A biological lipid membrane may be viewed as a two
dimensional (liquid crystal) fluid that is immersed in a
three dimensional water solution. The system is further
complicated by that the membrane is non flat,  undergo time
dependent undulations and have a thickness that fluctuates
in time and space. This gives rise to complicated
correlation functions in time and space. Experimentally some
of these functions can be probed by  inelastic scattering of
neutrons or light and more recently by neutron spin echo
experiments. Field dependent  NMR -relaxation experiments
give also important information. 
We report here about molecular dynamics simulations that
indicate that many of these correlation functions are
stretched exponentials rather than ordinary exponentials and
discuss different ways to interpret.