In article <[email protected]>,
[email protected] (Steve Pope) writes:
> I'm particularly interested in
> whether there specific evidence that a GMC has passed by on
> average every 500 million years, or if this is an extrapolation
> of some sort.
I'd guess it's a statistical estimate from knowing how many GMCs
there are and how fast things are moving. If so, it's probably good
to a factor of two or so unless there's a mistake. ("Passed by" is
vague; one would have to have a precise definition to do a
calculation. However, at first glance that encounter rate looks high
to me.) It isn't obvious to me what evidence there would be for a
specific GMC passage. Maybe there would be something in the
radioisotope record? Absent some signpost, correlating GMC passage
with specific mass extinction events seems impossible.
Where did you read about all this? Do they give any quantitative
information? If not, I'd tend to place it in the category of
"speculation" rather than anything more substantial.
Steve Willner Phone 617-495-7123 [email protected]
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)