In an email with a colleague, I postuled on the significance that gamma ray bursts in the young milky way might have retarded the growth rate of life in the entire galaxy. In a longer email response I listed out the mechanism for my idea. Essentially, GRB's and mettalicity may lead to conditions that periodically restart life processes on a galactic scale. Combined with the stellar level events (asteroids, comets and proto planetary body incursions) this explains a great deal of the reason why we haven't been visited by LGM's. It could simply be the fact that the galaxy and the solar system has only quited down enough for life and intelligent life in general to advance to the point that they would discover advanced technology and the ability to propel themselves between the stars. We are all (those forms of life in elementally conducive parts of the galaxy that is) at roughly the "same" level of technological advance (even if "same" can have a gap of thousands of years) and since we can't travel between the stars, neither can any of the many species that likely only recently (again can span thousands of years..which on a galactic time scale is nothing) achieved technological advancement as we have due to the quiescence in extinction events both on a galactic and planetary level. Below I include the entire email response to the original letter.
April 30, 2006
1) Prior to writing the first email where I postulated the connection between black hole merge induced GRB's and extinction events on our planets history, I had no previously researched information on the subject. I'd not seen it written in any journal, magazine, web page or any other place. ie. I was extrapolating the connection on the fly. So you'll excuse me if I, in a moment of shallow and rapid thought did not reflect more deeply upon the mitigating factor of galactic evolution (to be explained subsequently) on the occurance of GRB's. ;)
2)The connection between GRB formation and metal rich and metal poor galaxies specified in the article is likely true, and the chances of our galaxy having new GRB's may be retarded by this fact but that doesn't mean that our galaxy was not a source of such GRB's in the past. Our fossil record (as mentioned before) is strewn with currently inexplicable extinction events. Moreover, all metal rich galaxies were once metal poor ones by definition, this is how galactic evolution works according to currently accepted cosmology. Hydrogen burned in the furnaces of the original (mostly globular) clusters of stars, and aided by the crushing forces of gravity burned into the heavier elements ...helium, lithium, berylium and so fouth leading to the metals over subsequent generations of star death and rebirth. Our galaxy is metal rich only because it's gone through it's metal poor days (when it almost certainly was producing GRB's if the current black hold merge trigger is to be accepted) billions to hundreds of millions of years ago. Thus galactic core GRB production I assert is a state that all galaxies with sufficient mass have gone through at some time in the past. Note this does not factor in the other potential sources postulated for life killing gamma events like super novae, and rotating or consumed neutron star and black hole systems that are likely to be far more likely in our own galaxy.
3) There is no research readily available on exactly how a galaxy goes from being metal poor to metal rich, how quickly does this transition occur ? I gather it should vary with the density and distribution of the matter that composes the galaxy in question but is the state transition a sudden (on an astronomical time scale) one? If so, our galaxy might have changed state only in the last 500 million years or was in it's last throws of state change (and core GRB production) this would allow any remaining core GRB events to coincide with the development of early life on earth and could explain the extinctions. It would also again allow us to address the Fermi paradox mentioned in the linked article, if the galaxy only in the last 400 million years stabilized into it's metal rich (ie. slower GRB producing phase) phase any evolutionary processes on suitable worlds else where in the galaxy would have been subject to the same extinction events (likely more violent and complete ones) prior to the emergence of suitable life on earth.
4) The mentioned connection between metal content in galaxies and GRB's provides no calculation for determining the relationship between GRB likelihood and the metal content of a galaxy. Such a calculation would have to take several very amorphous factors into consideration such as the density of stellar matter in the galactic core, weather or not the metal distribution in the galaxy is homogeneous or not. These factors would be extremely difficult (if not impossible) to quantify on a galaxy by galaxy basis. And weather or not direction is important to registering the GRB. Therefor the conclusion that GRB's "only" occur in metal poor galaxies (which implies homogeniety in my view) can not be drawn without first knowing exactly what recipe is required to produce them which aside from the evidence of merging black holes of sufficient size is all that is known. The important qualification on any such statement should be, "we tend to find GRB's coming mostly from galaxies with a high metal content".
5) You are assuming that any gamma ray event that occurs in our galaxy need be as potent as the ones we detect in galaxies hundreds of millions of light years away (ie way back in time) We do know as fact that gamma rays are easier to propagate in a space that isn't filled with heavy nuclei, after all the heaver the nuclei is the more energetic particles are needed to move it or modify it. In so doing the source particle looses energy, gamma rays hit heavy atoms and turn to lower energy photons like x rays and scatter atom bits about. Also, if a galaxy does produce a core gamma event the momentum of the produced nuclei would be quickly retarded by the heavy nuclei in the surrounding galactic material...this indicates that the intensity of a core gamma ray event (to distinguish them from "bursts" which we'll uniquely define as detectable from neighbor galaxies) will vary non linearly with galactic evolution. In other words, as the galaxy goes from metal poor (high levels of low atomic weight elements) to metal rich (after a few generations of star death and rebirth to produce heavier nuclei) it also goes from being a great catalyst to the distribution of gamma rays to being a poor catalyst to such distribution. Does that mean that the core becomes incapable of producing gamma events sufficient to cause extinction events on planetary systems that might be evolving life ? NO It only means that the extent of the gamma events distribution may not travel through the galactic void in such a way as to signal sentients in other galaxies that "hey that milky way looking galaxy just had a really bad day" ;) So, the situation would be that gamma ray conduction and intensity will reduce as more heavy elements (not just metals IMO) are produced in a galaxy as it undergoes galactic evolution. Since gamma rays are impeded more readily by heavier nuclei, the more heavy elements there are the less likely enough gamma rays produced by a merge event would ever make it to the edge of the galaxy and be ejected out into intergalactic space to be picked up in distant galaxies. Thus, simply because we don't see them coming from what appear to be "metal heavy" galaxies does not preclude their occurance in the form of weaker events within such galaxies with sufficient virulence to extinguish life on systems in such galaxies without us every knowing a thing about it. Bolstering this view is the fact that gamma events (both core and non core based) likely have preferential directions in all the theorized possible sources(see 6), if we aren't in the path of the burst astronomers won't even know it is happening.
6) Finally, host galactic core events are not the only candidates for life killing gamma bursts. rotating neutron stars and even super novae have been implicated as sufficiently energetic sources capable of producing life killing gamma ray events that don't have to be in the galactic core of the host galaxy.
So I'd say gamma ray events (from inside our own galaxy) are still very relevant as a possible explanation for the apparent dearth of intelligent life forms in our galaxy. (namely the "why don't we get alien radio signals" question) I think it is way too early to go claiming that we are safe from GRB events as was implied in the original article you sent which admitted that it only studied a 4 galaxy sample of galactic burst sources to make it's conclusion! Now don't get me wrong I am not saying we should be worried (You republicans are paranoid enough!), I am just pointing out that I think the study cited in that link is leaving a LOT out of the picture of your purpose in sending it was to down play the possibility that a core GRB event in our galaxy could wipe us out.