>As far as I know the sun passes through the spiral arms in its orbit,
>so there should be a few passages every galactic year (250 million
>years, if I'm not misremembering).
Are you sure? According to Lem:
"The protosolar gas cloud, moving along an orbit at an angle of seven or eight degrees to the plane of the galaxy, entered the arm for the first time about 4.9 billion years ago. For 300 million years the cloud underwent the stromy conditions of passage through the entire width of the arm; since it left the arm, it has been traveling through calm space. The trip has lasted much longer thant he passage through the arm, because the corotational circle, along which the sun moves, intersects the spiral arms at a sharp angle, making the arc of the solar orbit between the arms longer than the arc within the arm.".
>although he might still argue that it is likelier for intelligence to
>evolve on stars in quieter areas sinply because of a lower risk of
>mass extinctions (but on the other hand, they tend to cause
>evolutionary radiations).
It seems that large scale meteor strikes are all we need to occassionally shake things up (if evolution hasn't created space-farers in this evolutinary cycle lets crash-and-burn and start again!).
best, patrick
Patrick Wilken http://www.csse.monash.edu.au/~patrickw/ Editor: PSYCHE: An International Journal of Research on Consciousness Secretary: The Association for the Scientific Study of Consciousness http://psyche.cs.monash.edu.au/ http://www.phil.vt.edu/ASSC/