The name vector boson arises from quantum field theory. The component of such a particle's spin along any axis has the three eigenvalues − ħ, 0, and + ħ (where ħ is the reduced Planck constant), meaning that any measurement of its spin can only yield one of these values. (This is true for massive vector bosons; the situation differs for massless particles such as the photon, for reasons beyond the scope of this article. See Wigner's classification.)