Thanks for the article Jorrie.

I've always had trouble understanding what frame-dragging. I can picture space curving due to massive objects (gravity), but I'm unclear why it would twist around spinning massive objects. Why does the spinning effect the space around a massive object at all?

Hi Roger.

The "how" is not too difficult to tell; the "why" is always a problem.

I keep my sanity by thinking as follows: the gravitational field of a linearly moving mass actually 'moves with the mass', just like a magnetic field 'moves with a magnet'. Add a N-S rotation to the magnet and you have a rotating magnetic field. In a more or less analogues way, even a perfectly spherical rotating mass causes a slight rotation of the gravitational field. This is called 'frame-dragging'.

Please don't quote me too widely on this one! It may have some degree of truth, but I won't try and defend it too vigorously.

-J

I'll read that link in detail. I kind of get what you're saying but I'm still confused. I'll think about it. In the mean time I have a followup question:

So if you are in the frame dragged space (not on the spinning mass), what is the effect? Do you see effects as though you are in a rotating reference frame yourself (coriolis effect, centrifugal force, euler force)?

Roger, you asked: "So if you are in the frame dragged space (not on the spinning mass), what is the effect? Do you see effects as though you are in a rotating reference frame yourself (Coriolis effect, centrifugal force, Euler force)?"

I think you get similar effects, but they are fundamentally different. To start with, if we drop an object from a large distance straight to a rotating black hole, it may miss the black hole's event horizon and get a slingshot boost around it, as shown in my Garbage-In-Gigawatts-Out thread. This is similar to the Coriolis effect, but clearly not the same thing.

The equivalent of Euler force may appear, because frame-dragging is more severe the closer you get to the event horizon, almost like a rotating frame that increases in angular speed. Centrifugal force may actually be viewed as decreasing the radial force of gravity (as on object is swung around the hole). As an example, objects can be in stable circular orbit much closer to the equator of a spinning black hole than for a non-spinning one (provided the orbit is with the direction of spin).

-J

Hi Jorrie,

Yes that's what I was thinking. In other words, if you are in the frame dragged space you are probably already being dragged with it, so it's the equivalent of being in a rotational frame of reference.

You've already anticipated my thought process a bit with your statement:

"Centrifugal force may actually be viewed as decreasing the radial force of gravity (as on object is swung around the hole). As an example, objects can be in stable circular orbit much closer to the equator of a spinning black hole than for a non-spinning one (provided the orbit is with the direction of spin)."

Fascinating. Am I correct then in thinking that the tidal forces around a spinning black hole would be less than a non-spinning one?

Hi Roger, you asked: "Am I correct then in thinking that the tidal forces around a spinning black hole would be less than a non-spinning one?"

If the conditions for "less than" are carefully defined, then yes for radial tidal forces, simply because the effective radial force is less. As I wrote to packrat561 above, there can also be rotational tidal forces, which are not present in the non-spinning case.

You Wrote:"As I wrote to packrat561 above, there can also be rotational tidal forces, which are not present in the non-spinning case."

I see, like shearing forces I would imagine. How interesting. Essentially that means an object falling into a spinning black hole will experience a torque. If I'm thinking about this correctly, the torque will be in the opposite direction as the spin of the blackhole. So if a black hole is spinning clockwise, a beam falling towards the blackhole will experience a counterclockwise torque.

Also, it occurs to me that a blackhole will have a spin axis and I would imagine therefore that the frame dragged space shares that same axis, is that correct? If so, the approach to the spinning black hole would matter for these effects, is that true?

Hi Roger.

Yes, I think you have it right. The spin axis of a black hole definitely matters. Conditions near its equator are quite different from near its poles.

Most dramatic are the accretion disks forming around the equator, while jets of plasma are normally ejected from near the poles of most massive, rotating black holes, caused by their "twisted" magnetic fields - not from within the event horizon, but from just above it. In the case of supermassive black holes emitting plasma, they are called 'blazars', because the jets are so luminous.

Even non-rotating holes can have the accretion disks, but AFAIK, only rotating ones can have the jets.

Is a non-rotating black hole possible? I always thought they were a theoretical construct. I would assume, and correct me here if I'm wrong, that all black holes spin to one degree or another. Also I assume all stars, planets, moons etc. spin.

It's interesting to think that all matter falling into a rotating black-hole experiences an increasing torque opposite to the spin of the black hole.

Do you know if the spins of black holes change?

I think black holes of a few solar masses will all spin, but super-massive BHs (SMBHs) may not - depending on how they formed. SMBHs are normally at the center of massive galaxies, which may have formed by mergers of many galaxies and many known ones have no detectable spin.

AFAIK, even our own Galaxy's central black hole has no detectable spin either, but I may be mistaken. Maybe it has just not been detected (or reported)...

Jorrie,

After your response I still don't feel that non-spinning black holes are possible. I did a google search and couldn't find anything definitive on the subject one way or the other, but I did find some interesting articles that gave hints (and were interesting in their own right):

http://www.newsdesk.umd.edu/scitech/release.cfm?ArticleID=1447 (SMBH spinning at 98.7 percent of the relativistic limit. They measured the spin by looking at the shift in the iron spectral line.

This article has nothing to do with our discussion regarding spin and non-spin, but was really cool and I thought something you might be interested in. It details the problems with viewing our local (galaxy center) black hole and a current project trying to overcome these difficulties.

http://spie.org/x35461.xml?ArticleID=x35461

There is a process called the "Penrose Process" that reduces the angular momentum of a black hole. Though my personal feeling is this effect is tiny compared to spin increasing due to accretion.

http://en.wikipedia.org/wiki/Penrose_process

I suspect that all black holes must rotate, if only a little bit, but I can't find anything that says that definitively so I'll let it go.

This was another great post Jorrie, thanks.

Roger

Possibly a tumble is possible.

Hi Jorrie,

Is it analogous to fluid coupling , as in hydraulic torque converter; the fluid and housing in this case representing space,rotating mass exerts gravitational torque on space, bad analogy maybe?

Now I'm wondering, if rotating mass influences space, does this mean that space has mass for the gravity to act on?

Pack Rat

Hi packrat561, yup, in a way frame-dragging is analogous to fluid coupling. There is a transverse component of tidal gravity that tends to rotate an extended object, just like with fluid coupling.

You asked: "Now I'm wondering, if rotating mass influences space, does this mean that space has mass for the gravity to act on?"

I would not say that, but it is as if the gravitational field itself gravitates, i.e., one can say the field has mass. This is one of the reasons why relativistic gravitational accelerations are stronger than Newton's, at least in the 'local frame'. The GM/r² law of gravity still holds, but the effective mass is increased by a factor (1-2M'/r)^-½, where M' = GM/c².

-J