a fair bit on other Fermi Paradox "filters" (intelligent life isn't too common, the majority of intelligent species never develop technological civilizations, interstellar colonization is really expensive and unrewarding so most civilizations tend to give it up sooner or later or just never attempt it, and a lot of technological civilizations don't last too long).

Just how damaging is a GRB to a biosphere? I know one is suspected of being the culprit of the Ordovician mass extinction. Would they just wipe out terrestrial life, or would species in the shallow oceans be at risk as well?

Is the idea of a long GRB-induced (or otherwise) "dead era" before a "intelligence era" that begins in relatively recent cosmic time plausible?

[JointStrikeFighter]

Just throwing it out there but this was the plot of a stephen baxter book. [space?]

[Junghalli]

Just throwing it out there but this was the plot of a stephen baxter book. [space?]

Manifold: Space, yes. Sort of, anyway. The explanation for the paradox in Manifold: Space was that there were galactic "reboot" events that destroyed all the existing civilizations, this theory is more that intelligent life simply couldn't emerge until recently in cosmic time.

[starslayer]

The main problem with Annis' paper is that he assumes GRBs are lethal to the entire galaxy no matter where they occur. He also considers only one way in which they happen (the neutron star collision). In the time since his paper was written, we have identified another method: Type II supernovae under particular conditions. This somewhat curtails the rate at which lethal GRBs occur, because the long bursts (the supernovae) occur as two relatively narrow beams, not a spherical burst, and therefore appear to most of the galaxy to be an ordinary supernova. He himself also says that he assumes that absorption and scattering due to dust is insignificant; it may very well be significant (I don't know the specifics of dust's absorption spectrum, beyond that it absorbs optical, but not infrared). Free hydrogen absorbs gamma rays too. And while most collisions would occur towards the center, a burst on the outskirts would definitely leave the other side of the galaxy untouched.

But I guess that would still work well if we assumed before that GRBs were surpressing the development of complex biospheres, so older planets than Earth didn't get much of a head start. It would probably also help if intelligent life took longer to evolve on most worlds than it did on Earth (I can think of at least one good reason that might be - not to mention in this universe I assume most biosphere worlds never develop intelligent life).

The problem with that is life started in the seas, and hitting a planet before it develops complex life really does nothing to it unless the GRB is really close (i.e., close enough for photon pressure to strip away most of the atmosphere). What does deep-sea life care if you reduce the ozone layer to a shred of its former self? Hell, if this is before Earth-like photosynthesis develops (or it just never did), you've done absolutely nothing even to surface dwelling life, because there's no ozone to begin with (well, okay, there's the smog layer, but still)!

Just how damaging is a GRB to a biosphere? I know one is suspected of being the culprit of the Ordovician mass extinction. Would they just wipe out terrestrial life, or would species in the shallow oceans be at risk as well?

It very much depends on how close it is; the effects can range from instant death to nothing at all. IIRC, they start to get somewhat dangerous within 10-20,000 light years. Beyond that range, they do nothing except provide a very bright gamma signature to things like Swift, and maybe mess up some electronics (or possibly give everybody on the ISS cancer). As it gets closer, nothing immediate happens to life on the surface; the atmosphere is an extremely good absorber of gamma rays. However, in the process of absorbing them, large amounts of nitrogen oxides can be formed. These chemicals destroy ozone, and at around 5-6,000 light years, half the ozone is gone within weeks, and it doesn't recover for more than five years. This has catastrophic effects; phytoplankton, the base of the marine food chain, simply dies en masse. Plants are killed, and everybody on Earth gets a really, really bad sunburn. The deep oceans are largely unaffected for a while; since they mostly feed off descending carcasses and detritus, and are protected from the increased UV flux by the immense amount of water above them, they gorge themselves. However, the creatures down there start to die off after a few years, because now they have much less food. But they recover as well, after the ozone is back to normal and the surface biosphere recovers.

So what happens as the burst gets ever closer? Eventually, you get to the point where the entire ozone layer is destroyed (I estimate that to be about 1,000 light years, but this is a SWAG), and then the real fun begins. Now the burst has enough energy to start stripping away the planet's atmosphere. Get close enough, and half the atmosphere is blown away (the other half is protected by the planet itself). The oceans start to boil immediately, and life on Earth is just fucking screwed. What if it's even closer? Eventually, the gamma ray flux is so strong that it completely sterilizes the planet; not even potential deep crust extremophiles are left alive. The ones inside nuclear reactors died long ago; even the intense radiation flux inside reactors doesn't even come close to what's hitting Earth now. And of course, if the blast is close enough, the planet itself comes out badly (or may even be destroyed).

As for the frequency of these events, they are quite rare. Ignoring the long ones for a moment, collisions between stars are very rare, even in the galactic center. Collisions between neutron stars are rarer still, as they are smaller targets and are much less common than ordinary stars. One every hundred million years seems plausible, but it may be much longer than that, and it probably wasn't orders of magnitude more common in the past. Overall, I would think it possible for a GRB to sterilize a large sector of the galaxy, but unlikely.

Oh, one note on the Ordovician extinction: while a GRB is plausible, far more likely are glaciation events caused by the movement of Gondwana over the South Pole, which disrupted ocean circulation. This would have not only caused extinction from the cooler temperatures, but also from the retreat of sea levels, destroying shallow water habitats.

[starslayer]

Some corrections/additional notes: I said that the neutron star collisions produced spherical bursts; they don't. They produce twin beams, just like the hypernovae. The only differences are that those bursts are shorter (less than two seconds long) and produce higher-energy gamma rays. This means that a planet getting hit by a GRB is even less likely.

There are also two more damage mechanisms that I came across: heat and cosmic rays. The absorption of so much energy by the atmosphere from the gamma ray photons ends up being converted into a large amount of heat. If the burst is close enough, it can cook those unfortunate enough to be directly under it (I don't know how close this is, though). Cosmic rays aren't very dangerous in and of themselves, as they are generally stopped in the upper atmosphere, but they produce large numbers of muons, which zip straight through the atmosphere. They also can zip through as much as a mile of water or half a mile of rock, and present a very dangerous radiation hazard. The absolute worst-case scenario for a burst 7,500 light years away is about 10 times the lethal dose. However, this figure is very controversial. Interestingly, the muon shower occurs several hours after the initial burst, because subatomic particles can't actually go at lightspeed, and lag behind the gamma rays. Therefore, areas of the planet that were safe from the initial burst are hit by the muon shower. I find that this method of destruction is probably less likely than all the other effects of a GRB, however. Our magnetic field may be able to protect us from most of the onslaught of cosmic rays. Whatever the case, I think I've given enough reasons why a nearby GRB is bad.

[Kitsune]

Curious, would the Earth's magnetic field deflect part of a Gamma Ray Burst?

[Wyrm]

Curious, would the Earth's magnetic field deflect part of a Gamma Ray Burst?

No. Gamma rays are EM radiation, and EM radiation goes straight through magnetic fields, whatever it is. Sizzle.

[Kitsune]

No. Gamma rays are EM radiation, and EM radiation goes straight through magnetic fields, whatever it is. Sizzle.

How else are gamma particles guided or focused? X-ray pulsars are suppose to produce their gamma rays through magnetic field decay. There would appear to be some interaction at least.

[Wyrm]

How else are gamma particles guided or focused? X-ray pulsars are suppose to produce their gamma rays through magnetic field decay. There would appear to be some interaction at least.

The interaction is with the charged matter in the magnetic fields. The nature of the interaction is such that the gammas are emitted in preferred directions. Once the gammas are formed, however, it does not feel magnetism, only charge — as the gamma smacks into it.

[RedImperator]

The problem to me seems to be that unless GRB frequency fell off very recently, that still leaves plenty of time for the galaxy to fill up with aliens. There hasn't been a GRB in Earth's vicinity since at least the Ordovician (and frankly, even that's sketchy). In the 400 million years since then, either no other intelligent life has evolved, or it did evolve and then went extinct, or it evolved and got stuck in a trap that has kept it bottled up in its own solar system. Because if they did evolve and did expand, even without FTL you can cover the entire galaxy in a few million years--and once you start expanding, there's no good reason to stop.