# 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 ...
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 ...
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 ...
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'...

### 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-...

### 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 ...
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 ...
Accepted

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

Yes, you are taking the metaphor too literally. When we visualize spacetime like a flexible rubber sheet there is an accidental and erroneous impression that a twist will lead to spacetime getting ...
Accepted

### Are rotating black holes producing a frame dragging effect inside the event horizon?

Krešimir Bradvica asked: "Are rotating black holes producing a frame dragging effect inside the event horizon? Is that effect moving space inside the event horizon at speeds far greater than the ...

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

Moments of inertia are defined about a given axis of rotation. Moment of inertia is the name given to rotational inertia, the rotational analog of mass for linear motion. It appears in the ...