# [ExI] Drake Equation Musings

Anders Sandberg anders at aleph.se
Tue May 17 10:59:17 UTC 2016

```On 2016-05-17 12:05, BillK wrote:
> That's logical thinking if you have no evidence for either probability.
>
> But we do have evidence! Life on earth started immediately, as soon as
> the newly formed planet had cooled down sufficiently. Unless you have
> evidence that Earth is unique in the universe, (the Rare Earth
> hypothesis), then life should be everywhere, in every niche where it
> can survive, just like in extreme environments on Earth.
Nope :-)

Remember the Carter argument: the probability of life emerging may be
very low, and there could be secondary low-probability steps between it
and intelligence. So most planets never get it, and among those that get
it the secondary steps never happen. The planets with observers on them
will be the ones that have the unlikely combination of getting all the
steps, and if the natural rate of them happening is slow, we should
expect them to be about equidistant across the time interval the planet
is habitable. So early life is not evidence of easy life unless there is
no reason to think there are any hard steps. However, we do have some
evidence for hard steps (e.g. at least one transition in genetic coding,
which is something we know tends to be stable over 10^80 cell divisions).

In short, observer selection bias makes data from Earth suspect in
updating our probabilities.

Note that your idea is nicely testable: if we do find life on Mars,
Europa or Ceres, then we have reason to think life is indeed common and
not a great filter. That is bad news for our future, though: while the
probability of intelligence evolving factor decreases the most given
this, the expected lifespan of civilization decreases second most.

> We don't have any evidence for why advanced civilisations might be
> 'quiet', though we can think of many possibilities. As you say it
> would be nice if we could think of a universal reason that *must*
> yet. When the Singularity hits us, it will probably become obvious.
> Duh!

Isn't this a rather backward way of reasoning? "It would be nice if we
could figure out what property of the lumiferous aether that makes the
Michelson-Morley experiment fail - there are so many possibilities - but
I have no doubt that when we find it it will be a totally obvious
property of the aether."

--
Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University

```