Kretschmann Scalar: Flat Spacetime & Singularities

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SUMMARY

The Kretschmann scalar, defined as K = R_abcd R^abcd, is utilized to identify singularities in spacetime, particularly in the context of Schwarzschild black holes where K is proportional to 1/r^6, indicating a singularity at r=0 but not at the event horizon. As r approaches infinity, K approaches zero, raising the question of whether K=0 signifies flat spacetime. However, it has been established that K=0 does not necessarily indicate flat spacetime, as certain pp-waves exhibit curvature singularities while maintaining K=0. References to this topic can be found in "Exact Solutions of Einstein's Field Equations" by Stephani et al. and works by Hawking and Ellis.

PREREQUISITES
  • Understanding of the Riemann tensor and its contractions
  • Familiarity with Schwarzschild black holes and their properties
  • Knowledge of curvature singularities in general relativity
  • Basic comprehension of pp-waves in the context of spacetime
NEXT STEPS
  • Study the Riemann tensor and its applications in general relativity
  • Explore the concept of curvature singularities in detail
  • Read "Exact Solutions of Einstein's Field Equations" by Hans Stephani et al.
  • Investigate the implications of K=0 in various spacetime geometries
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Physicists, mathematicians, and students of general relativity seeking to deepen their understanding of singularities and curvature in spacetime.

Cusp
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The Kretschmann scalar (the full contraction of the Reimann tensor K = R_abcd R^abcd) is often used to identify singularities - i.e. for a Schwarzschild black hole, K \propto 1/r^6, so we have a singularity at r=0, but not at the Schwarzschild horizon).

Clearly, as r->\infinity, K->0. Is K=0 a measure of flat spacetime in general? Is there a reference that shows this?

Cheers
 
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Cusp said:
The Kretschmann scalar (the full contraction of the Reimann tensor K = R_abcd R^abcd) is often used to identify singularities - i.e. for a Schwarzschild black hole, K \propto 1/r^6, so we have a singularity at r=0, but not at the Schwarzschild horizon).

Clearly, as r->\infinity, K->0. Is K=0 a measure of flat spacetime in general? Is there a reference that shows this?

Cheers

The person best able to answer this has stopped posting, but, from previous posts of his, the answer is "No." There are pp-waves that have curvature singularities, and that have K = 0. I suspect that this somewhere in Exact Solutions of Einstein's Field Equations by Hans Stephani, Dietrich Kramer, Malcolm MacCallum, and Cornelius Hoenselaers.

See 4. in
https://www.physicsforums.com/showthread.php?p=1351759#post1351759

1. in
https://www.physicsforums.com/showthread.php?p=1124707#post1124707

and the last paragraph of (the first post)
https://www.physicsforums.com/showthread.php?p=1176876#post1176876
 
Last edited by a moderator:
According to Hawking and Ellis, page 260, it was pointed out by Penrose that curvature can be non-zero even when stuff like K is zero.
 
Thanks George - will check it out.
 

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