Article":Interstellar Deathray Not Likely to Hit Earth

[wildone_106]

What really annoys in me about this is they're so gleefull to say that there's virtually no chance of a GRB hitting earth, fine! But then goes on to say GRB's may be the cause that there's no life been discovered in the universe yet...COME ON. Which is it? The chances of a GRB hitting another sufficently advanced civilisation is the same as ours then, sure some might actually get hit but given the potential number of advanced life in the universe I highly doubt they all got wiped out by GRB as this article is so happy to proclaim. GRRRRRRRRRRRRRRRRRRR I know they LOVE to tie in any hair brain theory to deny existence of ET's but this is just pathetic....

 

[vogon13]

'Deathray' not likely to hit earth.<br /><br />Heh, heh, heh.<br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[robnissen]

I thought that article was wildly unpersuasive. So the odds are less than .3%. In what time period? A year? 12 billion years? Until the end of time? That statement standing alone says nothing. Also it seems to me, that one reason why GRBs might appear to be in metal poor galaxies is because current theory requires GRBs to come from the remnants of very large stars that became black holes. But current theory also posits that as you get closer to the BB, you are more likely to have massive stars as first generation stars. Consequently, GRBs are going to tend to come from very young galaxies, which, by definition are metal poor. But the fact of the matter is, there are a few extremely large stars in the Milky Way, when (not if) they become black holes, there is certainly the possibility of GRBs.

 

[doubletruncation]

The article is actually based on observations rather than any theories of how GRBs occur. Basically, there are four local (redshift less than 0.17), long-duration GRBs that have been observed, and for which there is an identified host galaxy. In all cases, the host galaxy is faint and metal poor compared to the local population of star-forming galaxies. If GRBs are not correlated with metallicity, then the chance of having the four nearby ones that have been seen show up in the galaxies they did show up in is only .15%. Moreover, they found that the energy released by the GRB is strongly anti-correlated with the oxygen abundance of the host galaxy. So observations seem to be supporting the theoretically based hypothesis that GRBs only occur in rapidly rotating low-metallicty massive stars. While there are very massive stars in the Milky Way, they are not low-metallicity stars, so it would seem that they do not pose a GRB threat. You can see the actual paper that this is based on at:<br /><br />http://xxx.lanl.gov/abs/astro-ph/0604113 <div class="Discussion_UserSignature"> </div>

 

[robnissen]

I went and looked at the abstract you posted, but I'm still not convinced. The authors noted that the four closest GRBs were from galaxies whose oxygen level was less than half of our galaxy. As only 20% of all galaxies have oxygen levels less than half of ours, they computed that the random chance of low metallicy (sp?) being unrelated to GRBs is .2*.2*.2*.2 = .16% or about .15%. But that is a statistics 101 freshmen mistake. Correlation does not mean causation. For example, if skydivers are twice as likely as non-sky divers to get lung cancer, that does not mean that sky diving causes lung cancer (if skydivers were twice as likely to smoke as non-skydivers, that would be the causation). It is a very broad leap to state that because there is a CORRELATION between low metallicy and GRBs, that low metallicy CAUSES GRBs. That is an especially weak argument to make, with only 4 data points. At best, the researchers may have found that whatever causes a galaxy to be low in metals, also increases the likelihood of GRBs. But without knowing what that underlying cause is, there is really no way to speculate on how that finding impacts the Milky Way.

 

[doubletruncation]

I should say, I'm not an expert in this at all, I just thought it was a kind of neat paper.<br /><br /><i>Correlation does not mean causation</i><br /><p><br />Good point. After thinking about it some more, it does seem like the conclusion is probably quite a bit stronger than warranted. But, I don't know that they necessarily rely on metallicity being the cause of GRBs to come to the conclusion that we don't have to worry about GRBs in our galaxy. I think that, at the very least, they can conclude that the fraction of all GRBs that occur in galaxies like our's is less than the fraction that occurs in lower metallicity galaxies. So you could predict that the rate of GRB occurance in our galaxy is probably less than the rate that you might expect just from the observed frequency of all GRBs. (If skydivers are twice as likely as non-sky divers to get lung cancer, and if all you know is that you are a sky-diver, then you could predict that you are twice as likely to get lung-cancer as a non-skydiver and you probably would be right even if sky-diving is just a tracer for some other variable that causes the lung-cancer). It doesn't seem like you can conclude that the rate of occurance in galaxies like our's is zero (particularly since the work is only based on four points, and there is no positive detection in a galaxy like our's), and I don't know how they come to the conclusion that we have a .3% chance (or whatever it was), or any specific number, of being hit by a GRB.<br /><br />By the way, it seems that there is a theoretical reason why people do think that low metallicity is actually a prerequiste for a GRB. Apparently you need a lot of angular momentum to create GRB jets, however high metallicity massive stars will lose angular momentum due to extensive mass loss (I guess the winds also carry away angular momentum) whereas lower metallicity ones don't. I don't know though if there are other ways to get around that (there are papers suggesting that binarity or ejection</p> <div class="Discussion_UserSignature"> </div>