 Hence, as an approximative measurement of the position \(\bQ_t\), one could propose the observable \(\bQ'_t = \frac{t}{m}\bP\). It is known that under the stated conditions (and with \(m\) and \(t\) large) this approximation holds very well, i.e., we do not only have  \(\expval{\bQ'_t -\bQ_t}_\psi =0\), but also  \(\expval{(\bQ'_t -\bQ_t)^2} \approx 0\), as nearly as we please. But since \(\bQ'_t\) is just the momentum multiplied by a constant, its measurement will obviously not disturb the momentum of the system. In other words, for this example, one has \(\epsilon_\psi (\bQ)\) as small as we please with zero disturbance of the momentum. Therefore, any hopes that there could be a positive lower bound for the product \(\epsilon_\psi (\bQ) \eta_\psi (\bP)\) seem to be dashed, even with the simplest of measurement schemes, i.e. a free evolution.