## May 11, 2011

### Probabilistic heroism: how road inspectors and planetary defense are alike

I am attending the 2011 IAA Planetary Defense conference. A.C. Charania has a planetary defense blog with plenty of interesting material.

Some ponderings amid the mission planning diagrams, impact-porosity discussions, asteroid statistics and nuclear detonations:

Someone who saves a life is a hero. I don't care about whether this is done at personal risk or not; it is the altruist consequence that matters.

What about somebody who saves a life probabilistically? Imagine seeing a rock poised above a busy road: it might fall, leading to a landslide killing several people. I would argue that you are just as much a hero if you roll the rock back and prevent it from ever falling as if you were to jump in just when the rock was already falling (or somehow shielding the victims from the landslide). Where in the causal process you intervene doesn't matter, except that the uncertainties increase (so you might become a tenth of a hero if you remove the risk of one death with probability 10%)

Now, what about checking unsafe rocks above roads? There are many rocks, some might be dangerous, but most are safe. But if we do not do anything we should expect that there could be a sizeable risk - our a priori estimate of the fall probability implies a non-zero expected number of harmed people. As we investigate the rocks we might find that none are unsafe. The risk has become zero. I think this should also be regarded as a probabilistic heroic action.

Sure, nobody was ever in risk in this case, but we did not know that. One can distinguish between objective probability (the "actual" probability in the world) and subjective probability (a Bayesian view, where probability denotes our state of knowledge of the world). The objective probability was never affected, but we cannot base our actions on objective probabilities since we do not know them - as rational beings we can at best act according to our subjective probabilities.

It is a bit similar to the legal hair-splitting about incompetent murderers. Suppose you try to kill somebody, stupidly thinking that sugar is a deadly poison, so you spike his food with sugar from a jar. In this case (I have been told), the Swedish legal system would not consider it attempted murder since there was no way it could have succeeded, despite the ill intention. If you instead try to use rat poison but the mislabelled jar contains sugar, then it is attempted murder - it was just averted due to dumb luck. The expected number of victims would have been close to one.

So by this line of argumentation, checking the rocks with the intention of trying to fix the dangerous ones is heroic. It might be heroic because you find a rock you fix, reducing the objective risk, or because you find that all rocks are safe, reducing the subjective risk. The end result is the same: a safe road.

I think we can make an analogy from this to stopping asteroids. Thanks to current surveys we know around 90% of all near Earth asteroids large enough to cause massive disaster if they were to hit. None of them are dangerous over the next century. A bit more surveying and we will be able to completely retire this risk (there will be some remaining risk from long periodic comets and non-global risks from smaller asteroids, of course). Before this started there was an unknown risk, bounded only by historical evidence (biased by observer selection effects). Today the risk estimate is on the order of a few thousand expected dead per year, mostly due to very rare but deadly large impactors. This risk is falling fast as more and more are surveyed.

So in this slightly quirky way the people around me at the conference are saving thousands of lives per year just by checking what is out there. It is not the active heroism of actually rushing out there and deflecting a NEO or stopping a falling rock, but the discreet and large scale probabilistic heroism of keeping roads safe.

By this line of argument I am sitting among a whole bunch of impressive heroes.

Posted by Anders3 at May 11, 2011 04:22 PM