PURPOSE: Mechanical model of unusual quantum mechanical transition.

DESCRIPTION: The gizmo shown in the photograph above is known as the "Dirac Rope Trick." It derives its name because it describes certain quantum mechanical transitions that are unusual in the following sense: Most transitions, consisting of various rotations, translations, and other such leave the object in a position such that it can be returned to its original state by a set of simple rotations. For instance, after undergoing most combinations of twists and turns, giving the object up to a single 360^o rotation will restore it to its original orientation. In general, a rotation of 360^o will render an object back to its original state before the rotation. However, there are some transitions from which the object can only be restored to its original state by two 360^o rotations. This type of transition is illustrated by the device in the photograph above in an mpeg video that can be seen by clicking your mouse on the photograph.

Try this sequence of transitions.

SUGGESTIONS: See Question of the Week #269 for information on using this demonstration to enhance class involvement.

Click here to see Krishna perform this transformation as described in a paper: Edgar Rieflin, Some mechanisms related to Dirac's strings, AJP47(4), 379-380 (1979).

REFERENCES: (PIRA 1A50.60) See Demonstration Reference File for papers dealing with these gizmos.

EQUIPMENT: Rope assembly as shown.

SETUP TIME: None.