A Spacetime model of evaporating black hole

[PeterDonis]

The flow lines of the null dust are parallel to the bottom right and top left sides of the box

That also makes it simpler for me to see the key difference between this and the OS case: in the OS case, the boundary is itself a fluid flow line! Whereas in this case, of course, it isn't.

That might be a valid general condition for spotting cases where a vacuum to non-vacuum boundary won't work, at least for models where the nonzero stress-energy can be modeled as a fluid.

 

[Dale]

The box I'm visualizing has four null sides. Its left corner and bottom corner are in the Schwarzschild region, and its right corner and top corner are in the Vaidya region. So the boundary between the regions passes through the bottom right and top left sides of the box.

Yes. That will make the calculations easier.

If I've got that right, then you just saved me a bunch of tedious calculation.

You are welcome!