No, it varies in a gravitational field. This means pretty much everything Penrose ever said about black holes is wrong. Make sure you read what Einstein said:

1912:

“On the other hand I am of the view that the principle of the constancy of the velocity of light can be maintained only insofar as one restricts oneself to spatio-temporal regions of constant gravitational potential”.1913:

“I arrived at the result that the velocity of light is not to be regarded as independent of the gravitational potential. Thus the principle of the constancy of the velocity of light is incompatible with the equivalence hypothesis”.1914:

“In the case where we drop the postulate of the constancy of the velocity of light, there exists, a priori, no privileged coordinate systems.”1915:

“the writer of these lines is of the opinion that the theory of relativity is still in need of generalization, in the sense that the principle of the constancy of the velocity of light is to be abandoned”.1916:

“In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity”.1920:

“Second, this consequence shows that the law of the constancy of the speed of light no longer holds, according to the general theory of relativity, in spaces that have gravitational fields. As a simple geometric consideration shows, the curvature of light rays occurs only in spaces where the speed of light is spatially variable”.

Optical clocks go slower when they're lower, Victor. We call it gravitational time dilation. At the event horizon gravitational time dilation goes infinite, so the optical clock stops.

The relativistic jets get their energy from infalling matter erupting into a gamma ray burst as it approaches the event horizon. Again, see Friedwardt Winterberg's 2001 paper.

Make sure you read this: Kerr black holes as wormholes:

"This second exterior is sometimes thought of as another universe. On the other hand, in the Kerr solution, the singularity is a ring, and the curve may pass through the center of this ring. The region beyond permits closed time-like curves. Since the trajectory of observers and particles in general relativity are described by time-like curves, it is possible for observers in this region to return to their past". It's all a load of baloney. It's science fiction. So is everything else about the Kerr-Newman metric. I've written an article about black holes if you're interested. See this.

That is what one author says about it which does not make that author correct that, I cannot follow the link to "Kerr black holes as wormholes" but geometry wise the Kerr-Newman best explains SMBH's, meaning it is charged and rotating, whether or not there is a wormhole within these black holes is indeterminable.

**Edited by VictorMedvil, 04 December 2019 - 09:23 AM.**