*Berry phase*, is a phase difference that a given physical system acquires during a cycle in which the system itself is under the action of an adiabatic process. This phase is linked to the geometric properties of the system itself (which is a simplification, but for our purposes there is no need to go into too much detail).

It was discovered independently by

**Shivaramakrishnan Pancharatnam**in 1956

^{(1)},

**Hugh Christopher Longuet-Higgins**

^{(2)}in 1958 and subsequently generalized by

**Michael Berry**

^{(3)}in 1984. This phase, although geometric, has measurable physical effects, for example in an interference experiment. An example of a geometric phase is

*Foucault's pendulum*.

The most famous version of this experiment, designed by

**Léon Foucault**, dates back to 1851 when the French physicist, with the aim of showing the rotation of the Earth around its axis, suspended a ball of 28 kilograms of lead coated with brass over a surface of sand using a 67 meter cable hooked to the top of the dome of the

*Panthéon*in Paris. The plane of the pendulum was observed to rotate clockwise at approximately 11.3 degrees per hour, completing a full circle in 31.8 hours. A more refined examination shows that after 24 hours there is a difference between the initial and final orientation of the trace left on Earth which is equal to \[\alpha = -2\pi \sin \varphi\] where \(\varphi\) is the latitude. In other words, if on a closed path I associate a vector to the initial position and see how its orientation varies along the entire path, when I return to the starting point at the end of the tour, I find that the final orientation is different from the initial one.

All this to say that even black holes are subject to the existence of a Berry phase under the action of adiabatic variations of supergravity, which, as the name suggests, is linked to string theory and supersymmetry. And perhaps, then, it is no coincidence that this effect is practically impossible to measure for a black hole, an object that is itself difficult to detect...

de Boer, J., Papadodimas, K., & Verlinde, E. (2009). Black hole berry phase.

*Physical review letters*, 103(13), 131301. doi:10.1103/PhysRevLett.103.131301 (arXiv)- Pancharatnam, S. (1956, December). Generalized theory of interference and its applications. In
*Proceedings of the Indian Academy of Sciences-Section A*(Vol. 44, No. 6, pp. 398-417). Springer India. doi:10.1007/BF03046050. ↩︎ - Longuet-Higgins, H. C., Öpik, U., Pryce, M. H. L., & Sack, R. A. (1958). Studies of the Jahn-Teller effect. II. The dynamical problem.
*Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences*, 244(1236), 1-16. doi:10.1098/rspa.1958.0022. ↩︎ Berry, M. V. (1984). Quantal phase factors accompanying adiabatic changes.

*Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences*, 392(1802), 45-57. doi:10.1098/rspa.1984.0023. ↩︎

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