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JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Black Hole At Our Doorstep: Trapezium?
      #5506493 - 11/06/12 08:51 AM

Next time you aim your scope at Orion's nebula, consider the fact that you might be peeking into a black hole hundred times (or more) the mass of our sun:

http://phys.org/news/2012-11-international-massive-black-hole-sword.html

Scientific publication: http://arxiv.org/pdf/1209.2114.pdf



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deSitter
Still in Old School


Reged: 12/09/04

Re: Black Hole At Our Doorstep: Trapezium? new [Re: JKoelman]
      #5506516 - 11/06/12 09:13 AM



-drl


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EJN
Carpal Tunnel
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Reged: 11/01/05

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Re: Black Hole At Our Doorstep: Trapezium? new [Re: JKoelman]
      #5506871 - 11/06/12 01:04 PM

This seems to be based on a whole chain of assumptions, and if one of the assumptions
is wrong then the conclusion collapses like a house of cards.


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Jarad
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Re: Black Hole At Our Doorstep: Trapezium? new [Re: EJN]
      #5507067 - 11/06/12 03:21 PM

Well, I could accept that there could be some large stars hidden in the gas and dust that we can't see that could be providing the mass needed.

But it seems to me that if there were a large black hole embedded in the nebula, it would probably be accreting some of that gas and dust, and we would be seeing a strong X-ray source in there somewhere.

Not so sure about this one.

Jarad

Edited by Jarad (11/06/12 03:22 PM)


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PhilCo126
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Reged: 01/14/05

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Re: Black Hole At Our Doorstep: Trapezium? new [Re: EJN]
      #5511722 - 11/09/12 01:53 PM

Quote:

This seems to be based on a whole chain of assumptions, and if one of the assumptions
is wrong then the conclusion collapses like a house of cards.




Well most theories are based on assumptions which allow for certain parameters to be chosen in computer programs to run simulations (e.g. accretion theory of the solar system)


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Sean Cunneen
Let Me Think
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Reged: 08/01/07

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Re: Black Hole At Our Doorstep: Trapezium? new [Re: PhilCo126]
      #5511871 - 11/09/12 03:35 PM

I have a computer model of birds angrily launching themselves at pigs... Do I win?

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llanitedave
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Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: Black Hole At Our Doorstep: Trapezium? new [Re: Sean Cunneen]
      #5513038 - 11/10/12 11:17 AM

Quote:

I have a computer model of birds angrily launching themselves at pigs... Do I win?




As long as it's testable.

I saw a pack of ravens angrily launching themselves at a couple of coyotes yesterday. It was pretty cool to watch.


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BillFerris
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Re: Black Hole At Our Doorstep: Trapezium? new [Re: JKoelman]
      #5514017 - 11/11/12 06:16 AM

Quote:

Next time you aim your scope at Orion's nebula, consider the fact that you might be peeking into a black hole hundred times (or more) the mass of our sun:

http://phys.org/news/2012-11-international-massive-black-hole-sword.html

Scientific publication: http://arxiv.org/pdf/1209.2114.pdf






The ApJ article is an interesting read and, given both the close proximity and almost constant attention given the Orion Nebula cluster, I wouldn't be surprised if the kind of detailed kinematic study needed to confirm or refute the presence of a 150 solar mass black hole is already under way.

Bill in Flag


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JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: Black Hole At Our Doorstep: Trapezium? new [Re: BillFerris]
      #5514046 - 11/11/12 07:24 AM

Quote:

given both the close proximity and almost constant attention given the Orion Nebula cluster, I wouldn't be surprised if the kind of detailed kinematic study needed to confirm or refute the presence of a 150 solar mass black hole is already under way.




Indeed. No doubt a number of teams across the world are now busy analyzing existing data. We will soon know more.

The laws of physics tell us that stars are transient phenomena, and that black holes are the inevitable (classical) equilibrium configurations. Yet, people less familiar with General Relativity find it difficult to accept black holes. "How can one be so sure black holes exist if they remain per definition hidden from eyesight?"

It would be wonderful if Theta Orionis is confirmed to contain a black hole. I imagine amateur astronomy public outreaches in which the view of the Giant Orion Nebula is accompanied by an explanation of the dance of the trapezium stars around a dark massive object. This would bring 20th century science very close to direct experience.


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deSitter
Still in Old School


Reged: 12/09/04

Re: Black Hole At Our Doorstep: Trapezium? new [Re: JKoelman]
      #5514057 - 11/11/12 07:49 AM

Quote:


The laws of physics tell us that stars are transient phenomena, and that black holes are the inevitable (classical) equilibrium configurations. Yet, people less familiar with General Relativity find it difficult to accept black holes. "How can one be so sure black holes exist if they remain per definition hidden from eyesight?"





This has nothing at all to do with black hole skepticism, which is based on this:

1) GR has not been tested in any way in dense matter conditions. The only tests are the accurate prediction of the orbits of test particles around a massive object (a satellite in earth orbit, a planet around the Sun..), the accurate prediction of gravitational redshift, and the accurate prediction of the rotation curves in spiral galaxies. The first two cases are extreme weak field tests (essentially the one-body problem) while the latter is simply weak field (non-linearity of equations respected). No strong field test of any sort exists. There is little reason to doubt that GR is correct as one enters the strong field regime, but a great deal of reason to doubt that it is correct in the extremely strong field regime. The main point is...

2) It is certain that gravitation and the other interactions have a common origin, in the sense that a theory must exist in which all play essential roles and all are accounted for by one over-arching principle. These relationships are certain to become manifest in extreme conditions, while they may be entirely missing in the weak field case. That is true for example of electromagnetism - the relationship of electricity and magnetism does not become entirely clear until one has high field densities, high (relativistic) velocities, etc. In the weakest regime they are entirely disconnected (static charge and simple magnets). The black hole religion fails entirely to account for this certainty that the other interactions manifest alongside gravity in extreme conditions, and takes GR to be literally correct down to the smallest levels. This however is impossible for a third, and deadly reason..

3) There is no conservation law for energy and momentum for gravitation. It is therefore impossible even to make naive arguments based on simple energy conservation unless very special circumstances are at hand, all of which preclude a strong-field condition.

Black hole skepticism has nothing at all to do with something so naive as "I can't see it, it must not exist". It has to do with understanding how the rest of physics works, how it emerged historically, and what is the ground common to all physical theories. The black hole option is invoked in every conceivable circumstance because it is, for one thing untestable and therefore irrefutable, and for another, amounts to an automatic publication-generation algorithm. Thinking hard takes time and effort and both are in short supply.

-drl


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Ira
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Loc: Mitzpe Ramon, Israel
Re: Black Hole At Our Doorstep: Trapezium? new [Re: deSitter]
      #5514456 - 11/11/12 01:05 PM

So, are you saying black holes don't exist? They're a simple-minded mistake? Or that THIS black hole in M42 doesn't exist?

/Ira


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JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: Black Hole At Our Doorstep: Trapezium? new [Re: deSitter]
      #5514468 - 11/11/12 01:15 PM

Quote:

GR has not been tested in any way in dense matter conditions. The only tests are the accurate prediction of the orbits of test particles around a massive object (a satellite in earth orbit, a planet around the Sun..), the accurate prediction of gravitational redshift, and the accurate prediction of the rotation curves in spiral galaxies. The first two cases are extreme weak field tests (essentially the one-body problem) while the latter is simply weak field (non-linearity of equations respected). No strong field test of any sort exists. There is little reason to doubt that GR is correct as one enters the strong field regime, but a great deal of reason to doubt that it is correct in the extremely strong field regime.



This is incorrect. GR has been tested extensively in conditions that go well beyond the weak-field (the so called post-Newtonian approximation) limit. On top of that, the Hulse-Taylor system and PSR J0737-3039 provide superb strong field tests.

More importantly, the formation of black hole horizons doesn't require extreme gravity. This is something few people realize. Black holes are gravitational horizons: boundaries that define space-time regions causally disconnected from our universe. Such horizons form under mild gravity conditions.

One last point is that as higher-order gravity effects have been tested, there is absolutely no reason to doubt the strong field limit. The analogy with electromagnetism is instructive, but for reasons opposite to what you suspect:

Quote:

electromagnetism - the relationship of electricity and magnetism does not become entirely clear until one has high field densities, high (relativistic) velocities, etc. In the weakest regime they are entirely disconnected (static charge and simple magnets).



Any current no matter how small creates a magnetic field, and any change in a magnetic field no matter how small creates a current. It is absolutely untrue that the unification between electricity and magnetism was achieved only after stronger currents or higher magnetic fields were achieved.

It is correct that the full integration of electricity and magnetism was achieved only under Einstein's special theory of relativity, but this integration changed nothing whatsoever in the predictions of the theory.

Quote:

There is no conservation law for energy and momentum for gravitation. It is therefore impossible even to make naive arguments based on simple energy conservation unless very special circumstances are at hand, all of which preclude a strong-field condition.



Local energy and momentum conservation are part and parcel of GR. What is highly nontrivial, however, is to translate this into global conservation laws. But this is in no way a demonstration of problems inherent to GR. It is just a manifestation of the complex implications of GR. It is almost a century since Einstein wrote down the equations for GR, yet working out all its implications is still continuing. Physicists struggled for decades with the Schwarzschild solution, and for many years they refused to accept the physical reality of black holes. Yet, with all the evidence building up over the years, there is no single scientist knowledgable in the area that continuous to doubt the reality of black holes.

The bottom line is: black hole skepticism is crackpottery, What exactly happens deep beyond black hole horizons is dependent on the integration of gravity with the other forces, but the physics up to and including black hole horizons is well understood and unanimously accepted by experts (who became experts thanks to their inquisitive mind and their skeptical nature).


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deSitter
Still in Old School


Reged: 12/09/04

Re: Black Hole At Our Doorstep: Trapezium? new [Re: JKoelman]
      #5515052 - 11/11/12 08:38 PM

Quote:


Local energy and momentum conservation are part and parcel of GR.





Show me. Well this is not fair, but I feel feisty. Give it your best shot. Write a local law of energy momentum conservation in GR right here. I don't care where you get it from, just write it down, or link it, or whatever. Point out to me this thing that is "part and parcel of GR". And when I show you that you are wrong, what then? What will you do? Will you learn? Or will you just become entrenched in what you have failed to understand?

-drl


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Neutrino?
sage
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Reged: 12/14/09

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Re: Black Hole At Our Doorstep: Trapezium? new [Re: JKoelman]
      #5515099 - 11/11/12 09:14 PM

Quote:

It is absolutely untrue that the unification between electricity and magnetism was achieved only after stronger currents or higher magnetic fields were achieved.




I don't need to defend drl, as he can defend himself quite easily, but that is not what he said.

Quote:

It is correct that the full integration of electricity and magnetism was achieved only under Einstein's special theory of relativity, but this integration changed nothing whatsoever in the predictions of the theory.




Why would the predictions of the theory change? I am surprised this is even part of your argument, because we all know this and you missed the point. The classical equations by luck were already invariant under the Lorentz group. So of course formulating SR and finding these symmetries later had no change on the theory. It only let us better understand what was fundamentally happening. If the classical theory didn't adhere to these symmetries, it would have to be modified and then the predictions would have changed.

This could be happening with a bunch of things. That is what he is looking at.


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EJN
Carpal Tunnel
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Reged: 11/01/05

Loc: 53 miles west of Venus
Re: Black Hole At Our Doorstep: Trapezium? new [Re: Sean Cunneen]
      #5515111 - 11/11/12 09:28 PM

Quote:

Local energy and momentum conservation are part and parcel of GR.




Not true. Energy/momentum is conserved only in cases where the covariant
derivative of the energy-momentum tensor equals zero.


Quote:


I have a computer model of birds angrily launching themselves at pigs...Do I win?



I have a computer model of angry pterodactyls launching flying toasters at
flying monkeys.
I win.


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JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: Black Hole At Our Doorstep: Trapezium? new [Re: deSitter]
      #5515166 - 11/11/12 10:11 PM

You react to and dispute solely one of the many clarifications I have given you. I take it therefore that you have withdrawn all your objections against black holes, except for one. That is good. That means we are getting to the point of embracing black holes as inevitable consequences of the laws of physics.

Ok, your last remaining concern: energy-momentum conservation in General Relativity. You pose a rather trivial challenge:

Quote:

Write a local law of energy momentum conservation in GR right here. I don't care where you get it from, just write it down, or link it, or whatever. Point out to me this thing that is "part and parcel of GR".



You can open any textbook on General Relativity. Each of them will explain that locally energy-momentum conservation holds. This is trivial as soon as you realize that GR represents nothing more than a patchwork of Minkowskian space-times.

The preferred reference ("the bible of general relativists") is Wald's textbook "General Relativity" (ISBN 0-226-87033-2). Wald discusses energy-momentum conservation on p.69-70. It suffices to quote a single sentence:

Quote:

Thus equation (4.3.6) may be interpreted as a local conservation of material energy over small regions of spacetime.




The equation Wald refers to is Nabla(sup,a) T(sub,a,b) = 0. The quantity T is the energy-momentum pseudotensor. Wikipedia has the following to say about this pseudotensor:

Quote:

In the theory of general relativity, a stress–energy–momentum pseudotensor, such as the Landau–Lifshitz pseudotensor, is an extension of the non-gravitational stress–energy tensor which incorporates the energy-momentum of gravity. It allows the energy-momentum of a system of gravitating matter to be defined. In particular it allows the total of matter plus the gravitating energy-momentum to form a conserved current within the framework of general relativity, so that the total energy-momentum crossing the hypersurface (3-dimensional boundary) of any compact space-time hypervolume (4-dimensional submanifold) vanishes.




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JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: Black Hole At Our Doorstep: Trapezium? new [Re: EJN]
      #5515217 - 11/11/12 10:33 PM

Quote:

Quote:

Local energy and momentum conservation are part and parcel of GR.




Not true. Energy is conserved only in cases where the covariant derivative of the energy-momentum tensor equals zero.




Yes, this covariant derivative has to be zero. And it is zero. See my response above to DeSitter referring to equatiion 4.3.6 in Wald's book.


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EJN
Carpal Tunnel
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Reged: 11/01/05

Loc: 53 miles west of Venus
Re: Black Hole At Our Doorstep: Trapezium? new [Re: JKoelman]
      #5515233 - 11/11/12 10:45 PM

Quote:



Yes, this covariant derivative has to be zero. And it is zero. See my response above to DeSitter referring to equatiion 4.3.6 in Wald's book.



The covariant derivative is only equal to zero under special conditions,
with static spacetimes and asymptotically flat spacetimes. Also, using a psuedotensor
makes the result dependent on the coordinate system which is used, which is
not the case with "true" tensors.

Gravitational collapse satisfies neither of the above conditions. Neither does
the FRW metric of the expanding universe. The covariant derivative of the
energy-monentum tensor cannot be zero when the spacetime background is dynamic.

See:
http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

"The Schwarzschild metric describes spacetime around a massive object,
if the object is spherically symmetrical, uncharged, and 'alone in the universe'."

"For these reasons, most physicists who work in general relativity do not
believe the pseudo-tensors give a good local definition of energy density, although
their integrals are sometimes useful as a measure of total energy."


Quote:

The preferred reference ("the bible of general relativists") is Wald's textbook "General Relativity"



"Gravitation" by Thorne, Misner, & Wheeler is considered the true "bible" of GR.


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JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: Black Hole At Our Doorstep: Trapezium? new [Re: EJN]
      #5515292 - 11/11/12 11:32 PM

Quote:

Quote:

The equation Wald refers to is Nabla(sup,a) T(sub,a,b) = 0. The quantity T is the energy-momentum pseudotensor.



The covariant derivative is only equal to zero under special conditions, with static spacetimes and asymptotically flat spacetimes. Using a psuedotensor makes the result dependent on the coordinate system which is used, which is not the case with true tensors.

Gravitational collapse satisfies neither of those conditions. Neither does the FRW metric
of the expanding universe. The covariant derivative of the energy-monentum tensor cannot be zero
when the spacetime background is dynamic.

See:
http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

"The Schwarzschild metric describes spacetime around a massive object, if the object
is spherically symmetrical, uncharged, and 'alone in the universe'
."

"For these reasons, most physicists who work in general relativity do not believe the
pseudo-tensors give a good local definition of energy density, although their integrals
are sometimes useful as a measure of total energy."




You are confusing global energy conservation with local energy conservation. As I said in my first post: translating local energy conservation into a corresponding equation for global energy conservation is highly non-trivial. Static spacetimes and asymptotically flat spacetimes are examples where global energy conservation can easily be derived from local energy conservation.

Baez, in the text you linked to, is careful to distinghuish the two. He choses the following words when describing local energy conservation:

Quote:

The differential form says, loosely speaking, that no energy is created in any infinitesimal piece of spacetime. The integral form says the same for a finite-sized piece.




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deSitter
Still in Old School


Reged: 12/09/04

Re: Black Hole At Our Doorstep: Trapezium? [Re: JKoelman]
      #5515496 - 11/12/12 04:26 AM

You are missing the point completely.

To have a conservation law, you must be able to convert a integral throughout a volume into one on the surface of that volume via Stoke's theorem. There is nothing at all complicated about this. It does not depend on "interpretation". It is just a simple fact. In order for the amount of a conserved quantity contained within a volume to change, it has to go through the surface that bounds the volume.

But you cannot apply Stoke's theorem in GR unless a special condition is met - namely, you must be able to convert a covariant derivative into an ordinary derivative. This is always possible for a vector field;

Ja;n = (1/sqrt g) d/dxn (Ja sqrt g)

where g is the determinant of the metric tensor. This extra term that shows up may be thought of as the correction that appears from the changing volume element within the manifold. In general this rearrangement of terms by bringing in the volume element is always possible for a totally antisymmetric tensor of any rank. But the energy tensor is symmetric. It is not in general possible to convert the covariant derivative of the energy tensor into an ordinary divergence so that Stoke's theorem may be applied.

GR has the form

Rmn - 1/2 gmn R = 8 pi Tmn

The covariant divergence of the left side is identically zero (Bianchi identity). Thus

Tmn; n = 0

and since this is a covariant divergence of a symmetric tensor, it does not represent a conservation law. There is no conservation of energy and momentum in general relativity.

By the way, I own Wald and it's an awful book.

-drl


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