[zby]

So they do something like this: Let's choose a human in random - he is more probable to be from Pakistan than from Slovakia. (OK) Now let's choose a country - now an average country like Slovakia is more probable than a country as big as Pakistan. (OK)

So if you are a human - then it is most probable that you live in a country that is more populous than the typical country. (OK)

Now they say - ok - so now instead of choosing humans let's do the same thing with sentient beings. If you are a sentient being it is more probable that you live on a planet where there are many other sentient beings rather than on a planet that there are few of them. But if you go to some random planet with sentient life - then the expected number of sentient beings there would be average.

Then it goes on that "Physically larger species will on average have lower population densities." - so most probably the random alien planet will have fewer and larger sentient beings than us.

I don't know if I buy that whole argument - but I am too lazy to write the bayesian equations to nail it down.

[leblancfg]

My only gripe is that he's only used vertebrates in his dataset (based on http://journals.plos.org/plosone/article?id=10.1371/journal....). It's "only human" to include close members on the phylogenetic tree, but I see it as a flaw.

What if our imaginary aliens are plant-like, squid-like, or something we can't even begin to fathom, like symbiotic unicellulars? Sentient, self-replicating machine clusters the size of planets? Take a minute to think about this.

I think his conclusion should then have been worded like: the random alien planet will have fewer and larger VERTEBRATES than us. And that's a probably very meaningless conclusion.

I'm not saying his intuition is wrong, though. In fact I quite like it. But in this scenario, I don't think "whatever remains, however unlikely" lifeforms can even begin to be imagined by our tiny little human brains.

[wrsh07]

The statistics really doesn't care what type of sentient species we are.

If you're confused by the examples given [for the approximate size], that's understandable, but the examples don't change the analysis. That's like saying "All of the countries in the example are in the northern hemisphere, so the analysis is flawed."

[mombul]

Well yes the location of the country doesn't matter, but that's not at all what he said. He said that it's all based on vertebrates which is ridiculous. We can't begin to imagine what type of life form constitutes an alien, and if what we'll ever find will constitute a life form as we know it.

We know what a country is. And there's very (very, very, trust me I'm an engineer) little chance we'll find new countries with characteristics unknown to us...

[wrsh07]

My point is that the initial discussion wasn't limited to anything except "sentient life." It wasn't limited to vertebrates or mammals or any such thing.

Thus, the statistics are purely for the discussion of "what should other sentient life look like?"

Finally, it's ridiculous to complain in a discussion about aliens "that we can't begin to imagine what type of life form constitutes an alien." You have to make some fundamental assumptions. For instance, we should be able to assume that they obey the physical laws of the universe.

Anything else puts you in a totally unscientific world of discussion [your claims are no longer falsifiable], and that's not one I care to participate in.

[leblancfg]

>[W]e should be able to assume that they obey the physical laws of the universe

I don't think anyone is assuming otherwise... all I see is statistics and sampling being argued. You'll have to endure my wall of text, though, sorry XD.

Let's take the planets again. We don't know the size of every single planet in the galaxy (i.e. the planet size histogram), so we need to make a guess. So we've been using Kepler's exoplanet observations, knowledge of planetary geology, etc. to fit a model, and it's well developed (http://exoplanetsdigest.com/2014/07/25/exoplanet-statistics-...). The size distribution is quite well understood, its bounds and modes are well defined. It's unlikely that our best guess of what the average size of exoplanets is will change drastically the more we know about planets. That is why the article's first conclusion is legitimate.

Now back to (sentient) species, and my original point. The definition of life in the SETI context has to be VERY wide. It has to encompass any scenario for a species that we might consider "intelligent" -- and not just little green men. It might have (some would say, inevitably) evolved beyond the biological, and still be considered sentient. After all, life on Earth has only existed for ~4By, compared to ~13By for the Milky Way. As a base for extrapolation, just think how different to humans extreme life on Earth is. (http://www.livescience.com/13377-extremophiles-world-weirdes...) In any case, there are bound to be some weird-as-shit species out there, whose composition still obeys the laws of physics. In other words, there is almost certainly many statistical modes of life out there that are, or can become, sentient.

With that in mind, there's a very good chance that the overall size distribution of (sentient) species does not match the one the author used (that of vertebrates only). In statistics-ese: if the distribution is multimodal, the average of our unimodal sample is not a good guess as to what the true average really is.

Maybe we can agree upon that?

[VanillaCafe]

But why are we arbitrarily reducing the set from "life" to "sentient"? Right now, we humans are looking for _any_ other life in the galaxy.

But, when considering life, if applying the Big Alien logic, I should expect to be a single bacteria. And if not that, then, say, some insect.

But, instead, I'm a human. Or a mammal. Or a vertebrate. Any of these very fundamental descriptions of myself already put me in the SMALL group not the LARGE group when classifying the population of all life on this planet.

It feels like confirmation bias to select "vertebrate" or "mammal" to show that we are already indeed in a large group.

And indeed you can argue scientists are already applying this logic by looking for signs of simple life, say, or Mars. Or Europa. Because, that's the most likely to be found.

Maybe we should call it the Bacterial Alien Theory. Of course, it wouldn't be very controversial.

[wrsh07]

Right -- it's much more likely that alien life is not sentient.

But when we restrict our search to sentient life, here is what the statistics suggest [and as you correctly point out, that restriction is what makes this an interesting discussion].

[wrsh07]

If you want the math, that's here: http://mnrasl.oxfordjournals.org/content/456/1/L59