
A new theorem predicts stationary black holes must have at least one light ring - dnetesn
https://phys.org/news/2020-06-theorem-stationary-black-holes.html
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arbitrage
Some actual details from the article once you get past the initial five
paragraphs of breathless popscience-journalism:

> "In other words, we do not assume that one law of gravity is correct, but
> only assume that the correct law of gravity (i.e., whichever that is) allows
> the existence of black holes," Cunha and Herdeiro said. "Then, forcing the
> spacetime structure to obey to some regularity requirements, the existence
> of a black hole implies that there must be a light ring outside the horizon.
> In fact, a rotating black hole must have at least two light rings: one for
> light circling the black hole in the same spinning direction as the black
> hole and another one for light circling the black hole in the opposite
> direction."

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6gvONxR4sf7o
What's a light ring from a time perspective? Seems as close to time travel as
it gets, but that's just my pop intuition around timelike curves in spacetime.
If it's a closed timelike curve outside the event horizon, is that a big deal
for causality formalisms?

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fhars
It is a closed lightlike curve. There is no impact on causality, as time does
not pass on a lightlike curve.

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6gvONxR4sf7o
Thanks. I'd been dividing it between spacelike and timelike. I forgot there
was a third option.

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mark-r
What does "stationary" mean in this context? There's no fixed reference point
in the universe, so everything should be moving.

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evanb
Stationary means a blackhole that is not evolving in time---one that is in a
stationary state
[https://en.wikipedia.org/wiki/Stationary_state](https://en.wikipedia.org/wiki/Stationary_state)
Note that the article focuses on the QM meaning of stationary, but there's a
perfectly good classical analogue: something which is not evolving in time.

That doesn't mean that the BH can't spin, it's that the mass, charge, angular
momentum isn't changing.

