If $\overrightarrow{\alpha}$ and $\overrightarrow{\beta}$ are non zero and different vector such that $|\overrightarrow{\alpha} + \overrightarrow{\beta}| = |\overrightarrow{\beta} - \overrightarrow{\alpha}|$, then the angle between $\overrightarrow{\alpha} $ and $\overrightarrow{\beta}$ is :