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GR does not imply causality nor does it enforce it. In fact GR works in a non-causal universe without a problem. 2 very sensitive measurements conducted within
by valarauca1 10y ago
GR does not imply causality nor does it enforce it. In fact GR works in a non-causal universe without a problem.
2 very sensitive measurements conducted within the past year seem to suggest (if GR is true), that we are in a universe that lacks causality. 2/3 LIGO detections imply one of the merging pair of black holes should be a naked singularity.
GR allows naked singularities. It models them fine. GR just stops being globally deterministic.
If you look up the history of GR some mathematicians in 50's made some really weird proposals for non-causal universes that would appear locally causal. But there isn't a way to test this. So it is more pure mathematics or philosophy then physics.
- eaq 10y ago> 2/3 LIGO detections imply one of the merging pair of black holes should be a naked singularity. Where did you get this impression? All of the LIGO results are consistent with "standard" GR black holes whose event horizons merge.
- andrewflnr 10y agoPossibly referring to this? http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.116.171101 http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.116.... Background: https://en.wikipedia.org/wiki/Gravastar https://en.wikipedia.org/wiki/Gravastar
- eaq 10y agoThanks for the links. I find it unfortunate that often in science (and especially with the LIGO data) much is written about what could possibly be lurking in the data but isn't actually favored over our current understanding. This creates more interest, but can obfuscate what the real situation in the field is. In this case, while Gravastars are certainly something many scientists actively do and should consider, there is no real evidence from the LIGO data that favors the hypothesis of "we are in a universe that lacks causality" over the observation of the merger of two Kerr black holes.
- andrewflnr 10y agoI think you're being a bit harsh. Is there any empirical evidence that favors classic black holes over gravastars, or is our "current understanding" just a matter of what we thought of first? If the latter, take a chill pill and let us enjoy the possibilities. :)
- raattgift 10y ago> Is there any empirical evidence that favors classic black holes over gravastars Yes. http://journals.aps.org/prd/abstract/10.1103/PhysRevD.94.084016 http://journals.aps.org/prd/abstract/10.1103/PhysRevD.94.084... (preprint: https://arxiv.org/abs/1602.08759 https://arxiv.org/abs/1602.08759) So you're still left with gravastar models that can co-exist with black holes.
- nonbel 10y ago>"really weird proposals for non-causal universes that would appear locally causal" link?
- hansen 10y agohttps://en.wikipedia.org/wiki/Closed_timelike_curve#General_relativity https://en.wikipedia.org/wiki/Closed_timelike_curve#General_...
- raattgift 10y ago> there isn't a way to test this http://mnras.oxfordjournals.org/content/162/4/307.abstract http://mnras.oxfordjournals.org/content/162/4/307.abstract discusses observables of such metrics, especially Gödel's. A 2009 overview (postdating COBE, for example): http://iopscience.iop.org/article/10.1088/0004-637X/703/1/354/meta;jsessionid=C50D86D551078F7362D891E41716CA7B.c4.iopscience.cld.iop.org http://iopscience.iop.org/article/10.1088/0004-637X/703/1/35... (preprint: https://arxiv.org/abs/0902.4575 https://arxiv.org/abs/0902.4575) > 2/3 LIGO detections imply one of the merging pair of black holes should be a naked singularity. Where did you get that from? It's sure not prominent on the caltech or mit sites, or indeed in Pretorius et al. https://arxiv.org/abs/1603.08955 https://arxiv.org/abs/1603.08955
- hansen 10y ago> GR does not imply causality nor does it enforce it. In fact GR works in a non-causal universe without a problem. One of the key assumptions of GR is that spacetime is globally hyperbolic. This implies causality. You can’t guarantee solutions of the Einstein or Maxwell equations w/o this assumption.