Holonomy in quantum mechanics
Let’s start with the definition of holonomy: if you walk in closed loop and the object you carry changes when you complete the loop then you have experienced a holonomy.
Now this sounds plain crazy so a simple example can illustrate it. Suppose you are a hunter living on the Equator and you go on a quest to explore the Earth. You walk a quarter of the Earth circumference on the Equator going east, you travel north all the way to the North Pole and then you go straight down to the starting point of your journey. During your journey you carry with you your spear always making sure it is pointing in the same direction. For definiteness sake, let’s say that originally your spear was pointing towards the North Pole. When you walk on the Equator and then towards the North Pole your spear is pointing north. However, on the last part of the journey, your spear will be pointing west. So upon your arrival the spear has a different orientation even though you always carried it parallel with itself. This is the result of Earth’s curvature.
Now something very similar happens in Einstein’s general relativity: the presence of mass curves the space-time and although you travel on geodesics in a straight line, nearby curves are not parallel. We feel this lack of parallelism as gravity.
Fine, we understand this, but what does holonomy have to do with quantum mechanics? Suppose I have a box with a quantum device which when I press a button can flash either a red light or a blue light. Suppose that every time I press the button only the red light will flash. Now I go on a similar quest on a closed space loop and when I press the button only the blue light is flashing. This would happen every time I would circle a zone of magnetic field although I cannot detect any magnetic field anywhere on my path.
Now this is downright freaky: there are no forces whatsoever along my path and still there is a measurable effect. Welcome to the wonderful Bohm Aharonov effect.
Mathematicians usually refers to geometric phases and call this effect a topological one. But surprisingly it has a nice mathematical explanation in terms of boundary conditions and domains in standard quantum mechanics. When learning quantum mechanics, pesky boundary conditions and domains tend to be ignored as pedantic crossing of t’s and dotting the i’s. This is the typical cavalier physicist’s attitude towards mathematical rigor. Don’t believe me? Ask any physicist to tell you the difference between Hermitean and self-adjoint. If one in one hundred knows the difference you are lucky.
The point is that self-adjointness demands the domains of the operator and its hermitean counterpart (complex conjugate and transposed) to be identical!!!
A hermitean operator may have even an infinity of different self-adjoint extensions. And the eigenvalues (the observed values) are all distinct.
Here is an example of how sloppiness can get you into trouble: http://www.mth.kcl.ac.uk/~streater/lostcauses.html#VIII
Now for the Bohm-Aharonov effect, it is the boundary condition which selects the eigenvalues. And this boundary condition comes from the magnetic flux carried by the solenoid.
An excellent review of those topics can be found in Asher Peres’ classic: “Quantum Theory: Concepts and Methods” and I hope I wetted your appetite to read this outstanding book.
Now, although true, changes without forces bother a lot a people. Last year a proposal was made to restore the role of forces in the Bohm-Aharonov effect: http://arxiv.org/abs/1110.6169 The paper has a clever idea: the apparatus and not the particle feels the force, but it is fundamentally flawed because any forces will give out the “which way information” which will destroy the interference pattern. Holonomy is a fundamental property of Nature and it is not explainable away.