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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.
Hm. Various initial thoughts here.

1) Per the Wikipedia page on GRBs, no observed GRB event has taken place within our galaxy. The nearest GRB so far observed was 130 million ly away. So this would seem to argue for them being very rare in a local sense or otherwise operating within limits or parameters that make an impact on Earth unlikely.

Note also that the article also talks about potential effects of a GRB on Earth and doesn't seem to be concerned with Earth being cooked or the like, but rather with environmental effects as the gamma rays are absorbed by the atmosphere and induce chemical reactions that result in smog, acid rain, and other negative stuff that could adversely (albeit perhaps temporarily) impact the biosphere.

That all said, I suppose the references you provide could be based on more recent information. However, having said that and coming to my next thoughts:

2) You talk about how much energy the Earth would be subjected to, but don't include how much energy we are starting with. So we don't have complete information to consider your argument with. If you could please add the starting amount of energy you calculated from, that would be very helpful.

3) When you did your calculations, did you take into account the Inverse Square Law?

My 2c worth,

Todd
(08-05-2018, 02:31 PM)Drashner1 Wrote: [ -> ]Hm. Various initial thoughts here.

1) Per the Wikipedia page on GRBs, no observed GRB event has taken place within our galaxy. The nearest GRB so far observed was 130 million ly away. So this would seem to argue for them being very rare in a local sense or otherwise operating within limits or parameters that make an impact on Earth unlikely.

Note also that the article also talks about potential effects of a GRB on Earth and doesn't seem to be concerned with Earth being cooked or the like, but rather with environmental effects as the gamma rays are absorbed by the atmosphere and induce chemical reactions that result in smog, acid rain, and other negative stuff that could adversely (albeit perhaps temporarily) impact the biosphere.

Hi Todd, thanks for the response. This is a very important subject for me, and I welcome everyones feedback.

Gamma ray bursts used to be much more common in the past. These eruptions undoubtedly happened many times in our own Galaxy. A google search reveals that there are somewhere between 10 million to 1 billion black holes in the Milky Way alone. Why does this matter? Because black holes are the corpses of blue giants. Before these massive stars collapse and die, they will emit either a supernova or gamma ray burst.

Even if you take the lower estimate of 10 million black holes, and assume that only 1% of these are the remnants of a GRB, that still leaves you with 100,000 eruptions inside our own Galaxy. Its true that most of these would have happened early in the life of our Galaxy, which is over 13 billion years old. But even so, there were enough GRBs left over to routinely sterilise all corners of the Milky Way until now (seemingly).

You are right to point out that Brian Thomas' study had a narrow focus area, being concerned only with the direct effects of ozone formation in the aftermath of a gamma ray burst. But even here, his use of an unrealistically low energy level undermines his own conclusions.

(08-05-2018, 02:31 PM)Drashner1 Wrote: [ -> ]That all said, I suppose the references you provide could be based on more recent information. However, having said that and coming to my next thoughts:

2) You talk about how much energy the Earth would be subjected to, but don't include how much energy we are starting with. So we don't have complete information to consider your argument with. If you could please add the starting amount of energy you calculated from, that would be very helpful.

3) When you did your calculations, did you take into account the Inverse Square Law?

My 2c worth,

Todd

Apologys for that, I edited my post to include that info. Gamma ray bursts typically emit 1E44 joules of energy in a jet that is 2 to 20 degrees wide. The jets emanate from their north and south poles, allowing them to irradiate large areas.

I didn't take the inverse square law into effect, for reasons of simplicity. I wasn't sure how strongly that would apply to a GRB, as their energy is highly collimated. The true energy level received by the Earth may be lower than I predicted, but its not going to be lowered to the level suggested by Brian Thomas.

Before I ran my calculations, I figured that Thomas' numbers were off by a factor of only 50 or so. I was quite surprised to find that it was off by a factor of 760. Thats a really significant difference that limits the usefulness of his study.
Anders Sandberg looked at this question a few years ago.
http://aleph.se/andart2/tag/gamma-ray-burst/
His rather basic analysis of the problem indicates that there is a risk, but it is fare from uniform all over the galaxy, and isn't even 100% in the galactic centre.

Mapping this onto the densities of stars in the galaxy shows that there is a cluster of (probably) unaffected stars in the Galactic Habitable Zone near the plane, which is where we are.

This does mean that any civilisation near the Galactic Hub probably migrated there, or it must somehow have evolved to be highly resistant to radiation events.
(08-05-2018, 01:28 PM)Avalancheon Wrote: [ -> ]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.

Looking through the paper it is annoying that the authors don't explain that assumption, there are plenty of citations however so have you checked those?

(08-05-2018, 01:28 PM)Avalancheon Wrote: [ -> ]-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.

Your maths is right but it is unclear from the paper if the authors were referring to total energy or the energy that reaches the ground (as most of the energy is absorbed by the atmosphere). The easiest thing to do to check is to just email the authors and ask for clarification on the assumption. It's quite common in academia to do this, so long as you are polite about it and do not take up too much of their time.
(08-05-2018, 07:36 PM)stevebowers Wrote: [ -> ]Anders Sandberg looked at this question a few years ago.
http://aleph.se/andart2/tag/gamma-ray-burst/
His rather basic analysis of the problem indicates that there is a risk, but it is fare from uniform all over the galaxy, and isn't even 100% in the galactic centre.

Mapping this onto the densities of stars in the galaxy shows that there is a cluster of (probably) unaffected stars in the Galactic Habitable Zone near the plane, which is where we are.

This does mean that any civilisation near the Galactic Hub probably migrated there, or it must somehow have evolved to be highly resistant to radiation events.

Interesting article. The Galactic hub is still not a promising spot for life, though. The Milky Way used to have an active nucleus that spewed radiation in all directions. It only went inactive a couple billion years ago.

Random GRBs can sterilise large swaths of the Galaxy, but not all of it. Even so, it would tend to explain why we don't observe alien civilisations. Lets say that in the lifetime of our Galaxy, there were 2000 biospheres that managed to evolve to pass the eukaryotic filter. If they were randomly distributed across the Milky Way, a large number of them would be exterminated by GRBs.

The few eukaryotic biospheres remaining would then face the 'porous filters' of cephalisation and civilisation. That would cut down the number of sentient races to a tiny number that includes us (at least for now).
(08-05-2018, 08:47 PM)Rynn Wrote: [ -> ]Looking through the paper it is annoying that the authors don't explain that assumption, there are plenty of citations however so have you checked those?

Your maths is right but it is unclear from the paper if the authors were referring to total energy or the energy that reaches the ground (as most of the energy is absorbed by the atmosphere). The easiest thing to do to check is to just email the authors and ask for clarification on the assumption. It's quite common in academia to do this, so long as you are polite about it and do not take up too much of their time.

Sandburgs article stated that an energy level above 100 kilojoules per square meter would lead to an extinction level event. I think he got that impression from the Piran and Jimenez paper, which Brian Thomas cited. So in that regard, it seems like his study was a best case scenario for the Earth.

I'm also not sure whether or not the 100 kilojoule figure was the initial beam energy, or what reached the ground after atmospheric absorption. I'll send an email to the authors and ask them to clarify on this point, since its a key assumption.
Finally got an update. After reading this paper, things are more clear to me.

The energy fluence of 100 kilojoules per square meter was chosen based on a probability estimate within a 500 million year period. Piran & co. looked at three different fluence levels, from 10 KJ, to 100 KJ, to 1000 KJ .

The probability of the Earth being hit by a GRB within that time period was 95% for the 10 KJ scenario, 50% for the 100 KJ scenaio, and 4% for the 1000 KJ scenario.

But if you include a 'metallicity bias', the probability goes up significantly. 99.8% for the 10 KJ scenario, 90% for the 100 KJ scenaio, and 25% for the 1000 KJ scenario...

In my humble opinion, though, these estimates are still too optimistic. They all seem to be based on the assumption that the erupting GRB will have a very wide jet (wider than 2 degrees), resulting in a more diffuse spray of gamma rays.
In my humble opinion, though, these estimates are still too optimistic. They all seem to be based on the assumption that the erupting GRB will have a very wide jet (wider than 2 degrees), resulting in a more diffuse spray of gamma rays.

Do you mean you think such high power GRBs are more likely, or less likely?
(08-24-2018, 07:32 AM)tmazanec1 Wrote: [ -> ]Do you mean you think such high power GRBs are more likely, or less likely?

In my opinion, the probability of the 1000 KJ scenario (high energy) is being underestimated. At least when compared to the 100 and 10 KJ scenarios (medium and low energy). Its probability is much higher than 4%.

That is especially apparent when you understand that their study focused on a GRB at a distance of 2 kiloparsecs. My scenario had the erupting GRB at a distance of 2.45 kiloparsecs.

This conclusion is unaffected by the 'metallicity bias', I.E., the scarcity of elements heavier than hydrogen and helium. Why? Because if a GRB were ever to erupt within 2 kiloparsecs of us, then the Earth will almost invariably be subjected to a high energy fluence.

But to be fair, my scenario of a 76 MJ fluence isn't exactly realistic. Thats because not all of the energy from an erupting GRB is focused into the jets. It was an 'upper estimate' of mine.