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Nornagest comments on [Link]: Anthropic shadow, or the dark dusk of disaster - Less Wrong Discussion

10 Post author: Stuart_Armstrong 04 July 2013 07:52PM

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Comment author: Nornagest 04 July 2013 08:16:45PM *  11 points [-]

I'm not an astronomer, but my understanding is that when it comes to impact events, the probability of a planet-killer probably is going down over time: after an impact event, the impactor isn't around to pose a threat anymore, and there are only so many large objects with Earth-crossing orbits. At this point we're far enough out on a long enough tail that the probability density isn't changing much over time, but that's not true over the timescales being graphed; if we're dating lunar rocks accurately, extinction-scale impact events last peaked around 3.9 Gya, and were rare after 3 Gya. I'd be surprised if that trend hasn't to some extent continued.

More generally, if you want a good estimate of the near-term probability of impact events, you probably want to survey one or several of the other bodies in the solar system. They have an impact record relatively untainted by anthropic bias, and also have the advantage of being a lot easier to read, as most of them lack the plate tectonics that wipe out a lot of older geology on Earth. That said, though, there are extinction events that wouldn't be affected by the later evolution of the solar system: nearby supernovae, for example, or gamma-ray bursts.

Comment author: JoshuaZ 04 July 2013 08:19:59PM 7 points [-]

More generally, if you want a good estimate of the near-term probability of impact events, you probably want to survey one or several of the other bodies in the solar system. They have an impact record relatively untainted by anthropic bias, and also have the advantage of being a lot easier to read, lacking the plate tectonics that tend to wipe out a lot of older geology on Earth.

And this has been done. We have good records of impact levels on the Moon, Mars and Jupiter (although Jupiter is a little weird). It doesn't look like there's heavy anthropic bias there.

Comment author: satt 08 July 2013 07:25:01PM 1 point [-]

We have good records of impact levels on the Moon, Mars and Jupiter (although Jupiter is a little weird).

Ćirković, Sandberg & Bostrom refer to these briefly in the paper, but seem to think they're not adequate or as relevant:

For instance, the asteroidal and cometary impact history of the solar system is, in theory, easier to obtain for the Moon, where the erosion is orders of magnitude weaker than on Earth. In practice, this is still not feasible for obtaining the fair sampling of the impactors because: (1) precise dating of a large set of lunar craters is beyond our present capacities and (2) most of the large lunar craters are known to originate in a highly special epoch of the so-called Late Heavy Bombardment, ca. 4.0-3.8 billion years B.P., thus strongly skewing any attempt to plot the empirical distribution function of impacts for "normal" times. In practice, in the current debates about the rates of cometary and asteroidal impacts, it is the terrestrial cratering rates that are used as an argument for or against the existence of a dark impactor population, thus offering a good case on which the anthropic model bias can, at least potentially, be tested. [Footnote & endnote references omitted.]

Comment author: Manfred 05 July 2013 06:08:50AM 0 points [-]

It doesn't look like there's heavy anthropic bias there.

Citation?

Comment author: JoshuaZ 05 July 2013 01:55:15PM 3 points [-]

I don't have a citation for this, more a general familiarity with the literature on the subject, and that no one has ever said "hey it looks like we should have seen a lot more impacts on Earth than we've apparently gotten" or anything similar.

Comment author: Manfred 05 July 2013 02:07:44PM *  1 point [-]

and that no one has ever said "hey it looks like we should have seen a lot more impacts on Earth than we've apparently gotten"

Wouldn't this be a (weak, since humans have lots of reasons) piece of evidence that people see the same pattern of collision sizes on earth as on e.g. the moon?

Comment author: JoshuaZ 05 July 2013 02:12:58PM *  3 points [-]

Yes, and that's the point: that suggests that there's little anthropic bias at work here. A heavy anthropic bias would be if we didn't see the same collision patterns.

Comment author: Nornagest 05 July 2013 06:14:34AM *  2 points [-]

This paper seems to have some useful data. I'd be happier with a table of crater sizes and ages that I could plug into Octave and fit a regression to, but so far I haven't been able to come up with any decent-sized datasets.

ETA: The Lunar Impact Crater Database could probably do it, if you feel like doing some messy conversion.

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