Space-time 'foam' is a geometric picture of the smallest size scales in the
Universe, which is characterized mainly by the presence of quantum uncerta
inties in the measurement of distances. All quantum-gravity theories should
predict some kind of foam(1,2), but the description of the properties of t
his foam varies according to the theory, thereby providing a possible means
of distinguishing between such theories. I previously showed(3) that foam-
induced distance fluctuations would introduce a new source of noise to the
measurements of gravity-wave interferometers, but the theories are insuffic
iently developed(4) to permit detailed predictions that would be of use to
experimentalists. Here I propose a phenomenological approach that directly
describes space-time foam, and which leads naturally to a picture of distan
ce fluctuations that is independent of the details of the interferometer. T
he only unknown in the model is the length scale that sets the overall magn
itude of the effect, but recent data(5) already rule out the possibility th
at this length scale could be identified with the 'string length' (10(-34)
m < L-s < 10(-33) m). Length scales even smaller than the 'Planck length' (
L-P approximate to 10(-35) m) will soon be probed experimentally.