What happens when a Black Hole slows down?

I’ve often read/heard the description of a Black Hole as

The remaining bulk, still several times more massive than the Sun, collapses into a single point of infinite density, called a singularity

Often with some variation of “from which nothing can escape, even light”.

I was reading a reader comment in New Scientist asking “What about gravitons”, which made me think.

Presumably an object being pulled towards a black hole exerts drag on the black hole: the moon slows the earth by about 1.5ms a century (ref).

Black Holes are actually categorized as rotating and non-rotating, and the two kinds have very different properties. But what happens when one slows down and stops rotating? And can a large enough mass interacting with a non-rotating, can the drag cause it to being rotating?


I don’t think there is any occasion where one body of mass fails to exhibit it’s influence on another body of mass, regardless of distance. 13 billion light years or not, all matter counts (dark and otherwise).

The question is whether a black hole can exchange angular momentum with the outside. Black holes don’t have “rotation” per se, a point can’t rotate, but they do have angular momentum.

The moon slows down the rotation of the earth through tides, the fact that the earth is continuously being squished a little due to the different gravity on the sides nearest and farthest from the moon. That can’t happen with a black hole, because it has no extent.

Nevertheless, it does have an effect on the surroundings. The last stable orbit is different for a rotating black hole compared to a classic one, there’s an effect called “frame dragging” whereby a black hole can transfer angular momentum to its surroundings, and thereby slow down. I don’t really know how it works, but I think it has something to do with twisting space-time around the black hole such that the geodesics aren’t radial, which in effect amounts to a torque on the object.

Note that this is entirely different from simply exerting a gravitational force on a black hole. If you have a black hole and another object they will both feel a force of gravity and accelerate towards each other. The tidal angular momentum exchange only happens when the bodies are rotating relative to each other. Eventually, the earth’s rotation will be locked with the moon’s orbit, just like the moon’s is. At that point there will be no more angular momentum exchange and the system is stable forever.

only objects that fall *within* the black hole’s event horizon are cut-off from the outside world.

Stephen Hawking brought up the thought that if you threw a box of “entropy” into a black hole, that would violate physics by reducing entropy in ‘our’ universe, so he wondered, what would happen that avoids that violation?

Per Hawking, matching virtual particles, a part of of quantum mechanics (general relativity is not a part of quantum mechanics) are formed constantly, and those formed precisely at the boundary of the event horizon cause one of the matching virtual particles to go off into ‘our’ universe, and the other to go inside the event horizon. This is a hypothesis whereby the black hole ‘radiates’ virtual particles, reducing its size, and avoiding the loss of entropy. (I think I said that right).

Finally, the ‘singularity’ is called that because of the use of ‘points’. String theory, using strings instead of points, reduces the infinity of the ‘singularity’ to less than infinity, Infinities usually being a sign that something in the physic’s theory is not reflecting reality properly (mathematically speaking).

This used to be an interesting blog. Can’t we get back to the cat bathing and the pink ipod ?

You brainac types are to deep for me.

Do not look past the ability of pink nail polish and it’s attractiveness on the male toe. This hue can definately make a black hole spin up.

Hawkins never said b-h’s (black holes) are singular nor a regular scientist would do. Singular was only the beginning … in the Big-Bang-Theorie. String-Theorie here, M-Theorie there … masses just follow the rules of Keppler, in other cases we won’t know anything about them. The difference between rotating and non-rotating b-h’s is only the distance masses could get and been detected. Nearby rotating b-h’s the event horizon (Schwarzschildradius) is 5-times smaller than nearby non-rotating ones, cause spacetime got an extra twist due the rotation impulse. This results in different energy conversion efficiency, instead 8% of masses get transfered in energie (non-rotating), here 42% (rotating) get transfered (relativistic iron spectral lines)

so long slpr

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