Fast-spinning pulsar seen stealing - newscientistvideo

[Salv]

So I've just seen this video and I had a couple of somewhat interesting questions.

This pulsar (not the LoR Pulsar ) can be seen rotating 592 times a second. If we had to get closer to the pulsar we should see it spinning faster because of the huge gravitational forces in operation?

Got me thinking about black holes. It wouldn't make sense that a black hole is stationary. I reckon it's probably spinning around at crazy speeds. The only thing is that we couldn't possibly see this happening, and if we could see this happening, space-time is so warped beyond the event horizon that it would appear stationary to an observer anywhere outside the event horizon?


Edit: Youtube info

Read more at - http://www.newscientist.com/article/dn17175 A neutron star with a cosmic case of indigestion could help to explain why some of these ultra-dense stellar embers spin much more quickly than others.

 

[CupOfWater]

We don't know if black holes are spinning, since we can't get any information from it, except what we can indirectly observe. I'm not exactly sure, but I don't think it's possible to indirectly observe spin...

 

[Pulsar]

Meh, I can spin faster than that . Pulsars are cool...

About black holes, yes most of them (if not all) probably rotate, due to conservation of angular momentum when they form. And it is possible to see this spin indirectly, because rotating bodies distort space-time differently than stationary objects. A rotating black hole can be described by a Kerr metric. One of its consequences is a relativistic effect known as frame-dragging, and this disruption of space-time has an effect on the surrounding material. For instance, the jets emerging from active galaxies are thought to be caused by a combination of frame-dragging and electromagnetic forces on the ionized material orbiting a central supermassive black hole.

It's even becoming possible to calculate the spin of black holes. Due to the rotation of a black hole, close-by orbits are unstable, so that material gets consumed by the black hole. So the immediate area surrounding the event horizion of a rotating black hole should be almost void, and the closest material is orbiting at the radius of the "last-stable-orbit". If you know the black hole mass, and the size of this radius, you can determine its rotation. For more details, see here and here.

 

[Salv]

Oh wow, that's really interesting stuff. I read something similar to this a while back, about gravity waves. Two massive stars rotating around each other what I think was almost like concentrated spirals/arms of gravity extending outwards.

If we can measure amount of warping there is just outside the black hole, shouldn't we also be able to measure the density of the black hole? Wouldn't the warping be somewhat exponentially consistent? We could probably measure how much space time is actually being warped.

 

[Pulsar]

Yes, gravity waves are generated when two massive objects rotate around each other. There's a delay effect, so that the gravitational pull on one object doesn't point exactly towards the other, and vice versa. As a result, the system loses energy, which is carried off as gravity waves. Most likely this is what's happening to the Hulse-Taylor binary.

Black holes don't really have a density, their total mass generates an event horizon. This mass can be determined by measuring the rotational velocity of the surrounding material (gas or stars), or in some cases by gravitational microlensing effects.