# Tag Info

Accepted

### Do black holes have a moment of inertia?

The angular velocity of a Kerr black hole with mass $M$ and angular momentum $J$ is $$\Omega = \frac{J/M}{2M^2 + 2M \sqrt{M^2 - J^2/M^2}}$$ The moment of inertia of an object can be thought of as ...
• 6,475
Accepted

### Wouldn't Miller's planet be fried by blueshifted radiation?

Miller's world would be fried by a strong flux of extreme ultraviolet (EUV) radiation. The cosmic microwave background (CMB) would be blueshifted by gravitational time dilation and then would be very ...
• 135k
Accepted

### What do black holes spin relative to?

But if you start running around it, it will move faster or slower relative to you. In this case, the disc has a ground speed, 60 rpm, because it has something to spin in relation to, in this case, the ...
• 105k
Accepted

### Massless Kerr black hole

It's simply flat space in Boyer-Lindquist coordinates. By writing $\begin{cases} x=\sqrt{r^2+a^2}\sin\theta\cos\phi\\ y=\sqrt{r^2+a^2}\sin\theta\sin\phi\\ z=r\cos\theta \end{cases}$ you'll get good ol'...
• 5,951
Accepted

### Can a Kerr black hole become super-extremal?

TLDR; No, it cannot. The answer to this question has a somewhat storied history. Back in 1974 Wald considered a Gedanken experiment of what would happen if you would try to drop an object into a Kerr ...
• 12.6k

### Why does Roy Kerr claim that the Kerr black hole does not contain a singularity?

As far as I can tell, Kerr's point is that the singularity theorem doesn't specify whether or not a collapsing object will collapse to a "real" singularity. As a reminder, singularity ...
• 16.6k

### Why are complex coordinates outlawed in physics?

are complex coordinates completely outlawed in physics? Complex coordinates are “outlawed” specifically in general relativity, which is the relevant physics for your question. The mathematical ...
• 105k

### Massless Kerr black hole

This is presumably a flat spacetime described in funny coordinates. You can check this by calculating the Riemann tensor to see if it's zero. If I was going to do this, I would code it in the open-...
• 161

### Why does Roy Kerr claim that the Kerr black hole does not contain a singularity?

The central point of Kerr's paper is showing examples of inextendible null geodesics with finite affine length that do not end in a singularity. To construct such an example he looks at null geodesics ...
• 12.6k
Accepted

### How does the Penrose diagram for a spinning black hole differ in realistic scenarios (formed by stellar collapse)?

Your question basically boils down to a recognition of the following fact: The Schwarzschild metric, with spacelike $r=0$, admits an "eternal" BH to form by stellar collapse, like the one you've ...
• 1,456

### Why does Roy Kerr claim that the Kerr black hole does not contain a singularity?

There are some answers here that do a good job of exhibiting the curves that Kerr has pointed out, but I don't think any have outlined his core argument in a faithful manner, and most have missed key ...
• 1,300

### Does the twist of spacetime by a black hole increase over time?

The spacetime around a rotating black hole (Kerr metric) is stationary, this means that you can choose a coordinate system where the metric doesn't depend on the time coordinate. In layman's terms ...
• 2,919