Will a SMBH at the center of a rotating galaxy commonly be rotating in the same plane as that galaxy? I’m wondering if a SMBH that starts ‘feeding’ would shoot beams ‘up’ and ‘down’ in relation to that galaxy, or just any random direction that the galactic black hole’s poles are facing.
Actually, I may have phrase my question poorly. Will the magnetic poles (and potential energy beam emitters) of an SMBH tend face toward galactic north and south in a galaxy? Or just any old direction?
If the axes of rotation were different, gravitational/tidal dragging would eventually haul them into closer alignment.
(If the moon – or a moon-massed body – were set in a polar orbit around the earth, it would eventually be dragged into an orbit closer to the plane of the ecliptic by gravitational/tidal perturbation. Obviously, it doesn’t have to be exactly in the same plane – the moon isn’t! – but closer.)
Do black holes rotate? And I thought the energy emitted came from the destruction of matter falling into the black hole? That nothing was emitted from the black hole itself except Hawkins radiation?
Black holes can rotate (rotation is one of the very few properties they can have), and in practice, it’s believed that almost all of them do, at a very significant rate.
And you’re right that nothing can escape the hole itself except for Hawking radiation, but you can still get prodigious amounts of energy released from the close vicinity of the black hole, from matter falling in.
Did not know about the rotation - thanks!
But if the energy comes from stuff falling in, is it tied in any way to the rotation, so there is a direction from the poles?
Or does it just depend on the direction that the falling-in stuff is coming from?
Momentum is conserved, so as the material that comes to form the black hole condenses and the moment arm gets smaller (i.e. the moment of inertia decreases) the spin rate of the mass increases, hence why all black holes are assumed to have some finite amount of spin. Think of an ice skater pulling in her arms to spin faster and you get the idea. The off-axis torques that would cause gyroscopic motion in a quasi-rigid rotating body are off-loaded to the outer accretion disc in a phenomenon described by the Bardeen-Petterson effect. (A singularity cannot demonstrate gyroscopic behavior for reasons that should be obvious.)
Stranger
Thanks for the responses.
So to be clear - if matter were to fall into a galactic-center SMBH, the likely direction of the resulting high energy particle beams would be toward galactic north/south? At least in the vast majority of galaxies? In some rare galaxies would the beams emit toward the galactic plane (likely incinerating innumerable solar systems)? Or is that really unlikely?
Black holes have no hair. They have angular momentum, net charge, and mass, IIRC.
The beams couldn’t stay in the plane of the host galaxy indefinitely, but you could get things temporarily knocked off-kilter by a collision or near miss with another large black hole, such as might happen during or after a galaxy merger.
EDIT to Napier: And magnetic charge, too. Everyone always forgets about that one, just because our Universe appears to have no free magnetic charges in it.
Any radiation matter emits due to the tidal and EM forces will be visible until it crosses the event horizon (or detectable, at least, if it’s too far redshifted by the event horizon, or just intrinsically highly energetic, from UV through gamma), where the curvature of space all converges to the singularity beyond that point.
Hawking radiation is different, in that a black hole is an almost perfect blackbody object. So, due to Thermodyn law #2, it must radiate away its energy due to entropy; in this case, thermally, but we suppose there’s somekind of particle/anti-particle trade-off that’s going on at the quantum level, right on the event horizon to explain this “evaporation.”
Hawkins radiation is totally different. Today we just call it jazz.
Oh yes. I must confess to prejudice against free magnetic charge. Great way to make black holes seem weird and unfamiliar, that.
This was always my understanding as well. But, hasn’t it been recently decided that information is not destroyed when consumed by a black hole? I’m pretty sure I read that even Hawking has conceded this. Doesn’t this mean that black holes, in effect, do need a shave and a haircut?
What it means is that there’s a big gap in our knowledge, somewhere. Where exactly that gap is, we’re not sure, but the No-Hair Theorem is a reasonable guess. The problem is that a theorem is only as good as its axioms, and the No-Hair Theorem only applies to classical black holes, that is, black holes in a world without quantum mechanics. For most purposes, this is a reasonable approximation to our world, but our world does in fact have quantum mechanics, so it’s possible that there might be a subtle but real violation of the No-Hair Theorem.
Are you referring to 't Hooft/Susskind’s holographic principle?
The fact is we also have relativity too. My love for black holes is because of the event horizon, and the demonstrable and almost tangible incompleteness that exposes the gaps in modern physics (and offers clues that QM/GR are really two sides of one thing… Somehow…).
Rotating black holes may also lead to a ring singularity.
Although, this isn’t a hypothesis I seem to hear talked about much any more, so perhaps the maths don’t stack up?
Ring singularities aren’t talked about much for the simple reason that they’re uncontroversial. If the shape of space smoothly follows the same pattern inside the horizon as it does outside, then rotating black holes definitely have a ring-shaped singularity. Now, it’s possible, for all we know, that the shape of space doesn’t follow the same pattern, since we can’t observe anything inside the horizon to check. And in fact, it’s widely suspected that there aren’t even any singularities at all: That before you get to that point, there’s some quantum gravitational effect that takes over and prevents a singularity from forming. But that’s pure speculation, so the current best guess is the simplest one.