  In this work, we re-examined the ancient complex metric in the recent quantum
picture of black holes as Bose-Einstein condensates of gravitons. Both black
holes and particles can be described by the complex Kerr-Newman metric in a 6-D
complex space, which appears as a 4-D spacetime for a real or imaginary
observer because of the barrier of the horizon. As two kind of complex black
holes, particle and black hole are complex conjugated and can convert into each
other through a phase transition. From the view of an observer in 3-D real
space, an elementary particle with spin appears as an imaginary black hole in
an anti-de Sitter space. The self-gravitational interaction of a particle as an
imaginary black hole makes it obtain its wave-like nature in 4-D spacetime.
