John Carlos Baez on Nostr: The headline screams "𝗦𝗶𝗻𝗴𝘂𝗹𝗮𝗿𝗶𝘁𝗶𝗲𝘀 ...

The headline screams

"𝗦𝗶𝗻𝗴𝘂𝗹𝗮𝗿𝗶𝘁𝗶𝗲𝘀 𝗱𝗼𝗻’𝘁 𝗲𝘅𝗶𝘀𝘁,” 𝗰𝗹𝗮𝗶𝗺𝘀 𝗯𝗹𝗮𝗰𝗸 𝗵𝗼𝗹𝗲 𝗽𝗶𝗼𝗻𝗲𝗲𝗿 𝗥𝗼𝘆 𝗞𝗲𝗿𝗿

What's up with that?

As usual, it's less of a big deal than it looks. But Kerr is smart: he's the guy who first found solutions that describe rotating black holes. And the article is nice and clear. So it's more interesting than the usual nothingburger we've come to expect from screaming headlines about fundamental physics.

Kerr is talking about Penrose's famous singularity theorem, the one Penrose won the Nobel prize for. This says roughly that if space is infinite in extent, and light becomes trapped inside some bounded region, and no exotic matter is present to save the day, general relativity predicts that either a singularity or something even more bizarre must occur.

Of course I'm not stating the theorem precisely here. Each of the vague terms I just used must be made precise. But what's Kerr claiming?

Kerr isn't claiming Penrose's result is false. Instead, he's doing two things.

First, he's pointing out that the definition of "singularity" used in the theorem is not the only definition possible. A "curvature singularity" is - very roughly - a place where the curvature of spacetime approaches infinity. But that's not the kind of singularity that Penrose was talking about! Instead, he was talking about a place where the path of a particle can suddenly end. This may or may not be a curvature singularity.

This is not news.

Second, Kerr is arguing that rotating black holes have some singularities in Penrose's sense that aren't curvature singularities. Unfortunately this argument seems to be wrong.

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https://bigthink.com/starts-with-a-bang/singularities-dont-exist-roy-kerr/