Now consider an atom which is moving in the + z -direction. The Zeeman effect shifts the energy of the | J = 1 , m J = − − 1 ⟩ ⟩ state lower in energy, decreasing the energy gap between it and the | J = 0 , m J = 0 ⟩ ⟩ state; that is, the frequency associated with the transition decreases. Red-detuned σ σ − − photons, which only drive Δ Δ m J = − − 1 transitions, propagating in the − − z -direction thus become closer to resonance as the atom travels further from the center of the trap, increasing the scattering rate and scattering force. When an atom absorbs a σ σ − − photon, it is excited to the | J = 1 , m J = − − 1 ⟩ ⟩ state and gets a "kick" of one photon recoil momentum, ℏ ℏ k , in the direction opposite to its motion, where k = 2 π π ν ν 0 / c . The atom, now in an excited state, will then spontaneously emit a photon in a random direction and after many absorption-spontaneous emission events, the atom will have, on average, been "pushed" back towards the field-zero of the trap. This trapping process will also occur for an atom moving in the − − z -direction if σ σ + photons are traveling in the + z -direction, the only difference being that the excitation will be from | J = 0 , m J = 0 ⟩ ⟩ to | J = 1 , m J = + 1 ⟩ ⟩ since the magnetic field is negative for z < 0 . Since the magnetic field gradient near the trap center is uniform, the same phenomenon of trapping and cooling occurs along the x and y -directions as well.