Demonstrations
Resting a small plate flat on the palm, it is possible to perform two rotations of one's hand while keeping the plate upright. After the first rotation of the hand, the arm will be twisted, but after the second rotation it will end in the original position. To do this, the hand makes one rotation passing over the elbow, twisting the arm, and then another rotation passing under the elbow untwists it. In mathematical physics, the trick illustrates the quaternionic mathematics behind the spin of spinors. As with the plate trick, these particles' spins return to their original state only after two full rotations, not after one.The belt trick
The same phenomenon can be demonstrated using a leather belt with an ordinary frame buckle, whose prong serves as a pointer. The end opposite the buckle is clamped so it cannot move. The belt is extended without a twist and the buckle is kept horizontal while being turned clockwise one complete turn (360°), as evidenced by watching the prong. The belt will then appear twisted, and no maneuvering of the buckle that keeps it horizontal and pointed in the same direction can undo the twist. Obviously a 360° turn counterclockwise would undo the twist. The surprise element of the trick is that a second 360° turn in the clockwise direction, while apparently making the belt even more twisted, does allow the belt to be returned to its untwisted state by maneuvering the buckle under the clamped end while always keeping the buckle horizontal and pointed in the same direction. Mathematically, the belt serves as a record, as one moves along it, of how the buckle was transformed from its original position, with the belt untwisted, to its final rotated position. The clamped end always represents the null rotation. The trick demonstrates that a path in rotation space (SO(3)) that produces a 360 degree rotation is not homotopy equivalent to a null rotation, but a path that produces a double rotation (720°) is null equivalent. Belt trick has been witnessed in 1-d Classical Heisenberg model as a breather solution.See also
* Anti-twister mechanism * Spin–statistics theorem * Orientation entanglement * TangloidsReferences
* * {{Cite journal, last1=Pengelley, first1=David, last2=Ramras, first2=Daniel, date=2017-02-21, title=How Efficiently Can One Untangle a Double-Twist? Waving is Believing!, journal=The Mathematical Intelligencer, volume=39, language=en, pages=27–40, doi=10.1007/s00283-016-9690-x, issn=0343-6993, arxiv=1610.04680, s2cid=119577398External links