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Comment by pjdorrell | Hacker News Reader

The authors of the paper do various calculations to arrive at 36.

How do they determine f_l, the probability of life originating on a suitable planet?

They follow the "Principle of Mediocrity", ie life exists on Earth, and we are not special, so probably it exists anywhere it can exist and f_l = 1.

This is the exact opposite of the Anthropic Principle, which says (correctly) that we cannot deduce anything about the probability of life originating from the fact of our own existence.

If f_l isn't 1, and it's some smaller number, like 1^-1000, then there won't be 36 alien civilisations in the galaxy - there will only be zero.

Even in the observable universe, there will be zero alien civilisations.

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> This is the exact opposite of the Anthropic Principle, which says (correctly) that we cannot deduce anything about the probability of life originating from the fact of our own existence.

Your definition of the Anthropic Principle seems to clash with the Wikipedia definition [1], which states:

> The anthropic principle is the philosophical premise that any data we collect about the universe is filtered by the fact that, in order for it to be observable at all, the universe must have been compatible with the emergence of conscious and sapient life that observes it.

[1] https://en.wikipedia.org/wiki/Anthropic_principle

Also you use the word "correctly" for your definition, but as it isn't self-evident, some sort of proof should probably be required.

Well, even within the wikipedia article you see quite a few definitions under 'variants'. It's also nearly a word for word copy of how it is defined in contrast to the mediocrity principle ( https://en.wikipedia.org/wiki/Mediocrity_principle )

Either way, his point is fundamentally pretty close to even the WAP definition in the summary.

I think it's fairly self evident, though. If there is only one set of observers in the entirety of the observable universe, those observers would be inaccurate in assuming that they are unlikely to be special because they exist and therefor others likely to as well. We just can't make a deduction about the probability of life based on the fact we exist, because for us to make that observation we have to exist, regardless of (im)probable it was. You can reach the same conclusion just thinking about selection bias - we have selected for intelligent life out of necessity, because if we hadn't, we couldn't be around to make the argument either way.

If having a huge moon was an essential driver enabling development of advanced life and not, e.g., turning into Venus, then the expected number of others drops from 36 to well below 1.

If any exist (i.e., if other origins are possible, such as on Endor), we would only encounter expansionist cultures, because the others don't get out. If they all have controlled fusion (and how could they not?) they should have little interest in inner rocky planets, never mind the extreme primitives on them.

Look to the Kuiper belts, well supplied with frozen gases and extreme cold. Of the two, the latter is the far more valuable.

That’s not quite what the Anthropic Principle states. For instance, we can conclude from the fact that we exist that the universe is a place where life can exist, and that it is more likely we live in a universe where life arises frequently. However, the fact that we haven’t observed any aliens decreases the probability that life arises frequently.

Reality is frequently inaccurate.

Atypical, anyway.

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