  It is considered to re-formulate quantum theory as it appears: A theory of
continuous and causal time evolution, interrupted by discontinuous and
stochastic jumps. To develop the (missing) theory of jumps a
heuristic-phenomenological approach is suggested. Relying on a global reduction
process, a hypothesis is introduced postulating spontaneous collapse of
superpositions of states which describe spontaneous absorption or emission. The
collapse probability determines a mean collapse time $\tau^c = b \hbar /
\triangle E$, where $b$ is a dimensionless constant and $\triangle E$ is the
difference in energy distribution between alternative branches. Ramsey atomic
beam spectroscopy yields a lower bound on $b$ and avalanche photodiodes give an
upper bound, such that $1.35\cdot 10^{11} < b < 3.8\cdot 10^{21}$. Ample
opportunities for experimental improvements exist.
