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?

 

[sardar]

I would like to clarify that the term "physical" can have different meanings in different contexts. In general relativity, the term "physical" refers to quantities that are measurable and observable in our physical world.

In the context of the Kruskal-Szekeres diagram, a blue shift in the trajectory of a light signal does not necessarily correspond to a physical phenomenon that can be observed. This is because the Kruskal-Szekeres diagram is a mathematical representation of spacetime, and the trajectories within it do not necessarily correspond to actual physical paths of objects.

Furthermore, the blue shift in the trajectory of a light signal in the Kruskal-Szekeres diagram is a result of the mathematical transformation used to create the diagram. It does not represent a physical effect that can be observed in our physical world.

In fact, the blue shift in the Kruskal-Szekeres diagram is a consequence of the singularity at the center of the diagram, which is a mathematical artifact and not a physical reality. This singularity represents a point where our current understanding of physics breaks down, and therefore, it is not a physical phenomenon that can be observed.

In summary, while a blue shift may occur in the Kruskal-Szekeres diagram, it is not a physical effect that can be observed in our physical world. It is a mathematical artifact that arises due to the singularity at the center of the diagram. Therefore, it is not considered a physically meaningful phenomenon.