Is causality in quantum physics also always valid?

Question

For different observers the laws of causality are the same. So the cause and the effect are clear for all observers in any space or time. But is this still valid in quantum mechanics?

Answer 1

I'll brave an answer.

In the paper you described in your comments, they are creating a wavefunction that is a superposition of two different possible outcomes: event A causes B and event B causes A. Until the event is observed, it is unclear which case will occur. However, once this event is observed, the superposition collapses and either A causes B is selected, or B causes A is selected. Both cannot be observed simultaneously.

In terms of observes, anyone viewing the final event will see the same outcome, regardless of reference frame. The simultaneous observation of events for each observer is one of the basic principles of special relativity.

It should be noted that predictability which is related to the uncertainty principle has no general link to causality. For instance, a roulette wheel is not predictable, but it is causal. The same is true for say a Stern-Gerlach experiment. It is unpredictable whether a particle will be spin-up or spin-down, but once observed, it cannot be both spin-up and spin-down.