We present simulation results comparing the SU(3) heavy quark potential ext
racted from the full Wilson loop expectation to that extracted from the exp
ectation of the Wilson loop fluctuation solely by elements of Z(3). The two
potentials are found to coincide. This agreement is stable under multiple
smoothings of the configurations which remove short distance fluctuations,
and thus reflects long-distance physics. It strongly indicates that the asy
mptotic string tension arises from thick center vortices linking with the W
ilson loop. (C) 1998 Elsevier Science B.V. All rights reserved.