  We show that Lorentz and gauge invariance explain the long proton lifetime
within the standard model in six dimensions. The baryon-number violating
operators have mass dimension 15 or higher. Upon TeV-scale compactification of
the two universal extra dimensions on a square $T^2/Z_2$ orbifold, a discrete
subgroup of the 6-dimensional Lorentz group continues to forbid dangerous
operators.
