Notionally, the cause of bit rot.
However, this is a semi-independent usage that may be invoked as a
humorous way to handwave away any minor randomness that
doesn't seem worth the bother of investigating. "Hey, Eric -- I
just got a burst of garbage on my tube, where did that come
from?" "Cosmic rays, I guess." Compare sunspots,
phase of the moon. The British seem to prefer the usage
`cosmic showers'; `alpha particles' is also heard, because
stray alpha particles passing through a memory chip can cause
single-bit errors (this becomes increasingly more likely as memory
sizes and densities increase).
Factual note: Alpha particles cause bit rot, cosmic rays do not
(except occasionally in spaceborne computers). Intel could not
explain random bit drops in their early chips, and one hypothesis
was cosmic rays. So they created the World's Largest Lead Safe,
using 25 tons of the stuff, and used two identical boards for
testing. One was placed in the safe, one outside. The hypothesis
was that if cosmic rays were causing the bit drops, they should see
a statistically significant difference between the error rates on
the two boards. They did not observe such a difference. Further
investigation demonstrated conclusively that the bit drops were due
to alpha particle emissions from thorium (and to a much lesser
degree uranium) in the encapsulation material. Since it is
impossible to eliminate these radioactives (they are uniformly
distributed through the earth's crust, with the statistically
insignificant exception of uranium lodes) it became obvious that
one has to design memories to withstand these hits.
