Special relativity excludes the existence of rigid bodies, since such bodies can be used to transmit signals instantaneously. It is pointed out that the uncertainty principle requires that a force of orderhc/l2must be available if a rigid body of lengthlis to be used to produce such a violation of relativistic causality; a condition which is particularly stringent in the domain of elementary particles. Thus, where a force of this magnitude is not available, the treatment of bodies of lengthlas rigid may be valid.