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A new theorem predicts stationary black holes must have at least one light ring
- arbitrage 6y agoSome 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."
- mark-r 6y agoWhat does "stationary" mean in this context? There's no fixed reference point in the universe, so everything should be moving.
- jagraff 6y agoI suspect it means "non-rotating"
- evanb 6y agoStationary 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.
- 6gvONxR4sf7o 6y agoWhat'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?
- tbabb 6y agoA light ring is not a closed timelike curve.
- cma 6y agoI’m not sure of the distinction between light ring and this, but probably a good place to read: Photon sphere https://en.wikipedia.org/wiki/Photon_sphere https://en.wikipedia.org/wiki/Photon_sphere
- fhars 6y agoIt is a closed lightlike curve. There is no impact on causality, as time does not pass on a lightlike curve.
- 6gvONxR4sf7o 6y agoThanks. I'd been dividing it between spacelike and timelike. I forgot there was a third option.