It is said that the measurement done on a particle instantly affects its entangled pair because Bell's theorem excludes a hidden variable. That means there is a cause and an instant effect at a distance. Say we have two entangled particles A and B. If there is no hidden variable then the state of the particles are not definite until measured. After we measure the first particle and find it with spin |up> for example, does it mean now particle B is in a definite state |down>? If we consider both particles as a single system, after the measurement of the first particle is the system state determined on the measurement axis?