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  Did Dirac ever publish anything about his string trick?

+ 0 like - 0 dislike

Paul Dirac often used scissors and strings to demonstrate the properties of spin 1/2 and of SU(2). The demonstration is variously called the "string trick" or the "belt trick" or the "scissor trick".

Did Dirac ever publish anything about it - in a paper or in one of his books on quantum theory?

Searching on Google scholar it seems that he didn't - but is this conclusion correct?

asked Feb 10 in Theoretical Physics by Johanna [ no revision ]

@Johanna , I am not sure if Dirac wrote about it; but you can find that in Roger Penrose's and Wolfgang Rindler's textbook called Spinors and space-time volume 1 on page 43.

1 Answer

+ 1 like - 0 dislike

P. Dirac's string trick does not and may not demonstrate the properties of spin 1/2 of SU(2), as a matter of fact. First, SU(2) transformations are not real rotations, but recalculation rules from one still reference frame to another one. Second, his trick works for real rotations whereas a rotation to $2\pi$ in SU(2) does not change physical (observable) results of spin 1/2 in QM. I heard that he demonstrated his "analogy" for fun, but, probably, he reconned it as an "analogy" rather than as a "proof". There is nothing about it in his list of publications.

answered Feb 10 by Vladimir Kalitvianski (102 points) [ revision history ]
edited Feb 10 by Vladimir Kalitvianski

Did Dirac write about the trick in any of his books on quantum theory?

As usual, Vladimir's statements are all wrong or misleading. Why does he waste his talents, his life and his time in this way?

@Frank: Frank, frankly, point out where I am wrong or misleading, please. I wrote sincirely what I knew about the subject. For example, I have a full list of publications and the publications themselves in Russian (they were translated in Soviet times).

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