Black Hole's Spin Revealed for 1st Time

Hi,

About the spin of the singularity ... I thought that was a kind of 0/0 situation. The singularity is badly behaved mathematically. I didn't think there is consensus by physicists as to the actual details of the singularity.

But "point structures" can have spin. Isn't an electron considered to be a point, but still has a spin?

I found some papers a few weeks ago that showed tables of spin measurements, so this doesn't seem to be the absolute first measurement.

Thx!

TomB

tomBitonti said:

About the spin of the singularity ... I thought that was a kind of 0/0 situation. The singularity is badly behaved mathematically. I didn't think there is consensus by physicists as to the actual details of the singularity.

Click to expand...

Yes, well, it has to be the thing that is spinning (or, more properly, the thing that has angular momentum), because there is nothing else there to have it.

But "point structures" can have spin. Isn't an electron considered to be a point, but still has a spin?

Click to expand...

An electron has a quantum spin number, which is a touch different, but you're in the right ballpark of ideas.

Umbran said:

The singularity, by relativistic definition, has no volume, correct. And the Event horizon is not made of matter, or anything like that. However, there is still spin.

There are a few things in this Universe that cannot be created or destroyed, and may only be transformed. Energy is one of them. Momentum is another. When we say that something is spinning, that means it has some rotational kinetic energy, some angular momentum. When a star or other mass collapses into a black hole, that energy and momentum have to go *somewhere*. Either the mass has to shed it into the universe before it collapses, or it keeps it after collapse. If it keeps it, well, then whatever the volume of the singularity, it sill has to be spinning, by virtue of the fact that stopping isn't an option.

Click to expand...

Umbran's right (and in ahead of me, as usual), but I'll add a bit more below.

tomBitonti said:

About the spin of the singularity ... I thought that was a kind of 0/0 situation. The singularity is badly behaved mathematically. I didn't think there is consensus by physicists as to the actual details of the singularity.

Click to expand...

Well, the hope (of most people anyway) is that the ultimate theory of gravity will smooth out the "singularity" into something nonsingular. The presence of a singularity is really telling us that we shouldn't trust GR near that location. But, you're right, there's no consensus about what the correct theory is (there are a number of good candidates, depending on how you count them).

But "point structures" can have spin. Isn't an electron considered to be a point, but still has a spin?

Click to expand...

That's true, but that's a quantum mechanical property of the electron. These black holes are classical objects, so you'd think it would need
size to carry angular momentum.

The catch is that it's not actually the singularity that carries the angular momentum, at least not in the GR way of looking at things. One issue is that, once you're working in gravity, defining angular momentum (or even energy) is a bit more subtle than what we're used to. The upshot, though, is that one way (maybe the best one) to think about things is that the spacetime itself outside the horizon carries the angular momentum. In fact, if you really get down to it, you can figure out the energy or angular momentum only by looking at spacetime an infinite distance away from the black hole. Anyway, one of the points is that measurements are a bit tricky in gravitational situations.

I found some papers a few weeks ago that showed tables of spin measurements, so this doesn't seem to be the absolute first measurement.

Thx!

TomB[/QUOTE]