Vagn Lundsgaard Hansen

DIRAC'S STRING PROBLEM

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TRANSLATION OF THE BACK OF THE BAG

The String Problem is not just a curiosity. It was invented by the famous English physicist P.A.M. Dirac, who was awarded the Nobel Prize in physics in 1933, to demonstrate the property half-spin of elementary particles, like neutrons. Dirac himself used a pair of scissors for the demonstration. The mathematics behind the impossibility of removing the twisting of strings with a single twist was almost clarified by the English mathematician M.H.A. Newman in 1942, and by that time the problem had already been known for some years.

The proof uses a deep theory of mathematical braids developed by the eminent German mathematician E. Artin in the 1920's. In 1962 the American mathematician E. Fadell in a scientific paper 12 pages long presented the first complete proof. How one can manipulate the elastic strings so that the twisting of strings with a double twist is removed, is shown in the series of pictures on the back of the bag.

There is a technical application, patented in 1971 by the American D.A. Adams, which exploits the principle behind the String Problem. Adams provided an ingenious solution to the problem of transferring electrical current to a rotating plate without the wires being entangled and breaking. The principle is exactly that after 2 full turns one has untwisted wires as in the initial position.

It should be remarked that the number of strings in Dirac's String Problem is without importance.