  We study the time evolution of the one-loop diagram in Sen's rolling tachyon
background. We find that at least in the long cylinder case they grow rapidly
at late time, due to the exponential growth of the timelike oscillator terms in
the boundary state. This can also be interpreted as the virtual open string
pair creation in the decaying brane. This behavior indicates a breakdown of
this rolling tachyon solution at some point during the evolution. We also
discuss the closed string emission from this one-loop diagram, and the
evolution of a one-loop diagram connecting a decaying brane to a stable brane,
which is responsible for the physical open string creation on the stable brane.
