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Could a gamma ray burst destroy life on Earth?
Recently, I have been wondering whether or not the cosmic phenomenon known as gamma ray burst (GRB) could explain the absence of alien civilisations in the Universe. My suspicion was heightened after reading a paper [link 1] by James Annis. He suggests that GRBs have both the correct rates of occurrence and the correct levels of energy to strongly (and negatively) affect the evolution of life throughout the Cosmos.

I wanted to know just how damaging a gamma ray burst would be to a planet like the Earth. I browsed through a paper [link 2] by Brian Thomas, who implys that the consequences of a GRB would be fairly minor. His study was very detailed, but based on a strange assumption. Namely, that during this cataclysmic event, the Earth would be subjected to an energy influx of (only) 100 kilojoules per square meter.

This number seemed far too low to me. So I went online and did some more searching. I found a reddit thread [link 3] which discussed the impact that a GRB could have on Earth. One of the commentors stated that if the gamma ray burst occurred at a distance of 8000 light years, and emitted a 2 degree jet, then its energy would be diffused across an area 136 light years in diameter (at the point of impact).

-With this information, I decided to do some calculations of my own. I know that a distance of 1 light year is equal to 9.46E12 kilometers. I also know that a typical GRB releases an energy equal to roughly 1E44 joules.
-I then found out that a circle 1 light year in diameter has an area of 7.03E25 kilometers. And that a circle 136 light years in diameter has an area of 1.3E30 kilometers.
-Therefore, an area of 1 square kilometer will receive an energy of 7.69E13 joules. Thats 76 terajoules! 
-And since there are 1 million square meters in a square kilometer, each square meter will receive an energy of 7.69E7 joules. Thats 76 megajoules.

So, what conclusions can we take away from my calculations? Simple. If the Earth were to suffer the monumental misfortune of being hit by a gamma ray burst, then it truly would be a catastrophe. The consequences would be far more serious than what was predicted by Brian Thomas. His paper assumed an energy influx of only 100 kilojoules per square meter. In the scenario posited in link 3, however, the Earth would be subjected to an energy influx of 76 megajoules per square meter.

In my humble opinion, if there was a GRB eruption 8000 light years from our planet, and its energy was focused in a 2 degree jet, then this would represent an extinction level event. At worst, such an eruption might even lead to the extinction of all multicellular life on land... At any rate, the results would be far worse than what Brian Thomas estimated in his paper. Thats what happens when you increase the energy level by a not-unrealistic factor of 760. 

In the end, it seems that James Annis was right to conclude that gamma ray bursts were a potential factor behind the absence of alien civilisations. These phenomenon were much more common in the early Universe than they are now, after all. It seems entirely possible that planets teeming with life were routinely sterilised by GRBs in the past. They were undiscriminating killers, targeting not only prokaryotic biospheres, but the much rarer eukaryotic biospheres as well.

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Could a gamma ray burst destroy life on Earth? - by Avalancheon - 08-05-2018, 01:28 PM

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