Hidden symmetries
(a) Compactifying a 3-D universe with two space dimensions and one time dimension. This is a simplification of the 5-D spacetime considered by Theodor Kaluza and Oskar Klein. (b) The Lorentz symmetry of the large dimension is broken by the compactification and all that remains is 2-D space plus the U(1) symmetry represented by the arrow. (c) On large scales we see only a 2-D universe (one space plus one time dimension) with the "internal" U(1) symmetry of electromagnetism.