When a spinor is rotated by 360° (one full turn), it transforms to its negative, and then after a further rotation of 360° it transforms back to its initial value again. This is because in quantum theory the state of a particle or system is represented by a complex probability amplitude (wavefunction) "ψ", and when the system is measured, the probability of finding the system in the state "ψ" equals , the absolute square (square of the absolute value) of the amplitude. In mathematical terms, the quantum Hilbert space carries a projective representation of the rotation group SO(3).