The notion of spacetime having more than four dimensions is of interest in its own mathematical right. Its appearance in physics can be rooted to attempts of unifying the fundamental interactions, originally gravity and electromagnetism. These ideas prevail in string theory and beyond. The idea of complex spacetime has received considerably less attention, but it has been considered in conjunction with the Lorentz–Dirac equation and the Maxwell equations. Other ideas include mapping real spacetime into a complex representation space of SU(2, 2), see twistor theory.