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Relative Likelihood for Life as a Function of Cosmic Time (arxiv.org)
24 points by privong on July 1, 2016 | hide | past | favorite | 12 comments

One of the unfortunate parts of our being the only intelligent species we know of is that while we can mathematically guesstimate life's appearance throughout the cosmos, we have no data on when intelligent civilizations die out.

In the final episode of Cosmos, Carl Sagan describes a nightmare in which he is traveling the universe and decides to return to earth. As he gets closer, his spaceship intercepts broadcasts that have been traversing space since the invention of radio, from the earliest radio messages to television signals and then eventually...the signals stop.

> We find that unless habitability around low mass stars is suppressed, life is most likely to exist near 0.1 solar-mass stars ten trillion years from now.

Microbial life perhaps, but as far as complex life is concerned, I claim habitability on those timescales will indeed be suppressed. Complex life needs plate tectonics, which depends on a planet's internal heat. A red dwarf star may shine for ten trillion years, but terrestrial planets will be stone cold dead long before then.

Complex life also need a very large moon to stabilise the poles. The Earth's moon is very unusual (in some ways the moon and Earth are double planets). The collisions that can can form a moon of sufficient size are very, very rare.

How about low-temp life based on superfluid in Helium, with impurities? In addition, you can get plate techtonics in a number of ways, such as being a large moon orbiting a giant planet.

Why would complex life depend on plate tectonics?

These are speculative and rather weak. Like most theory in this area, it takes the form "Look at these features of the development of life as we know at; if they weren't there, it couldn't exist!". But of course, they are simply unable to imagine, much less predict, what happens in dramatically different conditions.

Also, talk about correlation does not equal causation at your link:

> The basic structure of the Earth, aided by plate tectonics, makes possible the Earth's magnetic field. Neither Venus nor Mars has such a magnetic field. Venus has core and molten regions like the Earth, but a very slow rate of rotation. It produces neither plate tectonics or a magnetic field. Mars has volcanism, but limited to a small number of spectacularly large volcanoes like Olympus Mons. It's magnetic field is weak despite a rotation period similar to that of the Earth. This suggests that the molten nature of the Earth's mantle that facilitates plate tectonics is also essential for the operation of the dynamo that produces the magnetic field.

I mean, N=3 !

Okay, but which of the following two positions are you arguing for?

1. We don't have enough data about other solar systems, so we don't know where complex life might exist.

2. We don't have enough data about other solar systems, so complex life probably exists all over the place.

If you're arguing for the first position, fair enough; I might not entirely agree, but it is at least reasonable.

But I see too many people arguing for the second position, taking scarcity of data as a license to believe what they want to believe.

The first. I think the most likely resolution to the fermi paradox, which is not that paradoxical, is just that abiogenesis is a galactically rare event. Other proposed filters, like the advent of sexual reproduction, eukaryotes, or intelligence, seem much less robust to a failure of imagination.

Fair enough, that's a reasonable position. My opinion, for what it's worth, is that every step is a hard step, because it's not just a matter of time elapsed to the next one, but of each step having to be taken in just such a way as to set the right preconditions for all the subsequent ones.

There is scope for considerable refinement in the choice of the second factor p(life|HZ). [...] In our simplified treatment, this constant value has no effect on dP(t)/dt since its contribution is also cancelled by the normalization factor N.

Though this paper does provide some needed formalism around the Drake Equation, I don't understand how its results can be meaningful if it "cancels out" one of the largest sources of uncertainty.

If we were exceptional, we would expect the sun to be one tenth the size it is. Therefore we are not exceptional.

Fine, but that same argument says that most life in the universe will be coming along in trillions of years and a fat lot of good that will do for us.

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