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" ... The difference between a gauge symmetry and a global symmetry is that the presence of gauge charge can be measured from far away, while the presence of a global charge cannot. For example in elecromagnetism if we want to know the total charge in a region, we just have to measure the electric flux through its boundary. Similarly in gravity if we want to know the energy of something, we can measure the fall-off of the metric far away (basically looking for the M in the Schwarzschild metric). This should be compared with for example the Z_2 global symmetry of the Ising model, where there is no way to know that the spins are up in a region without going there and looking at them. ... "