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trakker3
2006-Dec-05, 04:29 PM
If nothing can escape the gravity of black holes, then what are the jets of material that stream from opposite ends of the (supper massive) black holes and why do we not see the material falling back into the black hole?

puzzled amateur

antoniseb
2006-Dec-05, 04:38 PM
If nothing can escape the gravity of black holes, then what are the jets of material that stream from opposite ends of the (supper massive) black holes and why do we not see the material falling back into the black hole?
The jets do not come from inside the black hole, but rather from the complex activities of things falling in (but which are still outside). The magnetic fields in that kind of environment can do some amazing things.

Fazor
2006-Dec-05, 05:02 PM
I'm sure this post will get followed by corrections or additions of the many members on here that are proficient in this field, but as I understand it, Stephen Hawking theorises a measurable ammount of radiation is thought to escape black holes (Hawking Radiation) (http://en.wikipedia.org/wiki/Hawking_radiation). Check out that wikipedia article for more info. Hope that helps

antoniseb
2006-Dec-05, 05:09 PM
Hawking radiation is not what makes the Jets.

Fazor
2006-Dec-05, 05:15 PM
Just pointing out that it's not, theoretically, true that "nothing can escape the gravity of a black hole".

Kaptain K
2006-Dec-06, 12:39 AM
The first thing you must remember is that although it dominates our everyday world, gravity is, by far the weakest of the four forces (strong, weak, electromagnetic and gravity). Like the gravitational field, the magnetic field of the black hole extends out into space. The rapidly rotating field is able to sweep up ions (mostly protons and electrons) from the accretion disk and fling them out at a large fraction of the speed of light.

Squashed
2006-Dec-06, 09:32 PM
The first thing you must remember is that although it dominates our everyday world, gravity is, by far the weakest of the four forces (strong, weak, electromagnetic and gravity). Like the gravitational field, the magnetic field of the black hole extends out into space. The rapidly rotating field is able to sweep up ions (mostly protons and electrons) from the accretion disk and fling them out at a large fraction of the speed of light.

Don't photons mediate the forces of magnetic fields ... or in other words it takes photons for a magnetic field to be created?

If this is true then how do the photons escape the blackhole to create a magnetic field?

Fazor
2006-Dec-06, 09:43 PM
as i understand it, the photons are coming from outside the black hole, not from within. altho i don't really undstand it. hm...hope that smoke i smell isn't my brain frying again.

publius
2006-Dec-06, 10:09 PM
Don't photons mediate the forces of magnetic fields ... or in other words it takes photons for a magnetic field to be created?

If this is true then how do the photons escape the blackhole to create a magnetic field?

Squashed,

The answers to how EM behaves in strongly curved space-time are not simple. And that's the understatement of the year. There are very people who understand and can do the calculations. All I can do is try to paint a picture of how terribly complex the equations are. :) Due to some discussions in various interesting threads here, I got a copy of Landau & Lifsh-itz's "Classical Theory of Fields" which is sort of a bible of classical field theory (Jackson is the bible for pure classical electrodynamics -- L&L add GR to it). L&L show how the metric modifies Maxwell.

There are several ways to do this. One is to take a background metric defined by a familiar mass-energy distribution, that is a regular gravitational field solution, and see what Maxwell looks like there. This does not take into account the affect of the EM energy and momentum on its own background. That couples Maxwell with the EFE, and makes the whole thing about the most complex, non-linear mess you ever saw. And there are other simplifications and approximations that are valid in various regimes. But the full, general thing is a complex mess indeed.

What I'm about to say is my rough understanding of this which at best will be incomplete, and at worst may be flat out wrong in some details. I'm not confident of anything when it comes to Maxwell and full GR together.

A black hole cannot have any intrinsic magnetic field due to "neutral currents" as we think of them. Consider your typical current loop making a magnetic field. What do you have? You have no net charge separation but charges of one sign are moving with repsect to the others and so you have an external B field with no E field (in that frame where we specified the motion :) ).

So imagine a battery powering a little current loop like that making a magnetic dipole. Now drop that sucker into a Schwarzschild black hole. What happens? That current we see *slows down* and comes to dead stop at the event horizon. The current and the B field cease. Now, that's a time varying B field, which couples to an E, and that makes radiation from our perspective! There are various rules (some are general, some specific) about "conservation of magnetic flux" and stuff like that I forget. This radiation we see sastifies that requirement. As the current loop falls, we see it "throw it's B loops off" as radiation.

You'll note that neutron stars can have honking huge magnetic fields by this conservation of flux rule when they collapse. But if they collapse down to a black hole, all that B will be thrown off as radiation. That occurs so rapidly that the radiation is astronomical. Those gamma rays bursts may be related, but this getting outside my knowledge.

That's what happens when we drop current into a black hole. But what if we drop net charge? We see that net charge freeze at the event horizon. It never crosses in finite time, so we always see that charge *and its electric field* sitting there. So lines of E can come out of a black hole. If magnetic monopoles existed, lines of B could as well, but as it is, no B lines can come out of a Schwarzschild black hole.

But now, what about a rotating black hole. Here is where my understanding is even less tha the above. But suppose a rotating mass with a net charge collapses into a Kerr black hole.

A *stationary* observer will see a magnetic field. One of the "poetic ways" to describe this is the frame dragging is "rotating the E field lines" around and so we see a B field. But if we go with the flow of space-time, follow our geodesics (into oblivion), we won't see a B field, just an E field.

Another related way to look at this is "non neutral currents". Imagine a net charge, and imagine it's moving. Both E and B. Now, take rotating sphere of charge. You get a B field there too, but that's different than a sphere within neutral currents. If that collapses, I think some of that B will be "thrown off" like above, but again this gets way beyond my understanding.

But at any rate, for a black hole to have a magnetic field, by itself, it must be rotating, and it must have a net charge. There is a limit to the angular momentum to mass ratio, which constrains the B you can get from a given net charge. The only way to get more B is more charge, and in that case the electric field would have to get very large.

Now, that's your classic black hole picture. In Mitra's ECO picture, an ECO can be a MECO, and have a much larger B field that a charged Kerr hole can.

-Richard

publius
2006-Dec-06, 10:25 PM
Well, reading what I just wrote, I see something wrong right off. I said we wouldn't see a B following our geodesics. Well, we would in general, cause we'd be moving radially and see a moving E field. If we transformed away the frame dragging locally, in such a way we were still radially stationary, we wouldn't see a B field, but if we did that, our notion of radius would be different from the full stationary observer in the Kerr metric anyway.

But basically, Squashed, Black holes can have no "intrinsic" B field in the sense of one being produced by internal currents, like stars and neutron stars and everything else.

It's a complicated mess, and there are very few who can do the calculations, but basically all the EM fields we see outside, are consistent with what we see of the infalling charge sources. They freeze at the horizon, and we see charge sitting there, and frame dragging can rotate the electric field and make magnetic field. But that field has limits.

-Richard

Squashed
2006-Dec-07, 06:29 PM
Well, reading what I just wrote, I see something wrong right off. I said we wouldn't see a B following our geodesics. Well, we would in general, cause we'd be moving radially and see a moving E field. If we transformed away the frame dragging locally, in such a way we were still radially stationary, we wouldn't see a B field, but if we did that, our notion of radius would be different from the full stationary observer in the Kerr metric anyway.

But basically, Squashed, Black holes can have no "intrinsic" B field in the sense of one being produced by internal currents, like stars and neutron stars and everything else.

It's a complicated mess, and there are very few who can do the calculations, but basically all the EM fields we see outside, are consistent with what we see of the infalling charge sources. They freeze at the horizon, and we see charge sitting there, and frame dragging can rotate the electric field and make magnetic field. But that field has limits.

-Richard

publius,

I must say you are infinitely wiser than me in regards to magnetism and electromagnetics.

I think part of my whole consternation with the blackhole subject is the fact that blackholes are of different masses and yet the infalling matter, from our perspective, never enters the event horizon.

Some of what you write and calculate seems to coroborate my angst and now you state that the electric charge never goes inside the event horizon which causes it to freeze in state and produce a magnetic field (or words to that effect, I'm out of my mind with ignorance here).

So now I see neither mass nor charge entering an event horizon.

Blackholes are fun, ain't they?

Cougar
2006-Dec-07, 06:53 PM
I think part of my whole consternation with the blackhole subject is the fact that blackholes are of different masses and yet the infalling matter, from our perspective, never enters the event horizon.
I just started reading Kip Thorne's book Black Holes and Time Warps: Einstein's Outrageous Legacy. I was pleased to see that he said that an infalling robot definitely entered the event horizon; it's just that the visual signal of that event doesn't reach us due to the extreme gravity and curvature close to the horizon. That may be rather simplified, but I believe it is essentially a correct view.

Squashed
2006-Dec-07, 07:20 PM
I just started reading Kip Thorne's book Black Holes and Time Warps: Einstein's Outrageous Legacy. I was pleased to see that he said that an infalling robot definitely entered the event horizon; it's just that the visual signal of that event doesn't reach us due to the extreme gravity and curvature close to the horizon. That may be rather simplified, but I believe it is essentially a correct view.

If the charge falls inside the event horizon then every blackhole would be absent a magnetic field.

publius
2006-Dec-07, 08:51 PM
I'm not going to get in another argument about whether something "really" crosses the horizon; I'll just state my position, and one can take or leave it. The position of anything free-falling (not the time it takes its light image) into a black hole does not reach the horizon (r = R, in the notation I've used in the various Schwarzschild threads) after any finite time on the clock of any external observer. If we're at some stationary postion r0 > R, and we plot the r(t) trajectory of something we drop, r(t) never gets to R in finite time. vents of the infalling object past the horizon *do not exist* in any external coordinate system. However, switch to the proper rules and clocks of the thing falling in, and he goes past that horizon in finite time.

The trouble here, I now realize is we want "falling into a black hole" to be an *absolute event* that applies to all frames of reference. Something falls in or it doesn't, so to speak. But unfortunately, space-time doesn't work that way, and falling into a black hole is not an absolute event. And it is not just an "illusion" of light travel time. Indeed, if light "can't here from there", then "there" does not exist in our coordinates.

The same thing applies in the Rindler metric. Accelerating away from earth at constant g, we never see the earth "fall" past the Rindler horizon. It doesn't happen in the coordinates of the accelerating observer. But, for us watching on earth, we see certainly cross that horizon (and in this case, a singularity does not await us). When that happens, we'll note that because the accelerating observer's clock is slow*ing* down, not just running slow as with constant velocity, but accelerating and slowing down, his clock will never "catch up", and never be able to record events past that point.

It's the same thing with black holes. Outside, we are the accelerating observer, and our clocks can never register events inside the horizon. It doesn't happen for us.

But, in other frames, it does happen. And that relativity of whether something happens or not is a philosophical conundrum. You want to impose that on all frames. But unfortunately, space-time doesn't allow you to do that. "Simple" inertial SR makes force us to abandon our strict Galiean/Newtonian intuition about absolute time and absolute simultaneity. We can sort of accept that things don't happen at the same time for different observers. But we still hold on, even if we think we don't. Going to non-inertial frames and full GR slaps us in the face even more, as it tells that not only do things happen at different times in different frames, but some things that happen in certain frames do not happen in others. That is hard to accept.

So if you want to say something "really falls in", I won't argue, because that does happen in someone's coordinate system. That definition of 'happen' means it occurs in some possible frame of reference. But if you want to make that absolute and demand it happen in all frames, at some finite time on external clocks for instance, I'm going to argue. :)


-Richard

phunk
2006-Dec-08, 04:42 PM
If the charge falls inside the event horizon then every blackhole would be absent a magnetic field.

Sure but the accretion disk could certainly have a huge magnetic field.

Kaptain K
2006-Dec-09, 11:40 AM
If the charge falls inside the event horizon then every blackhole would be absent a magnetic field.
Only if you are willing to accept that:
If the mass falls inside the event horizon then every blackhole would be absent a gravitational field.
is also true!