4 ms·

This quote from the article makes no sense to me: > Statistically, impacts the size of Hiawatha occur only every few million years, he says, and so the chance

by kijin 8y ago

This quote from the article makes no sense to me: > Statistically, impacts the size of Hiawatha occur only every few million years, he says, and so the chance of one just 13,000 years ago is small. If an event happens every one million years on average, for example, it's just as likely that the last one happened 13,000 years ago and the next one will happen 987,000 years in the future as that the last one happened 999,999 years ago and the next one will happen tomorrow. Even the interval can be highly variable.

prestonh 8y ago
But the total probability of the event happening "recently" (even if this is a fuzzy description) is far lower than the probability of it happening further away. If there's an event known to occur once in a trillion years and we observed it in our lifetimes, wouldn't you think that was remarkable?
todd8 8y ago
Assuming that a meteor impact is totally random (which might not be true because objects that impact the earth could conceivably influence each other through gravity, etc), it is probably best to model these impacts as a poisson process. A poisson process with an average arrival time of one million years means that we would find that if these size impacts happen on average once per million years that on any given day, like the day before this Greenland discovery, we should expect the next impact to happen a million years in the future and the previous impact to have occurred one million years in the past. If buses arrive randomly and independently on average every 10 minute, then we can expect a ten minute wait at the bus stop if we get there at a random time. Think of random points on the number line averaging a point every million years. Now throw a stochastic dart at the line. It’s much more likely to land in a large interval not the small ones—-they take up more space on the line. For this reason, the distance between the dart and the next random point is on average one million, not 500,000. So it is very unlikely that the meteor impact was only a few thousand years ago.
- peterashford 8y ago
  Yes... but averages are not overly meaningful when considering a single event.

by kijin 8y ago

prestonh 8y ago

But the total probability of the event happening "recently" (even if this is a fuzzy description) is far lower than the probability of it happening further away. If there's an event known to occur once in a trillion years and we observed it in our lifetimes, wouldn't you think that was remarkable?

todd8 8y ago

Assuming that a meteor impact is totally random (which might not be true because objects that impact the earth could conceivably influence each other through gravity, etc), it is probably best to model these impacts as a poisson process. A poisson process with an average arrival time of one million years means that we would find that if these size impacts happen on average once per million years that on any given day, like the day before this Greenland discovery, we should expect the next impact to happen a million years in the future and the previous impact to have occurred one million years in the past. If buses arrive randomly and independently on average every 10 minute, then we can expect a ten minute wait at the bus stop if we get there at a random time. Think of random points on the number line averaging a point every million years. Now throw a stochastic dart at the line. It’s much more likely to land in a large interval not the small ones—-they take up more space on the line. For this reason, the distance between the dart and the next random point is on average one million, not 500,000. So it is very unlikely that the meteor impact was only a few thousand years ago.

peterashford 8y ago
Yes... but averages are not overly meaningful when considering a single event.