  We discuss a recent provocative suggestion by Amelino-Camelia and others that
classical spacetime may break down into ``quantum foam'' on distance scales
many orders of magnitude larger than the Planck length, leading to effects
which could be detected using large gravitational wave interferometers. This
suggestion is based on a quantum uncertainty limit obtained by Wigner using a
quantum clock in a gedanken timing experiment. Wigner's limit, however, is
based on two unrealistic and unneccessary assumptions: that the clock is free
to move, and that it does not interact with the environment. Removing either of
these assumptions makes the uncertainty limit invalid, and removes the basis
for Amelino-Camelia's suggestion.
