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A carousel doesn't just spin you around in a circle — with each revolution, it also slightly screws itself into the ground. This corkscrew-like motion is the essence of the NUT charge, a special property of spacetime that physicists have been studying since the mid-20th century.

In the new work, researchers revisited old solutions to Einstein's equations describing curved space around rotating objects with such 'screwing' properties. They showed that mass (ordinary gravity) and NUT charge (a magnetic-like addition) are two sides of the same coin. It all depends on how we solve the main equation: one choice gives mass, the other gives NUT charge. It's like translating one phrase into two languages: the meaning is the same, but the words differ.

The most interesting part came when the authors looked at particles with different spins (internal rotation, like a spinning top). It turned out that the 'electric' version of a particle (associated with mass) and the 'magnetic' version (associated with NUT charge) are described by similar formulas. A special function plays a key role in them — the Bessel function. Only the order of this function changes sign: for mass it is positive, for NUT charge it is negative. It's like looking at the same wave but with the binoculars flipped.

In the end, physicists found no convincing evidence that self-dual gravity exists in higher dimensions — a special case where a particle is completely balanced with itself. In four dimensions, this happens and leads to surprising simplifications in calculations. But in worlds with more dimensions, this magic seems to disappear. This means our Universe is unique in this sense. Or we just haven't found the right 'key' to multidimensional doors yet.