Hi all, Another naive question related a previous post (where the topic diverged somewhat). I'm wondering about the following thought experiment: Consider the field associated with a single electron. Now, confine the field to a region (volume) of radius R - that is, field values outside of R are exactly zero (out to infinity, say). This would, in some sense represent a classical potential barrier of infinite height and infinite width. Next, reduce the width of the barrier (very quickly) to some finite value, say M. Now you have a situation in which the field, which was initially exactly zero beyond R, can assume non-zero values at radius R + M, but is still confined to be zero in the 'barrier' region between R and R + M (i.e., the barrier is now a spherical shell). The question: can anything approximating this situation happen in reality? If so, it would seem that, with the field forced to remain zero inside the 'barrier' region, there would be no way to propagate and obtain a non-zero field value outside the 'barrier' (because the field must evolve continuously).... the poor electron would remain forever bounded inside the sphere of radius R. If the field were somehow to evolve through the barrier - this would need to occur no faster than c? How does QFT handle the situation above (again, if possible)? You can't suddenly have a non-zero field value at radius R + M, without some type of propagation (presumably causally) through the barrier region. Thanks.