A block is at rest. The block is macroscopic so we can see that it is at rest and where it is, i.e. we know its position with certainty. Suppose we use the Heisenberg Uncertainty Principle to estimate the position uncertainty Δ x. Consider the following argument: Since the engine is at rest, its momentum p is zero. Moreover, since Δ p = 0, the uncertainty in its position is infinite. Therefore, the position of the locomotive is completely unknown. Resolve the supposed contradiction.