Roy Kerr - The Elephant in the Room. Kerr is its own Maximal Extension.

Abstract: Last year I showed that the Kerr metric, either ingoing or outgoing, contains light rays whose affine lengths are finite and yet they do not end at some singularity. This destroys all the singularity theorems as they assume this cannot happen. I was then told that the “singularity” exists in the maximal extension, say in Kruskal. I checked the derivation of these and found that the determinant of the alleged metric tensor is zero on all the horizons in Kruskal. Not only that but the protagonists all seem to think that this is OK! It isn't. Kerr and Eddington-Finkelstein are their own maximal extensions.
If time permits I will also show why "soft hair" is fool's gold and why the Kerr-Schild approximation method gives the Ligo curves in its first step.
www.bhio2021.com/

[@Achrononmaster]

This is the most entertaining GR webinar I've listened to in ages. @49:00 I wasn't sure what the point being made about the quadrupole approximation matching LIGO was, from Rodrigues and Rueda. Is that also some sort of refutation of the cult of singularity, or just a different curmudgeon issue? Was Roy just trying to say the black hole merger was simpler physics than the orthodoxy expected? So he's having another laugh at them, is all? In any case, go Kiwis. Great stuff, thanks for posting this.

[@gthooft]

I am not quite sure whether we are talking about the same thing. From my perspective, the singularity or singularities inside all black hole solutions known are unphysical anyway. The point is that, in those mathematical space-time structures, all singularities that I know about are hidden behind horizons, and this means that they cannot be observed and they cannot affect the behavior of matter and fields in any way that would be necessary to understand. . It’s not the affine lengths of the paths taken by material particles that are relevant for observations, but the question whether behavior of matter at and beyond singularities is of any relevance for understanding what we see,, and I claim the answer to such questions is negative. In any case, eventually, one has to understand also the quantum effects, and for these, the behavior of particles and fields beyond horizons cannot count because they cannot contribute to the Hilbert space of states.. Think of extrapolating physical models into complex extensions of the metric. Even planets moving in Newtonian ellipses generate cusp singularities in complex spacetimes; for mathematics these singularities are of interest, but not for questions about physically observable phenomena.

[@Achrononmaster]

Horizons can be leaky. Wormholes (at the Planck scale, likely unavoidable, no?) The interior sure matters for Bob who wants to take a one-way vacation there. Physics is not about what _you_ or I can only observe or see, but about what anyone _could_ observe (with rulers, clocks, etc.). I like your point though about the mathematical artefacts being a bit of a Siren trap for physicists.