Soul Physics are an odd sort of byproduct of religion. They are the interactions between souls and odd things like that. For example, the current view is that the soul enters the cell at the moment of conception. Only human cells though.
This differs from earlier notions that the soul entered the fetus at quickening (first kick) or that early on a fetus has a vegetable soul. However, the current view (at least in the Christian worldview) has a number of rather critical problems. First, identical twins. When the embryo splits does the soul double? Do they share the same soul? Does one clump of cells get the soul and the other clump not get any? What happens if the broken embryo recombines? Does it now get one soul, two souls, half a soul? What about Chimerism? Where rather than twins recombining we simply have two different fraternal twins recombine into one person. Do they have two souls? Or one soul that enters at the moment of combining? What about clones?
If embryonic stem cells are taken to their full potential it's easily possible that any stem cell could be made into an embryo. As is, it's easily possible to remove a few stem cells from the side of a embryo and make a new stem cell line. Growing the original embryo into a human being. What happens soul physics wise if we start growing thousands of clones out of that stem cell line? What if it turns out that rather than plurapotent adult stem cells have totopotency? We could take adult stem cells and make them into embryos. It's also feasible to take any cell and un-differentiate it. Any cell in a human has the potential to be human. If embryos are people too, aren't we committing genocide by scratching our asses? The only thing stopping many of these cells from being human is a lack of a womb. Clearly, worse than the murder of removing some potential life from a womb is having a womb without potential life.
An unoccupied womb is murder!