Why is a Blue Shift in Kruskal Szekeres not Physical

[Airsteve0]

In my intro. to GR class we recently covered the Kruskal Szekeres diagram and trajectories within the diagram. My question comes from a comment made by my professor about time-like trajectories emerging from v=0 and that if an emitter sends a light signal to an observer it will be blue shifted. He said that this is a classical case of instability in the Kruskal-Szekeres geometry; however, I am unsure as to why this is (i.e. why is a blue shift not physical).

 

[atyy]

Googling suggests http://jila.colorado.edu/~ajsh/insidebh/realistic.html

 

[pervect]

Googling suggests http://jila.colorado.edu/~ajsh/insidebh/realistic.html

Nice link, and the same thought came to my mind, but that's not a Kruskal black hole. It's a Reisser-Nordstrom black hole mimicking a Kerr black hole.


It *is* fairly well known that the Schwarzschild/Kruskal geometry probalby isn't stable, but I'm not sure how the blue shift argument would show that. Now if the professor was actually talking about the Kerr black hole, it would all make sense.

 

[atyy]

@Airsteve0: ask your professor and tell us the answer please?