# Highly symmetric spacetimes

1. Nov 30, 2006

### Stingray

In 4 dimensions, a spacetime can have a maximum of 10 linearly independent Killing vectors. Are there known examples of spacetimes (satisfying Einstein's equation) with 7, 8, or 9 Killing vectors? I know FRW cosmologies have 6 Killing vectors, but I'm looking for something a bit more symmetric that still has varying curvature.

2. Dec 1, 2006

### Chris Hillman

Solutions with 7 Killing vectors: two explicit examples, plus caveat

Hi, Stingray,

The book by Stephani et al., Exact Solutions of the Einstein Field Equations, 2n Ed., Cambridge University Press, 2001, is a gold mine of information for this classical topic, although it will require some work to extract all the relevant information. (This book offers some nifty tables which can help to quickly get the general idea, however.)

A complete answer would be very complex, but the very short answer is that there are many solutions with 0-4 Killing vectors, and of course an certain explicit vacuum solution has the maximal number of 10, but not very many exact solutions possess "intermediate" dimensional Lie algebras of Killing vectors. Indeed, there are various results to the effect that such and such a class is the only spacetime model with various properties and 5 or 6 Killing vectors (one of these refers to the famous Goedel dust, which has 5 Killing vectors but which escapes being isotropic as well as homogeneous). Similarly if you allow homotheties, affine collineations, or other generalizations of Killing vectors.

I seem to recall that there are some dimensions in the range 7-9 which do not occur at all, at least with some restrictions on the Ricci curvature (i.e. on the stress-energy tensor).

On a more positive note, I can offer a few explicit examples of exact solutions with 7 Killing vectors . The generic plane wave (EK9, the ninth class in the Ehlers & Kundt classification of vacuum plane waves, also SG10, the tenth class in the Sipple and Goenner classification of all plane waves) has a 5 dimensional Lie algebra of Killing vectors, but there are some interesting special cases which have one or two extra ones. In particular, SG16 and SG17 possess seven dimensional Lie algebras of Killing vector fields.

A specific example: the line element of SG16 can be written (in the harmonic or Brinkmann chart)
$ds^2 = -a^2 \, \left( X^2+Y^2 \right) \, dU^2 - 2 \, dU \, dV + dX^2 + dY^2,$
$-\infty < U, V, X, Y < \infty$
If you compute the Einstein tensor you find this is a "null dust solution" modeling something like "incoherent EM radiation" unaccompanied by gravitational radiation, since this exact plane wave solution happens to conformally flat! A simple choise of seven linearly independent Killing vector fields is:
$\partial_U, \; \partial_V, \; \partial_\Theta = -Y \, \partial_X + X \, \partial_Y$
$a X \, \cos(a U) \, \partial_V + \sin(a U) \partial_X$
$a X \, \sin(a U) \, \partial_V - \cos(a U) \partial_X$
$a Y \, \cos(a U) \, \partial_V + \sin(a U) \partial_Y$
$a Y \, \sin(a U) \, \partial_V - \cos(a U) \partial_Y$
where the first and third in this list are "extras".

You mentioned "varying curvature"; you'd probably consider this example to fail that test. For example, using the standard NP tetrad constructed from the Brinkmann chart,
$$\vec{\ell} = \partial_U - a^2/2 \, \left( X^2 + Y^2 \right) \, \partial_V, \; \vec{n} = \partial_V, \; \vec{m} = \frac{1}{\sqrt{2}} \left( \partial_X + i \, \partial_Y \right)$$
the Weyl scalars all vanish and the only nonvanishing Ricci scalar (other than the NP Lambda) is $$\Phi_{00} = a^2$$.

But you would probably admit SG17 as an example with "time-varying curvature". In a harmonic or Brinkmann chart, the line element can be written
$ds^2 = -\frac{m \, \left( X^2+Y^2 \right)}{U^2} \; dU^2 - 2 \, dU \, dV + dX^2 + dY^2,$
$0 < U < \infty, \; -\infty < V, X, Y < \infty$
The two extra Killing vectors here are
$$U \partial_U - V \partial_V, \; \partial_\Theta$$
(SG17 also admits an affine collineation which is not a Killing vector, by the way, $$U \, \partial_V$$.) This has $$\Phi_{00} = m/U^2$$ with respect to the standard NP tetrad.

I should stress that "invariant characterizations of curvature" can be quite tricky when radiation is present. That is, different observers, even different classes of inertial observers, might observe very different behavior and might even disagree on whether or not any curvature components diverge on some locus. So we naturally reach for curvature invariants, but these are no help at all, since ALL the curvature invariants of plane waves vanish identically, yet these are curved spacetimes. (This is analogous to the fact that in Lorentzian manifolds, the "length" of a nonzero null vector field vanishes identically.) This observation (due to Penrose) gives rise to the provisional rough classification of curvature singularities as scalar or nonscalar and strong or weak in various senses. Some of the other EK and SG classes in fact provide classic examples of plane waves exhibiting some of the heirarchy of strength (where "weaker" singularities are more survivable by small objects).

Chris Hillman

Last edited: Dec 1, 2006
3. Dec 1, 2006

### robphy

The mirror of "The On-line Exact Solutions Database at UERJ"
http://www.astro.queensu.ca/~jimsk/
suggests that there are known examples with "Maximal Isometry Group" from the list: 0 1 2 3 4 5 6 7 10

(does this search form actually work?)

Last edited by a moderator: Apr 22, 2017
4. Dec 1, 2006

### Chris Hillman

On-line database of exact solutions?

Hi, Rob,

It hasn't worked for me in quite some time. I have been unable to get responses to email inquiries to Jim Skea; as far as I can tell, he abandoned classical gravitation quite some time ago. Skea's database never included (as far as I know) more than a hundred or so metrics (some describing the same solutions, by design).

Years ago I was informed that MacCallum's group planned to make publically available a more portable implementation of the Karlhede algorithm as well as a searchable database of exact solutions. This intelligence impelled me to abandon my own work on presenting such a thing, but nothing seems to have happened, which I think is regretable. I have been unable to obtain any information about their current plans--- maybe you would have better luck?

I have a database holding thousands of frame field definitions in GRTensorII (this undoubably the most convenient package for computing observable quantities associated with specific solutions or classes of solutions), but I would need to do much work to make a portion of this available to others (maybe several hundred solutions, including dozens of different frame fields for some of the most important examples). Recently, I have been thinking of taking that up that project again. It would help if I had a better idea of how many serious students use GRTensorII; the fact that the fairly recent book by Eric Poisson, A Relativist's Toolkit, plays nice with GRTensorII might be helpful here, but the expense of maple obviously is not. Still, I'd like to assume that I can assume that working installations of maple will be readily available to registered university students.

I would insist on ducking responsibility for dealing with the onerous security issues associated with maintaining publically searchable databases, but I have considered making available a tar file which would allow interested parties to build a local database, if they have say MySQL installed.

Chris Hillman

Last edited by a moderator: May 2, 2017
5. Dec 1, 2006

### Stingray

Thanks for the interesting response. I did not know anything about the solutions or classification schemes you've mentioned. I'll take a look at Stephani's book.

As for GRTensor, it is used pretty commonly in my experience. And for those who haven't discovered it yet, I think pretty much anyone associated with a university has access to Maple anyway.

6. Dec 1, 2006

### Chris Hillman

Glad to hear GRTensor is getting used--- I use it all the time myself.

Chris Hillman