So when physicists measure the spin of a particle along the x axis as, for example, ⁠ ħ / 2 ⁠, the particle's spin state collapses into the eigenstate | ψ ψ x + ⟩ ⟩ . When we then subsequently measure the particle's spin along the y axis, the spin state will now collapse into either | ψ ψ y + ⟩ ⟩ or | ψ ψ y − − ⟩ ⟩ , each with probability ⁠ 1 / 2 ⁠. Let us say, in our example, that we measure − ⁠ ħ / 2 ⁠. When we now return to measure the particle's spin along the x axis again, the probabilities that we will measure ⁠ ħ / 2 ⁠ or − ⁠ ħ / 2 ⁠ are each ⁠ 1 / 2 ⁠ (i.e. they are | ⟨ ⟨ ψ ψ x + | ψ ψ y − − ⟩ ⟩ | 2 and | ⟨ ⟨ ψ ψ x − − | ψ ψ y − − ⟩ ⟩ | 2 respectively). This implies that the original measurement of the spin along the x axis is no longer valid, since the spin along the x axis will now be measured to have either eigenvalue with equal probability.