PBR theorem

The PBR (or Pusey-Barrett-Rudolph) theorem explains how to find out whether two different quantum systems, which might be far away from each other, are the same or different. To do this, you look at how they're related to each other. They're the same if they are related in the same way in all possible situations, and different if they are related differently in any of those situations. This theorem helps us understand if two different systems are the same.