# When does the wave function collapse in the EPR paradox experiment? Shouldn't this depend on the reference frame?

Suppose you have two entangled particles. Particle A, and Particle B. If the spin of particle A is up, then the spin of particle B must be down and vice versa.

Now suppose you measure the spin of A at $(xA, tA)$, and measure the spin of B at $(xB, tB)$.

We usually say that the wave function collapsed at $t=tA$, if $tA<tB$ and at $t=tB$, if $tB<tA$.

However, in a different reference frame, if the separation between $(xA, tA)$ and $(xB, tB)$ is spacelike, $tA<tB$ or $tA>tB$ is not uniquely determined.

So, when exactly a wave function collapsed is not clear.