Survey of two-time physics.

*(English)*Zbl 0990.83010Two-time physics (2T) is a general reformulation of one-time physics (1T) that displays previously unnoticed hidden symmetries in 1T dynamical systems and establishes previously unknown duality-type relations among them. This may play a role in displaying the symmetries and constructing the dynamics of little understood systems, such as M-theory. 2T-physics describes various 1T dynamical systems as different \(d\)-dimensional ‘holographic’ views of the same 2T system in \(d+2\) dimensions. The ‘holography’ is due to gauge symmetries that tend to reduce the number of effective dimensions. Different 1T evolutions (i.e. different Hamiltonians) emerge from the same 2T-theory when gauge fixing is done with different embeddings of \(d\) dimensions inside \(d+2\) dimensions. Thus, in the 2T setting, the distinguished 1T which we call ‘time’ is a gauge-dependent concept. The 2T-action also has a global \(SO(d,2)\) symmetry in flat spacetime, or a more general \(d+2\) symmetry in curved spacetime, under which all dimensions are on an equal footing. This symmetry is observable in many 1T-systems, but it remained unknown until discovered in the 2T formalism.