A body at rest in Doran and Boyer-Lindquist coordinates

  • #1
JimWhoKnew
33
11
Homework Statement
Must a body at rest in Boyer-Lindquist coordinates move (or not) in Doran coordinates?
Relevant Equations
see below
I read this question in the second edition of "Exploring Black Holes" by Wheeler, Taylor & Bertschinger. The book can be freely accessed and downloaded from the author's website.
Chapter 17 deals with Kerr's solution on the equatorial slice ## \theta = 0 ##.
In Doran coordinates, the 2+1 metric is given by $$ d\tau ^2 = dT^2 - [(\frac{r^2}{r^2 + a^2})^{1/2} dr + (\frac{2 M}{r})^{1/2} (dT - a d\Phi)]^2 - (r^2 + a^2) d\Phi ^2 $$ (Eq. 5 p. 17-4)
By the transformation (Eqs. 113-114 p. 17-36) $$ dT = dt + \frac{R \beta}{r H^2} dr $$ $$ d\Phi = d\phi + \frac{\omega R}{r H^2 \beta} dr $$ where ## r ## is the same in both coordinate systems, one gets the metric in Boyer-Lindquist coordinates (Eq. 112 p. 17-35) $$ d\tau ^2 = (1-\frac{2 M}{r})dt^2 + \frac{4 M a}{r}dt d\phi - \frac{dr^2}{H^2} - R^2 d\phi ^2 $$ with $$ R^2 \equiv r^2 + a^2 + \frac{ 2 M a^2}{r} $$ $$ H^2 \equiv \frac{1}{r^2} (r^2 - 2 M r + a^2) $$ $$ \omega \equiv \frac{2 M a}{r R^2} $$ $$ \beta \equiv (\frac{2 M}{r})^{1/2} (\frac{r^2 + a^2}{R^2})^{1/2} $$ (p. 17-27)
(the calculation is straight forward but lengthy; I have carried it out and got the desired result).

Now, on page 17-38 the question is asked:

BL-7. Not “at rest” in both global coordinates
Show that a stone at rest in Boyer-Lindquist global coordinates (## dr = d\phi = 0 ##) is not at rest in Doran global coordinates; in particular, ## d\Phi \neq 0 ## for that stone.

My problem is that from the transformation relations (presented above), ## dr = d\phi = 0 ## in B-L, trivially transforms into ## dr = d\Phi = 0 ## in Doran, and vice versa. What am I overlooking? How can this (apparent?) contradiction be resolved?
 
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  • #2
JimWhoKnew said:
Chapter 17 deals with Kerr's solution on the equatorial slice ## \theta = 0 ##.
I apologize for this error. ## \theta ## is the colatitude, of course, and the equatorial slice is at ##\theta = \pi /2 ## . I'm still looking for a solution to my question.
 
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  • #3
I agree with you. I also agree with you that it's an odd way to ask the question if it's a typo, so perhaps there's something I've missed.

I'm out of time this evening, but one thought I had was to note that "stationary" observers have non-zero four-acceleration. Can you compute those for the two families of "stationary" observers and transform one into the other system? I don't immediately see that doing so would get the answer they want, but it's at least different to mucking around with the metric (which is what I've been doing).
 
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  • #4
Thanks for helping. I appreciate it.

Ibix said:
I also agree with you that it's an odd way to ask the question if it's a typo
The question is phrased in a manner that it repeats itself 3 times, as if to eliminate any possibility for misunderstanding. That's why I find it hard to discard as a typo. It looks too intentional.

The book aims to be readable by undergrads, so the questions are not necessarily easy, but are not supposed to be extremely hard either. If we really miss something, I don't expect it to be very elusive.

Ibix said:
I'm out of time this evening, but one thought I had was to note that "stationary" observers have non-zero four-acceleration. Can you compute those for the two families of "stationary" observers and transform one into the other system?
That might bring on some advance! Since ##a_\mu a^\mu## is invariant, it can be calculated independently (for static observers) in each coordinate system, and then compared. If the values are different, It supports the book's claim (but as you said, it will not explain why I am wrong). I will do the calculations during the weekend and post the results.

Special thanks to @berkeman who pushes this wagon out of the mud. :smile:
 
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  • #5
JimWhoKnew said:
## r ## is the same in both coordinate systems
But ##dr## is not. To see this, note that if we look at ##d\tau^2## in each chart for an interval with only ##dr## nonzero, we get different quantities.

The above is also true in Painleve vs. Schwarzschild coordinates in Schwarzschild spacetime, which is the analogue of Doran vs. Boyer-Lindquist coordinates in Kerr spacetime. In the Schwarzschild spacetime case, the difference in ##dr## is due to the surfaces of constant coordinate time being "tilted" with respect to one another. But in the Kerr case, in addition to that, the "grid lines" of ##r## in the two charts are rotated with respect to one another (and the angle of rotation varies with ##r##). You can see this in Figure 7 where the "rest" vs. "static" frames are shown; the "rest" ##y## direction is the Doran global ##r## direction, but the static ##y## direction is the Boyer-Lindquist global ##r## direction. So saying that ##dr = 0## in Boyer-Lindquist is not the same as saying ##dr = 0## in Doran.

This by itself does not seem to affect the question, but it invites consideration of the fact that the global ##\phi## direction is also not the same in the two charts. Again, if we look at intervals with only ##d \phi = 0## in the two charts, we get different quantities for ##d\tau^2##, so the two directions cannot be the same. The question is whether any of the difference is due to the "rotation" of the grid lines described above, in addition to the difference due to the surfaces of constant coordinate time not being the same.

It seems to me that the above might be a possible clue.
 
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  • #6
Thanks for joining in. As I wrote to @Ibix above, I really appreciate your help and precious time.

You gave me a lot to think about (well, for me it's a lot), so I'll reply in bits in the coming days.

PeterDonis said:
But ##dr## is not.
As I understand it, given a chart on spacetime and an event ##A##, the coordinates of ##A## in this chart are 4 simple real numbers. Transforming into a second chart with ##r'=r##, says that the nominal value of ##r'_A## is the same number as ##r_A ## (otherwise, what ##r'=r## means?). If we have a second nearby event ##B##, then ##\Delta r## is just the simple subtraction of two numbers ##r_B-r_A##. If not, how can the coordinates be used to calculate any physical quantity? This leads me to infer ##dr'=dr##.

Another way to look at it: the coordinate ##r## is a function on spacetime, and the 1-form ##dr## does not depend on other coordinates or on the existence of a metric. If ##r'=r##, then ##dr'## should be the same 1-form as ##dr##.

However, ##\partial_{r'}\neq \partial_r## in general, because of the "tilts".

PeterDonis said:
To see this, note that if we look at ##d\tau ^2## in each chart for an interval with only nonzero ##dr##, we get different quantities.
Do you mean taking two nearby events for which only ##dr## is nonzero? If so, we can loosely say that these events are "simultaneous" in the first chart. But they will not be simultaneous in the second. So a pair of events with only ##dr'## nonzero, is necessarily different from the first pair, and I wouldn't expect them to have the same ##d\tau ^2##.

PeterDonis said:
The above is also true in Painleve vs. Schwarzschild coordinates in Schwarzschild spacetime, which is the analogue of Doran vs. Boyer-Lindquist coordinates in Kerr spacetime. In the Schwarzschild spacetime case, the difference in ##dr## is due to the surfaces of constant coordinate time being "tilted" with respect to one another.
Following this observation, I can think of a simple thought experiment in SR (where things are usually simpler and clearer), that supports (in my opinion) my understanding that ##x'=x## imposes ##dx'=dx##. I will describe it if anyone shows interest.

PeterDonis said:
You can see this in Figure 7 where the "rest" vs. "static" frames are shown; the "rest" ##y## direction is the Doran global ##r## direction, but the static ##y## direction is the Boyer-Lindquist global ##r## direction.
I will have to re-read these sections and think more about it. It will take me few days.
 
  • #7
JimWhoKnew said:
This leads me to infer ##dr'=dr##.
If we are considering a given physical displacement, we must hold ##d\tau^2## constant, and if we are considering a displacement in which only ##dr## is nonzero, the two line elements have different coefficients of ##dr##, so if ##d\tau^2## is the same, ##dr## must change if it is the only nonzero differential in both charts. Or, alternatively, an interval with only ##dr## nonzero in one chart must have some other differential also nonzero in the other chart. I should have mentioned the latter possibility before.
 
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  • #8
PeterDonis said:
Or, alternatively, an interval with only ##dr## nonzero in one chart must have some other differential also nonzero in the other chart.
It seems to me the same as what I wrote above (the paragraph that begins with "Do you mean...").

The invariance of ##d\tau ^2## doesn't forbid ##dr'=dr##, unless you insist that only ##dr## and ##dr'## are nonzero - a condition that can't be fulfilled simultaneously in both charts by the same pair of neighboring events.

If we go back to the OP, the assumption ##dr'=dr=0## along with the given ##d\phi=0##, leads to ##d\Phi=0## and ##dT=dt##, and ##d\tau^2## is the same in both charts. So we can question the assumption, but its consequence is compatible.
 
  • #9
JimWhoKnew said:
The invariance of ##d\tau ^2## doesn't forbid ##dr'=dr##, unless you insist that only ##dr## and ##dr'## are nonzero - a condition that can't be fulfilled simultaneously in both charts by the same pair of neighboring events.
Yes, that's the "alternative" that I said I should have mentioned in my earlier post. Sorry for the lack of clarity on my part.
 
  • #10
I've managed to do it in a different way to what I suggested above.

Deduce the Jacobian matrix from the two line elements and apply that to a unit vector in the "only time coordinate changes" direction in either coordinate system.

Edit: Actually, I think I messed up - see my next post.
 
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  • #11
Ibix said:
I've managed to do it in a different way to what I suggested above.
I'm not sure how this helps since if we restrict to ##dr = 0## the two line elements are the same.
 
  • #12
By "the Jacobian matrix" , do you mean $$\frac{\partial x'^\mu}{\partial x^\nu}$$ (or its inverse) of the coordinates transformation?
 
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  • #13
Ibix said:
"stationary" observers have non-zero four-acceleration. Can you compute those for the two families of "stationary" observers and transform one into the other system?
I have not done this computation, but the fact that the line elements in both charts are the same for ##dr = 0## indicates to me that the obvious Killing vector fields ##\partial_t## vs. ##\partial_T## and ##\partial_\phi## vs. ##\partial_\Phi## are the same KVFs in both charts. Since "at rest" worldlines are orbits of ##\partial_t## or ##\partial_T##, the fact that those KVFs are the same would indicate that the "at rest" worldlines are the same.

It is true that the two 1-form fields ##dt## and ##dT## are not the same, even though ##\partial_t## and ##\partial_T## are the same vector fields. However, the difference between them only shows up for intervals with ##dr## nonzero; if we restrict to a single value of ##r##, i.e., to a "cylinder" in spacetime, the difference in the two 1-form fields vanishes.

The more I think about this, the more I wonder if in fact there was a typo in this question in the book.
 
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  • #14
OK - I'm very confused. If you can see why my four-acceleration isn't perpendicular to my four-velocity when working in Doran coordinates then let me know. The Maxima code below derives the Doran metric tensor from the line element copied from (4) on page 17-3 in the link in the OP, uses ctensor to compute Christoffel symbols (in Maxima ##\Gamma^i_{jk}## is mcs[j, k, i]), defines a function to compute the covariant derivative of a vector, then computes ##A^a=U^a\nabla_aU^b## (which should be the four acceleration) and then ##g_{ab}U^aA^b## which should be zero. It works in Boyer-Lindquist coordinates (there's commented out code to override the metric tensor with the B-L form) but not Doran.


Edit: figured it out - the indices are inconsistent in the calculation of the covariant derivative. See next post. I've deleted the buggy code from this post.
 
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  • #15
Ibix said:
OK - I'm very confused. If you can see why my four-acceleration isn't perpendicular to my four-velocity when working in Doran coordinates then let me know. The Maxima code below...
I am not familiar with Maxima, so I can't help here. I have calculated the acceleration too (as promised) and got the same ##a^\mu a_\mu## for observers "at rest" in both metrics.

In either metric I get $$u^\mu=(1-\frac{2M}{r})^{-1/2}\: \delta^\mu_{\: 0}$$ and from $$a^\mu = \frac{du^\mu}{d\tau}+\Gamma^\mu_{\: \nu \rho}u^\nu u^\rho$$ I get $$a^\mu=(1-\frac{2M}{r})^{-1} \: \Gamma^\mu _{\: 00}$$ So it's all about calculating ##\Gamma^\mu _{\: 00}##.
The results I got do satisfy ##a^\mu u_\mu=0##, and have the desired property that ##a^\mu a_\mu## diverges at the static limit. However, in Boyer-Lindquist I got that only ##a^r## and ##a_r## are nonzero. I expected a ##\phi## component to oppose the frame dragging.

(In 3+1 dimensions, ##g_{0 \theta}=0##, so the results for ##a^\mu ## (## \mu\neq\theta##) should be the same whether calculated in 3+1 or 2+1. I tried both, just to verify).
 
  • #16
Fixed the code and deleted buggy version (turned out to be using indices inconsistently) from above. Corrected and expanded version is below.

In B-L coordinates I get ##A^t=A^\phi=0## and $$A^r=\frac{M(r^2-2Mr+a^2)^2}{r^5(r-2M)}$$In Doran coordinates I get $$\begin{eqnarray*}
A^T&=&\frac{\sqrt{2}M^{3/2}\sqrt{r^2+a^2}}{r^{5/2}\left(r-2M\right)}\\
A^r&=&\frac{M(r^2-2Mr+a^2)}{r^3(r-2M)}\\
A^\Phi&=&\frac{\sqrt{2}M^{3/2}a}{r^{5/2}(r-2M)\sqrt{r^2+a^2}}
\end{eqnarray*}$$
This matches the analysis in #15.

I think these two are actually the same vector - if you use ##(J^{-1})^T## to convert the BL ##A## into Doran coordinates their inner product is equal to the square root of the products of the magnitudes, so they appear parallel.
Code:
/* https://www.physicsforums.com/threads/a-body-at-rest-in-doran-and-boyer-lindquist-coordinates.1063139/ */

/* Some useful definitions, from Taylor et al Exploring Black Holes */
assume(r>0);
H:(r^2-2*M*r+a^2)/r^2;
R:sqrt(r^2+a^2+2*M*a^2/r);
omega:2*M*a/(r*R^2);
beta:sqrt(2*M/r)*sqrt((r^2+a^2)/(R^2));

/* Convert Doran coordinates (Kerr spacetime) line element into metric tensor */
dT^2-(sqrt(r^2/(r^2+a^2))*dr+sqrt(2*M/r)*(dT-a*dPhi))^2-(r^2+a^2)*dPhi^2;
doranMetric: collectterms(expand(%), dT, dr, dPhi);
doranTr: ratsimp(part(doranMetric, 1) / (dT*dr));
doranPhir: ratsimp(part(doranMetric, 2) / (dPhi*dr));
doranrr: ratsimp(part(doranMetric, 3) / (dr*dr));
doranPhiPhi: ratsimp(part(doranMetric, 4) / (dPhi*dPhi));
doranTT: ratsimp(part(doranMetric, 5) / (dT*dT));
doranTPhi: ratsimp(part(doranMetric, 6) / (dT*dPhi));

doranLG:matrix([doranTT, doranTr/2, doranTPhi/2],
		[doranTr/2, doranrr, doranPhir/2],
		[doranTPhi/2, doranPhir/2, doranPhiPhi]);

/* Boyer-Lindquist coordinates metric */
blLG:matrix([1-2*M/r, 0, 2*M*a/r],
		[0, -1/H^2, 0],
		[2*M*a/r,0,-R^2]);

/* Write down B-L -> Doran Jacobian matrix */
J:matrix(	[1,             0,       0],
		[-R*beta/(r*H), sqrt(H), -omega*R/(r*beta*H)],
		[0,             0,       1]);
radcan(J.blLG.transpose(J)-doranLG);

/* Transform "stationary in B-L coordinates" to Doran coordinates */
invert(transpose(J)).[1,0,0];

/* Setup to check proper accelerations - load ctensor package and provide */
/* functions for covariant differentiation of a vector.                   */
load(ctensor);
covdif(U):=block(
	[i,j,k,ans],
	ans:zeromatrix(dim,dim),
	for i:1 thru dim do block (
		for j:1 thru dim do block (
			ans[i,j]: diff(U[j],ct_coords[i]),
			for k:1 thru dim do block (
				ans[i,j]: ans[i,j] + mcs[i,k,j] * U[k]
			)
		)
	),
	ans
);

/* Compute 4-acceleration of hovering observer in B-L coordinates */
ct_coords:[t,r,phi];
dim:3;
lg:blLG;
cmetric(false);
christof(mcs);
blU:[1/sqrt(lg[1,1]),0,0];
blA:ratsimp(blU.covdif(blU));
ratsimp(blU.blLG.blA);  /* Check U orthogonal to A */

/* Ditto in Doran coordinates */
ct_coords:[T,r,Phi];
dim:3;
lg:doranLG;
cmetric(false);
christof(mcs);
doranU:[1/sqrt(lg[1,1]),0,0];
doranA:ratsimp(doranU.covdif(doranU));
ratsimp(doranU.doranLG.doranA);  /* Check U orthogonal to A */
 
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  • #17
PeterDonis said:
I have not done this computation, but the fact that the line elements in both charts are the same for ##dr = 0## indicates to me that the obvious Killing vector fields ##\partial_t## vs. ##\partial_T## and ##\partial_\phi## vs. ##\partial_\Phi## are the same KVFs in both charts. Since "at rest" worldlines are orbits of ##\partial_t## or ##\partial_T##, the fact that those KVFs are the same would indicate that the "at rest" worldlines are the same.
I am not sure about it, so please point out where I'm wrong:
##\partial _t## in B-L must be a linear superposition of the form ##A\,\partial _T+B\,\partial _\Phi##, ##A## and ##B## constant (because these 2 vector fields span the Killing subspace). So motion in B-L along a worldline which is an orbit of ##\partial_t##, should be viewed in Doran as motion along a worldline which is an orbit of ##A\,\partial _T+B\,\partial _\Phi##, and therefore, in this case, ##dr=0## in both charts.
 
  • #18
JimWhoKnew said:
I expected a ##\phi## component to oppose the frame dragging.
There isn't one for observers "hovering" at constant ##r##; the ##\phi## component only appears once ##dr / d\tau## is nonzero.
 
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  • #19
JimWhoKnew said:
I am not sure about it, so please point out where I'm wrong:
##\partial _t## in B-L must be a linear superposition of the form ##A\,\partial _T+B\,\partial _\Phi##, ##A## and ##B## constant (because these 2 vector fields span the Killing subspace).
Generally speaking, yes, but if you work it out for this particular case I think you will find that ##B## must be zero. Otherwise the KVF does not approach a Minkowski time translation at infinity and is therefore not a possible KVF for describing an "at rest" observer.
 
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  • #20
Ibix said:
This matches the analysis in #15.
In Doran, I got the same results as you. My ##a^r## in B-L is exactly the same as in Doran, and different from yours. I got the same ##a^\mu a_\mu## in both charts. Do you get it too?
 
  • #21
JimWhoKnew said:
In Doran, I got the same results as you. My ##a^r## in B-L is exactly the same as in Doran, and different from yours. I got the same ##a^\mu a_\mu## in both charts. Do you get it too?
I've double checked, and what's written is what I've got. The vectors are parallel, but B-L ##A^aA_a## is ##H## times Doran ##A^aA_a##. I'll have another look just in case there's another bug lurking there.
 
  • #22
I note that my B-L ##A^r=\Gamma^r_{tt}(1-2M/r)^{-1}##, where$$\Gamma^r_{tt}=\frac{M(r^2-2Mr+a^2)^2}{r^6}$$That's consistent with your #15. The B-L metric tensor I'm using is$$\begin{pmatrix} 1-{{2\,M
}\over{r}}&0&{{2\,M\,a}\over{r}}\cr 0&-{{r^4}\over{\left(r^2-2\,M\,r
+a^2\right)^2}}&0\cr {{2\,M\,a}\over{r}}&0&-r^2-{{2\,M\,a^2}\over{r
}}-a^2\cr \end{pmatrix}$$and the Christoffel symbol seems consistent with that.
 
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  • #23
Ibix said:
I note that my B-L ##A^r=\Gamma^r_{tt}(1-2M/r)^{-1}##, where$$\Gamma^r_{tt}=\frac{M(r^2-2Mr+a^2)^2}{r^6}$$That's consistent with your #15. The B-L metric tensor I'm using is$$\begin{pmatrix} 1-{{2\,M
}\over{r}}&0&{{2\,M\,a}\over{r}}\cr 0&-{{r^4}\over{\left(r^2-2\,M\,r
+a^2\right)^2}}&0\cr {{2\,M\,a}\over{r}}&0&-r^2-{{2\,M\,a^2}\over{r
}}-a^2\cr \end{pmatrix}$$and the Christoffel symbol seems consistent with that.
Note that ##H^2## is defined as $$H^2 \equiv \frac{1}{r^2} (r^2 - 2 M r + a^2)$$ so actually you are using $$-\frac{dr^2}{H^4}$$ instead of $$-\frac{dr^2}{H^2}$$
 
  • #24
PeterDonis said:
Generally speaking, yes, but if you work it out for this particular case I think you will find that ##B## must be zero. Otherwise the KVF does not approach a Minkowski time translation at infinity and is therefore not a possible KVF for describing an "at rest" observer.
Thanks. I did the calculation (which turns out to be very easy in this particular case), and got that indeed ##\partial_T=\partial_t## , ##\partial_\Phi=\partial_\phi## , and ##\partial_{r'} \neq \partial_r## , as expected.
 
  • #25
JimWhoKnew said:
Thanks. I did the calculation (which turns out to be very easy in this particular case), and got that indeed ##\partial_T=\partial_t## , ##\partial_\Phi=\partial_\phi## , and ##\partial_{r'} \neq \partial_r## , as expected.
To summarize what I think this means, in conjunction with Figure 7 in the book that I referred to earlier:

In both charts ##\partial_\phi = \partial_\Phi## points in the direction of the global ##\Phi## in the figure, which is also the "static" ##x## direction.

The Boyer-Lindquist ##\partial_r## points in the "static" ##y## direction in the figure. This direction is orthogonal to ##\partial_\phi##.

The Doran ##\partial_r## points in the "rest" ##y## direction in the figure. This direction is not orthogonal to ##\partial_\phi##.

##\partial_t = \partial_T## points out of the page of the figure, "tilted" relative to (i.e., not orthogonal to) ##\partial_\phi##, but orthogonal to the Boyer-Lindquist ##\partial_r##. (But not orthogonal to the Doran ##\partial_r##.)
 
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  • #26
I'm not quite sure how it addresses the referenced exercise in the OP, and maybe was pointed out before, but the following can be derived from either the metric forms or the differential coordinate transforms:

1) Holding r and ##\phi ## constant represents the same paths as holding r and ##\Phi## constant.
2) Holding t and r constant represents the same paths as holding T and r constant.
3) Holding t and ##\phi## constant represents different paths from holding T and ##\Phi## constant.

Maybe this is what the exercise was meant to bring out, but the wording seems wrong for this??
 
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  • #27
JimWhoKnew said:
Note that ##H^2## is defined as $$H^2 \equiv \frac{1}{r^2} (r^2 - 2 M r + a^2)$$ so actually you are using $$-\frac{dr^2}{H^4}$$ instead of $$-\frac{dr^2}{H^2}$$
Right. Way too many typos from me in this - sorry. With that correction, I agree with you.
 
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  • #28
PeterDonis said:
To summarize what I think this means, in conjunction with Figure 7 in the book that I referred to earlier:

In both charts ##\partial_\phi = \partial_\Phi## points in the direction of the global ##\Phi## in the figure, which is also the "static" ##x## direction.

The Boyer-Lindquist ##\partial_r## points in the "static" ##y## direction in the figure. This direction is orthogonal to ##\partial_\phi##.

The Doran ##\partial_r## points in the "rest" ##y## direction in the figure. This direction is not orthogonal to ##\partial_\phi##.

##\partial_t = \partial_T## points out of the page of the figure, "tilted" relative to (i.e., not orthogonal to) ##\partial_\phi##, but orthogonal to the Boyer-Lindquist ##\partial_r##. (But not orthogonal to the Doran ##\partial_r##.)
I am confused, so I'll start from the bottom line: do you think the question in the book (from the OP) is correct?

I still have to study Figure 7 carefully. But even now, it seems to me that the relations you describe are reasonable consequences of ##g_{\mu \nu}=\partial_\mu \cdot \partial_\nu\:## .
 
  • #29
PAllen said:
1) Holding r and ##\phi ## constant represents the same paths as holding r and ##\Phi## constant.
This is what the whole thread is about :smile:
If you are right, the book is wrong.

Thanks for joining.
 
  • #30
JimWhoKnew said:
do you think the question in the book (from the OP) is correct?
As I said in post #13, the more I think about it, the more I think the question as it appears in the book is not correct.
 
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  • #31
PeterDonis said:
As I said in post #13, the more I think about it, the more I think the question as it appears in the book is not correct.
Thanks for making that clear. The confusion is my fault. Sorry.
 
  • #32
PAllen said:
but the following can be derived from either the metric forms or the differential coordinate transforms:
How your 3 listed conclusions can be derived from the metric forms ALONE? (i.e. when the coordinates transformation relations are not provided)
 
  • #33
JimWhoKnew said:
How your 3 listed conclusions can be derived from the metric forms ALONE? (i.e. when the coordinates transformation relations are not provided)
Strictly speaking, you can't show the families of curves holding two coordinates constant are the same across the two metrics. All you can show is that metric interval along the curves is the same function the third coordinate. This is a necessary, but not sufficient condition. So, I was imprecise in my claim.
 
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  • #34
PAllen said:
Strictly speaking, you can't show the families of curves holding two coordinates constant are the same across the two metrics. All you can show is that metric interval along the curves is the same function the third coordinate. This is a necessary, but not sufficient condition. So, I was imprecise in my claim.
Showing that the conclusions hold in an infinitesimal vicinity of the starting point, is good enough for me.

Let's try:
If I use the property that each metric form has 2 explicit Killing directions, one timelike and one spacelike, I can use @PeterDonis' argument of the asymptotic form at infinity (post #19), and get that ##\partial_t## and ##\partial_T## are proportional. This is your first conclusion above. Moreover, by substitution in the metric forms, we can get ##\partial_t=\partial_T## .
By a similar procedure we get ##\partial_\phi=\partial_\Phi## . This is your second conclusion. By now we have determined 6 out of 9 components of the inverse transformation matrix (or it's transposed...).
Now we can see that in B-L, ##g_{r \phi}=\partial_r\cdot\partial_\phi=0## , but in Doran ##g_{r' \Phi}\neq 0## . This leads us to your third conclusion. ( ##g_{T \Phi}=g_{t \phi}## so we're good here).

Can it be done without employing the Killing symmetries?
 
  • #35
PAllen said:
Strictly speaking, you can't show the families of curves holding two coordinates constant are the same across the two metrics. All you can show is that metric interval along the curves is the same function the third coordinate. This is a necessary, but not sufficient condition. So, I was imprecise in my claim.
I thought about it today. By comparing the two metric forms, we implicitly assume ##r'=r##. Otherwise it's a mess. So we are left with 6 unknown components in the transformation matrix. The metric forms provide 5 quadratic equations, relating the differentials of the new coordinates to those of the old (the ##dr^2## equation is omitted, since it doesn't bear any additional information). We know that a solution exists. So, if I don't miss anything, there is possibly a 1-parameter family of transformations that carry the B-L form to Doran's (and maybe more, owing to different roots of the quadratic relations).
Must they all satisfy the 3 conclusions listed in post #26 ?
 

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