According to Einstein's thought experiment, imagine a clock but this time the distance between mirrors is one Planck length $\ell_p$. If we move the spaceship (with clock on board) at velocity $v$. What is the distance that the photon wavefunction must go to reflect from the point of view of an external observer? Obviously it should be $n\ell_p,n\in\mathbb{N}$ and can't be a non-integral multiple.
By the way I'm not a physicist so I will be glad if you explain with less math.
