State of Pulsar Theory & Observations

  • Thread starter Saul
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  • #26
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Usually it works best if you become a hyper-expert on something. If you are interested in hyper-velocity stars then spend the next five years of your life learning everything there is to know about them. If you are interested in fountain of youth stars, you can spend the next five years of your life learning everything about those. If you don't specialize in some topic, you aren't going to end up with enough deep knowledge to do anything useful with either of those topics.

There are just too many mysteries in the universe for any one person to comprehend. So I think it's useless to try to understand everything. Just know enough some that you can understand anything.
 
  • #27
Jonathan Scott
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Here is a review paper on black hole magnetospheres...

http://arxiv.org/pdf/0909.2580

If you go www.arxiv.org and type in "black hole" and "magnetic field" you get 842 hits.

What it boils down to is that Schild seems to think that the fact that AGN and quasars have very strong magnetic fields some how disproves the idea that they are powered by black holes. What I don't understand is why Schild seems to think that black holes can't have strong intrinsic magnetic fields when no one else seems to have any problem with the idea.

Before we go any further, I'd really like to understand why he seems to think that. This isn't a rhetorical statement. It's really like to understand what he is asserting.
I was taught (in around 1990) that it is a standard result that classical black holes themselves cannot have a significant intrinsic magnetic field because of the "no hair" theorem. This theorem means that a black hole can only have one sign of charge, so the magnetic effects can be calculated directly from the charge and the rotation. In addition, the overall charge of a black hole is limited by the fact that beyond a certain point an overall charge would cause further particles of the same charge to be repelled more strongly than the gravitational attraction. Together, these factors limit the maximum intrinsic magnetic field of a black hole to many orders of magnitude smaller than the typical field of a neutron star.

As far as I know, to get round this limitation, it was proposed that the strong magnetic fields around black holes arise from ionised material in the accretion disk. Given such a field, frame-dragging near the central black hole could intensify the field and account for jet production.

I have not fully understood exactly what Schild is saying, but I think from previous discussions that what is being suggested is that the overall magnetic moment of the system may have the opposite direction to the magnetic field between the central object and the accretion disk. This suggests that the central object itself is the source of a strong intrinsic magnetic field.
 
  • #28
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Together, these factors limit the maximum intrinsic magnetic field of a black hole to many orders of magnitude smaller than the typical field of a neutron star.
Part of the problem here is that this discussion tends to move in and out of my field of expertise. There are limits to the field that you can get as a result of internal behavior of the central object, which but you can get quite high magnetic fields from black holes.

The reason I bring up Kip Throne is that he has a mechanism that allows for the "intrinsic" magnetic field of the black hole to be quite high. What happens is that as material falls into the black hole, GR causes the infalling material to appear to freeze and the magnetic field lines to remain frozen. Technically speaking the magnetic field doesn't come from the black hole, but rather the material that is about to fall into the black hole which appears "frozen".

If you have a black hole that is sitting by itself in space, and a neutron star that is sitting by itself in space, then the argument holds, and the black hole will have a much lower magnetic field.

Once you have charged infalling material then all bets are off. The magnetic lines of force will attach themselves to the black hole and then all sorts of weird things can happen.

The papers by Schild appear to describe this sort of mechanism. Also from what I gather from his papers, his objections aren't an issue with field strength but with field location.

I have not fully understood exactly what Schild is saying, but I think from previous discussions that what is being suggested is that the overall magnetic moment of the system may have the opposite direction to the magnetic field between the central object and the accretion disk. This suggests that the central object itself is the source of a strong intrinsic magnetic field.
Which based on what I can gather from Schild's arguments, does not rule out a black hole. As far as I can tell, the equations he uses for what he calls a MECO are exactly the equations you would use to describe a black hole in the reference frame of a distant observers.

Again, this is all blind men feeling the elephant. I don't have particular strong expertise in accretion disk theory, and there may be something utterly stupid in something I said, but when I read Schild's papers, the impression that I'm getting is that he is describing a black hole.
 
  • #29
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Also last I heard we don't understand anything about jets.

The thing about black holes is that the physics is pretty clean. There isn't much place for some weird thing to hide. Once you introduce magnetic fields and plasma, then pretty much anything can happen.
 
  • #30
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I found this paper by Schild at

http://journalofcosmology.com/SchildLeiter1.pdf

He mentions the membrane paradigm, but he pretty clearly doesn't understand it. The membrane paradigm has nothing to do with Hawking radiation since Hawking radiation is pretty irrelevant to anything we are talking about. His grasp of black hole accretion physics seems pretty poor.

The other impression that I get is that the peer reviewers at Ap.J. make Schild and company look less nutty they they are. If you take his Ap.J. papers and mentally cross out MECO and write "black hole", you end up with papers that are actually quite interesting and useful.

The only reference he has is to the Scientific American article and he doesn't mention Thorne's book where Thorne has an entire chapter on how to apply the membrane paradigm to accretion problems.

Also the Journal of Cosmology seems to be a fun read. I might decide to post some of my nuttier ideas there.
 
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  • #31
Jonathan Scott
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The reason I bring up Kip Throne is that he has a mechanism that allows for the "intrinsic" magnetic field of the black hole to be quite high. What happens is that as material falls into the black hole, GR causes the infalling material to appear to freeze and the magnetic field lines to remain frozen. Technically speaking the magnetic field doesn't come from the black hole, but rather the material that is about to fall into the black hole which appears "frozen".
OK, this was new to me (as is your spelling of Kip Thorne).

I was aware that quite some time ago (1970s?) Kip Thorne published a paper which effectively pointed out that during gravitational collapse, magnetic flux would create resistance to collapse, and I think this idea was later used (possibly over-used) as part of the MECO concept. I've also seen references to a later paper about the "membrane paradigm". I guess I might need to look into those further.

(I am still personally unconvinced that Hilbert's revised choice of integration constant for the exterior Schwarzschild solution is better than Schwarzschild's original, so as far as I'm concerned it could still be that black holes don't exist anyway, without any need for mind-blowing math).
 
  • #32
Jonathan Scott
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The other impression that I get is that the peer reviewers at Ap.J. make Schild and company look less nutty they they are. If you take his Ap.J. papers and mentally cross out MECO and write "black hole", you end up with papers that are actually quite interesting and useful.
As I recently mentioned in another thread, I consider calling it a "MECO" unhelpful, as that is a specific idea which is not supported by the evidence. What the evidence suggests is that the central object has a strong intrinsic magnetic field. This is interesting, as it is well-known that a classical black hole cannot have such a thing. It means we have a new piece of physical evidence to incorporate into our theories.

I think it DOES provide evidence that what is in the middle is not just a classical black hole, which would probably otherwise have been the expected model. I don't know whether it means there's some "fossilised" magnetic field from infalling material or whether there's some reason why black holes don't happen.
 
  • #33
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For the book on the membrane paradigm see....

https://www.amazon.com/dp/0300037708/?tag=pfamazon01-20&tag=pfamazon01-20

This is interesting, as it is well-known that a classical black hole cannot have such a thing.
If you have a black hole in the middle of nowhere, those arguments apply. Once you put a black hole in a plasma or magnetic fields, then you get weird and complex interactions between several different types of physics and the basic argument that you use to limit the field strengths of black holes just doesn't work any more.

If Schild was writing papers saying that he has proof that the arguments about black holes don't have magnetic fields don't apply in AGN's, and that under some conditions black holes can indeed have strong magnetic fields, this isn't something people would find difficult to accept. The is because classical argument against black holes having strong magnetic fields is extremely, extremely brittle.

I think it DOES provide evidence that what is in the middle is not just a classical black hole, which would probably otherwise have been the expected model.
Personally I think it's evidence that Schild is out of touch with people that do the physics of black hole accretion.

I don't know whether it means there's some "fossilised" magnetic field from infalling material or whether there's some reason why black holes don't happen.
At this point you go with "principle of least astonishment". We basically have no clue how magnetic fields work in compact objects and AGN, so if you come up with some mechanism to attach a magnetic field to a central black hole, they people will shrug, and nod their heads. If you start arguing that what is in the AGN is not a black hole, then you basically have to rewrite the laws of gravity. The big problem is that Schild and Leitner are claiming they that aren't rewriting the laws of GR.
 
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  • #34
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Here is a review paper on BH magnetism.

http://arxiv.org/abs/0709.3895

Note figure 5 where you have a magnetic field coming straight from the black hole. There are also about another several dozen papers that have similar results.
 
  • #35
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I see three separate issues.

Massive Object has an intrinsic magnetic field
Schild and other authors involved in this research have completed a very detailed quasar analysis (explanation of radio loud/radio quiet states, gravitational lens observations of the quasar core that shows the inner region is swept clean, reverberation analysis to determine component sizes and locations, the discovery and explanation of the quasar's Elvis structures) all of which supports the assertion that a strong magnetic field is attached to the massive object. Their observational analysis is published. They are specialists in the observation and analysis of quasars.

The evidence presented to support the assertion that the massive object has an intrinsic magnetic field is multifaceted. There appears to be no published papers refuting the evidence and no physical reason to doubt the assertion. It is suggested that we accept their conclusion as an observational fact.

Physics of Massive Objects
There is a second issue as to what is happening in the massive object at a subatomic level to create the massive magnetic field. Typically the first step is more observation evidence to guide and bound the model and mathematics. The generation of strong magnetic field in other astronomical objects might be one place to look for observation data to discuss. I am having a look and will see if I can get something to start a thread with.

http://arxiv.org/abs/astro-ph/0307133

Origin and Evolution of Neutron Star Magnetic Fields

An up-to-date overview of the rich phenomenology (encompassing “classical” and millisecond radio pulsars, Xray binaries, “magnetars”, and “thermal emitters”) suggests that magnetic fields on neutron stars span at least the range 10^8−15 G, corresponding to a range of magnetic fluxes similar to that found in white dwarfs and upper main sequence
stars.

Research on magnetic fields in neutron stars is undoubtedly in one of its most interesting moments. Little is known about the strength, structure, origin, and evolution of the field, but there seems to be little doubt that it plays a fundamental role in determining the increasingly rich phenomenology of these objects.


History and Assumptions in Past Classical Analysis
There is a third issue concerning the assumptions made in a series of classical papers concerning the collapse of massive objects. Mitra argues that if the assumptions are changed to be physically real it is not possible to create a classical BH.

I think that everyone assumes because the mathematics is intimidating that the analysis assumptions are not open to criticism and do fundamentally affect the conclusions. The base assumptions should be stated and approved. Mitra has specific criticisms which seem fundamental. It would be interesting to discuss and clarify Mitra's criticisms and then to see if a GR specialist would respond.

http://arxiv.org/abs/astro-ph/0608178

Sources of Stellar Energy, Einstein- Eddington Timescale of Gravitational Contraction and
Eternally Collapsing Objects
 
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  • #36
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There is a second issue as to what is happening in the massive object at a subatomic level to create the massive magnetic field. Typically the first step is more observation evidence to guide and bound the model and mathematics. The generation of strong magnetic field in other astronomical objects might be one place to look for observation data to discuss. I am having a look and will see if I can get something to start a thread with.
Yes, and you'll find it a very rich and complex field with a huge amount of open questions. The other thing is that the same process that causes magnetic fields in neutron stars and black holes are likely to be more or less the same thing that causes the earth's magnetic field. One thing that is curious is that we don't have a complete understanding of what causes the Earth's magnetic field, and a lot of the models which we have for magnetic fields in astrophysical objects are "toy models" which may be very, very wrong.

One thing that does come out of the research is that the processes that are involved in creating neutron star magnetic fields are very different from what causes black hole magnetic fields. Basically neutron star magnetic fields could come from movements of materials within the neutron star, whereas black hole magnetic fields come from things that are happened at the "surface" of the black hole.

Also, it's generally believed that astrophysical magnetic fields don't come from subatomic processes, but from the motion of conducting fluids. One piece of evidence for this is that most objects (including the Earth) have temperatures that are above the Curie point which means that the magnetic field doesn't actually come from the atoms themselves. Also you can show that if the Earth's magnetic field did come from the material in the earth, that it would have disappeared a long time ago.

This is something that people spend their entire careers studying. It's really cool stuff.

I think that everyone assumes because the mathematics is intimidating that the analysis assumptions are not open to criticism and do fundamentally affect the conclusions.
I don't find the math in Mitra to be intimidating. The problem is that I look at the math and start foaming at the mouth and start screaming "this is utter rubbish" people who aren't familiar with the math start looking at me like I'm acting irrationally. Basically, I'm looking at his arguments, and to me it's as if you were to see someone say "2+2=5!!!!!!" and be proud of discovering "2+2=5!!!!!"

The base assumptions should be stated and approved. Mitra has specific criticisms which seem fundamental. It would be interesting to discuss and clarify Mitra's criticisms and then to see if a GR specialist would respond.
What Mitra is saying is that if you take the equations for a collapsing star and then put them into the equations for GR, then it takes an infinite time for the star to collapse. The problem is that he is using the wrong equations.

Go to equation 28 in that paper. If you are right outside the event horizon the the first part of the equation goes to zero. So if you use this as your coordinate system then the variable that you are calling "time" stops at the event horizon, and if you ask "how much time" does it take for anything to happen, the answer is the answer that Mitra gets, which is infinity. If you use equation 28 for your coordinate system then when you get close to the black hole, everything freezes, and it freezes precisely because you have chosen a coordinate system in which "time" stops at the surface of the black hole.

The problem is that you have selected the coordinate system so that this happens. It's like flying over the north pole in spherical coordinates, if you ask how fast your are moving in angular coordinates as you fly over the north pole, you get nonsense answers But even thought polar coordinates gives you silly and absurd answers of you try to use them over the north pole, it's a convenient way of locating points on a sphere.

It's the same principle here.

Now the reason I notice this is that when I write my simulations of supernova collapse, I use something like Mitra's equation 28 in my simulations. The reason I do this is that I want the simulation to be well behaved so that I intentionally choose a definition of "t" and "x" so that "t" slows down at the surface of the forming black hole. I do this because I'm not really interested in what happens inside the black hole (since I can't observe any of that), so I have the computer run the inside of the simulation at a slower and slower speed so that the inside of the simulation never actually becomes a black hole. But I intentionally define time and space this way so that I get this result. It gives me the right answers for things outside the black hole (which I care about), but it gives me misleading answers for anything near or inside the black hole (which I don't care about).

One other way of thinking about it is "bullet time". In the movie the Matrix, the film makers speed up or slow down parts of the movie for dramatic effect. Same sort of principle here. When I'm running supernova simulations, I try to have things where the "action is" happen at normal speeds where as I trying to "slow down" parts of the simulation that I'm not interested in. Since relativity says that I have some flexibility in coordinate systems, I choose a coordinate system in which this behavior happens, that what that is is equation 28.

If you look at equation 28, you'll see that the number in front of "du" goes to zero as you cross the Schwarzchild radius. What this means is that you've *intentionally* set up the coordinate system so that you never see something crossing the event horizon.

Also, I'm not a GR specialist. I know just enough GR to run computer simulations, but what I've just said I think is a pretty standard consensus view.
 

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