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see also Symmetry and Gauge Symmetry
You are sitting in a room with a friend and a ping-pong ball (perfectly spherical and perfectly white— the ping-pong ball, not the friend). The conversation gets around to Newtonian mechanics. You toss the ball to your friend. Both of you agree that, given the speed and direction of the toss, F = m A and the formula for the gravitational attraction at the surface of the earth ( F = −mg k, if the positive z-direction is up), you could calculate the motion of the ball, at least if air resistance is neglected. But then you ask your friend: “As the ball was traveling toward you, was it spinning?” “Not a fair question”, he responds. After all, the ball is perfectly spherical and perfectly white. How is your friend supposed to know if it’s spinning? And, besides, it doesn’t matter anyway. The trajectory of the ball is determined entirely by the motion of its center of mass and we’ve already calculated that. Any internal spinning of the ball is irrelevant to its motion through space. Of course, this internal spinning might well be relevant in other contexts, e.g., if the ball interacts (collides) with another ping-pong ball traveling through the room. Moreover, if we believe in the conservation of angular momentum, any changes in the internal spin state of the ball would have to be accounted for by some force being exerted on it, such as its interaction with the atmosphere in the room, and we have, at least for the moment, neglected such interactions in our calculations. It would seem proper then to regard any intrinsic spinning of the ball about some axis as part of the “internal structure” of the ball, not relevant to its motion through space, but conceivably relevant in other situations.
The phase of a charged particle moving in an electromagnetic field (e.g., a monopole field) is quite like the internal spinning of our ping-pong ball. We have seen that a phase change alters the wavefunction of the charge only by a factor of modulus one and so does not effect the probability of finding the particle at any particular location, i.e., does not effect its motion through space. Nevertheless, when two charges interact (in, for example, the Aharonov-Bohm experiment), phase differences are of crucial significance to the outcome. The gauge field (connection), which mediates phase changes in the charge along various paths through the electromagnetic field, is the analogue of the room’s atmosphere, which is the agency (“force”) responsible for any alteration in the ball’s internal spinning.
The current dogma in particle physics is that elementary particles are distinguished, one from another, precisely by this sort of internal structure. A proton and a neutron, for example, are regarded as but two states of a single particle, differing only in the value of an “internal quantum number” called isotopic spin. In the absence of an electromagnetic field with which to interact, they are indistinguishable. Each aspect of a particle’s internal state is modeled, at each point in the particle’s history, by some sort of mathematical object (a complex number of modulus one for the phase, a pair of complex numbers whose squared moduli sum to one for isotopic spin, etc.) and a group whose elements transform one state into another (U (1) for the phase and, for isotopic spin, the group SU (2) of complex 2 × 2 matrices that are unitary and have determinant one). A bundle is built in which to “keep track” of the particle’s internal state (generally over a 4-dimensional manifold which can accommodate the particle’s “history”). Finally, connections on the bundle are studied as models of those physical phenomena that can mediate changes in the internal state. Not all connections are of physical interest, of course, just as not all 1-forms represent realistic electromagnetic potentials. Those that are of interest satisfy a set of partial differential equations called the Yang-Mills equations, developed by Yang and Mills [YM] in 1954 as a nonlinear generalization of Maxwell’s equations.
page 22ff in Topology, Geometry and Gauge fields by Naber
For the case of regular spin, we had to take spin-space seriously because it was associated with a concrete, measurable, physical quantity—angular momentum. This was only mildly uncomfortable because, although spinspace has the somewhat hard-to-stomach property that you have to turn all the way around twice to get back to your original condition, it’s otherwise pretty much like regular space. Isospin space, however, is completely abstract; it bears no relation whatsoever (other than through analogy) to anything we can grasp with our faculties of perception. How could rotations in such a space possibly have anything to do with the physical world? And yet the physical manifestation of the invariance of the strong force with respect to rotations in this space, the conservation of isospin, is a solidly established fact in the world of experimental science. So, what then is isospin-space from a physical point of view? Physicists usually describe it as an internal symmetry space, but what’s that, really? It’s your old buddy again, telling you that your car’s carburetion system “works on a vacuum principle.” How’s that going to help you to understand and fix the thing? It isn’t. Regarding the physical interpretation of the notion of isospin space, again your guess is as good as mine. Perhaps its experimental manifestations are hinting at some new and deeper truth about the universe that lies just beyond the current limits of our comprehension. Perhaps not. But one thing, however, is true: The introduction of the idea of internal symmetry spaces, of which isospin space was the first example, was an essential step forward in our understanding of the universe and the nature of the laws that govern it." Deep Down Things by Schumm>
„What the heck is an internal space?” you ask. Good question. The best answer I have is “useful.” It’s what we invented to quantify the observed behavior of particles, a mathematical tool that helps us make predictions. “Yes, but is it real?” you want to know. Uh-oh. Depends on whom you ask. Some of my colleagues indeed believe that the math of our theories, like those internal spaces, is real. Personally, I prefer to merely say it describes reality, leaving open whether or not the math itself is real. How math connects to reality is a mystery that plagued philosophers long before there were scientists, and we aren’t any wiser today. But luckily we can use the math without solving the mystery.
Lost in Math by Sabine Hossenfelder
Internal symmetries are powerful that we use, for example, to derive the correct Lagrangians describing fundamental interactions.