Defense of the Standard Quantum Limit for Free-Mass Position

Measurements of the position x of a free mass m are thought to be governed by the standard quantum limit (SQL): In two successive measurements of x spaced a time apart, the result of the second measurement cannot be predicted with uncertainty smaller than ({m) }^1{2}. Yuen has suggested that there might be ways to beat the SQL. Here I give an improved formulation of the SQL, and I argue for, but do not prove, its validity.