imaginary mass(tachyons) m = 0 m = 0 p 0  = 0 p 0  = 1 m = 1 p 0  = 2 m = 2 p 0  = 3 m = 3 p 0  = 4 m = 4 p 0  = 5 m = 5 p 0  = 6 m = 6 p 1 p 0