The structure of spacetime around a Kerr black hole leads to lovely and complex orbits.

Check out @duetosymmetry’s app that lets you visualize the orbits of a photon outside a rotating black hole:

https://duetosymmetry.com/tool/kerr-circular-photon-orbits/

Here is a “cross section” of a Kerr black hole.

The ergosphere is the light blue region outside the horizon. One can move in and out of the ergosphere, but it is not possible to remain at rest once inside. One *must* move in the direction of the black hole’s rotation.

(Note: When thinking about black holes, it is often a good idea to view relativity as describing rules for how and where things are allowed to move, rather than sticking with Newtonian ideas like forces and accelerations.)

As it spins faster, the horizon gets smaller. There is an upper limit on how rapidly the black hole can spin. When it reaches this "extremal" limit, its horizon is just half the Schwarzschild radius.

But the really interesting stuff is happening outside the horIzon, where the black hole’s rotation drags space time along with it.

There is a region called the “Ergosphere” where this dragging is so powerful that it is not possible to remain at rest!

Like the Schwarzschild black hole, Kerr's solution has an event horizon demarcating a region of spacetime which a person, dog, or spaceship can enter but not leave.

(In fact, it has two — an inner horizon and an outer horizon. For our purposes only the outer horizon is relevant.)

The (outer) horizon for a Kerr black hole is smaller than that of a Schwarzschild black hole with the same mass. If it is spinning very slowly, they are about the same.

There are many interesting things to say about Kerr's rotating black hole solution of Einstein's equations. Let me highlight just a few.

First, the structure of spacetime around a rotating black hole is qualitatively more complex than the Schwarzschild black hole, which does not rotate.

Indeed, 47 years passed between the discovery of Schwarzschild’s and Kerr’s solutions.

Both Boyer and Lindquist were visiting the UT Center for Relativity at the time.

Just a few weeks later, on August 1, Boyer was walking across campus when he became the third victim of mass murderer Charles Whitman, the UT Clock Tower shooter.

Lindquist had to add the following note to their paper.

Before saying any more about Kerr's solution, there's a very sad story attached to this worl.

The Kerr metric is usually presented in "Boyer-Lindquist" coordinates, which makes its nature as a rotating, axisymmetric, spherical object more apparent.

They submitted their paper presenting these coordinates in July of 1966.

https://pubs.aip.org/aip/jmp/article-abstract/8/2/265/233738/Maximal-Analytic-Extension-of-the-Kerr-Metric?redirectedFrom=PDF

Kerr was already visiting the US Air Force's Aerospace Research Laboratories in Ohio before he arrived at UT.

It makes sense that UT had a relativity center. The field had been revived by Wheeler et al in the 1950s, and UT was flush with oil money. It was easy for Schild to convince them to support his research center.

But why was the Air Force hosting relativists? Bergmann and Goldberg had convinced them that relativity was essential to national defense, and there's always Defense money.

Relativists quickly realized the importance of what Kerr had done.

Subrahmanyan Chandrasekhar said that realizing the Universe was full of "untold numbers" of black holes described by Kerr's solution was "the most shattering experience" of his scientific career.

http://www.parrikar.org/essays/shakespeare-newton-beethoven-page3/

He announced his result at the 1st Texas Symposium on Relativistic Astrophysics (these mostly biennial meetings still take place). The meeting was convened to discuss quasars, which had only recently been discovered.

As the story goes, Kerr's esoteric topic prompted many in the audience to leave or ignore the talk!

Achilles Papapetrou, a giant of the field, was furious. He stood and demanded the crowd's attention, noting that he himself had tried and failed to solve the problem for 30 years.

Kerr's paper appeared in print on September 1st of that year, so I guess you could just as well call that "Rotating Black Hole Day.”

But why pass up an (admittedly gimmicky) opportunity to talk about black holes?

Kerr, who is from New Zealand, was spending a year at UT-Austin when he wrote and published his breakthrough paper. He was one of the first visiting scientists at Alfred Schild's new Center for Relativity.

Subrahmanyan Chandrasekhar, who established that stars of a certain mass would inevitably collapse into black holes, described his reaction to Kerr's solution as "shuddering before the beautiful."

He wrote:

Happy Rotating Black Hole Day!

Roy Kerr submitted his breakthrough paper describing the spacetime around a rotating black hole #OTD in 1963.

Physicists had been searching for decades for this solution to the field equations of Einstein's theory of general relativity.

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11.237

But here is the thing: the “rule” against posthumous Nobel Prizes was only codified in 1974, well after the Watson and Crick prize, and exclusively refers to “he.”

Three prizes have been awarded after the recipient died, including one in 2012.

https://www.newyorker.com/books/page-turner/should-death-stop-the-nobel

The Nobel Prize used to tweet about her every year, never missing an opportunity to point out that it wasn’t their fault, they surely would’ve given Franklin a share of the prize awarded to Watson and Crick if only she’d lived a little longer. 🙄

Rosalind Franklin was born #OTD in 1920. Her X-ray diffraction work was critical for establishing the helical nature of DNA.

Image: Vittorio Luzzati / Jewish Women’s Archive

@thomasfuchs what?!?! NO!!!

@thomasfuchs But maybe not that active

@thomasfuchs I think he’s here as @GreatDismal

@minouette love this print.