Have you ever seen someone who is physically and mentally abusive (possibly a narcissist or a person with BPD) change, especially if they've been this way to every partner they've had for years?

I can't speak personally really, but from my research, no. A VERY hard MAYBE with BPD, if they WANT to change and after MANY years of DBT Therapy, they COULD change but it most likely won't happen. Narcissists will never change. They don't think anything is wrong with them, they're getting all of their needs met no matter if it hurts anyone or not. They will deny being a narcissist and never seek treatment.

In layman's terms, what is spacetime in string theory?

You ask for an explanation in layman's terms but in the question details you talk about QFT, GR and position operators. So I'll give you an explanation that assumes basic knowledge of QFT and GR; it might not be suitable for an actual "layman".We only know how to describe string theory perturbatively. What this means is

What do you think about love marriage and arranged marriage?

Both of them can succeed or fail .Love marriages in the west have the highest divorce rate .Arranged marriages in India fails often . of-course , love marriages fails in India at considerable ratesSuccess of any marriage lies in bonding . It takes more than sex to stay together . companionship , friendship and mutual respect is very