Timeline for Mental model of general relativity
Current License: CC BY-SA 4.0
16 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Mar 9, 2022 at 13:16 | comment | added | m4r35n357 | Of course, I am not arguing against Penrose diagrams! | |
| Mar 9, 2022 at 13:11 | comment | added | m4r35n357 | Spacelike geodesics, yes. (In fact I have got so far as to simulate Kerr geodesics, but not to use the simulations to do any significant investigations: youtube.com/playlist?list=PLvGnzGhIWTGRuIsHzDLdeWS4GSZ9CIY-b) | |
| Mar 9, 2022 at 13:09 | comment | added | Prahar | The rubber sheet diagram (embedding diagram) is not the only type of drawings people use in GR to depict spacetimes. For instance, Penrose-Carter diagrams are used a lot to represent the geometries of spacetimes as well. That, too has its limitations. A Penrose-Carter diagram accurately depicts causal relationships, but not distances. Those diagrams never have any equations and yet they are useful to gain intuition. | |
| Mar 9, 2022 at 13:07 | comment | added | Prahar | Motion of particles can be described by drawing geodesics in that same diagram. If you want to show time evolution of the geometry itself you can convert it to a movie and that works too. | |
| Mar 9, 2022 at 13:05 | comment | added | Prahar | You can't - at least not in that diagram. As I said earlier, the important thing is to understand the limitations of these sort of diagrams. The goal of that particular figure is to give an explanation for the near horizon geometry at a particular instant of time and as a function of $a$ (a parameter in the Kerr metric). Each diagram has its own particular applications and should not be extrapolated beyond it! A good physicist must learn what conclusions one can and cannot draw from any given drawing. | |
| Mar 9, 2022 at 13:00 | comment | added | m4r35n357 | @Prahar thanks for the link! My problem with this is that, as I understand it, the embedding is a spacelike surface (t = constant, from the diagram), so I don't see how one can draw any conclusions regarding motion! | |
| Mar 9, 2022 at 11:38 | comment | added | Prahar | @m4r35n357 - Figure 2 of this paper - 202.38.64.11/~jmy/documents/publications/… describes the near horizon geometry of a Kerr black hole close to extremality. The equations in the paper describe all the details of course, but the diagram is a clean summary of all the results. | |
| Mar 9, 2022 at 9:50 | comment | added | m4r35n357 | @Prahar I am now intrigued. OK I am just a learner here but I haven't heard of any positive applications for the rubber sheet. Perhaps you could give a brief pointer. I trust you are not referring to en.wikipedia.org/wiki/…. | |
| Mar 8, 2022 at 15:15 | comment | added | Andrew | To add to what @Prahar is saying, it's not enough for most working physicists to just know the equations. The equations obviously give you the rigorous and exact description of what is going on, but are also very complicated and abstract. A successful physicist also needs physical intuition, which comes from solving problems, building toy models, and yes, using crude visualizations that capture some important aspect of reality. It is part of a good physics education to learn how to use such tools appropriately, which aren't rigorous but are often essential for building intuition. | |
| Mar 8, 2022 at 10:01 | comment | added | m4r35n357 | I'm sure the OP will appreciate that distinction ;) | |
| Mar 8, 2022 at 10:00 | comment | added | Prahar | I'm not talking about newbies. You mentioned in your answer that "Nobody who understands General Relativity uses it." and I'm pointing out that this is false. People who understand GR do use it to communicate with each other. I never said anything about using them to teach newbies. | |
| Mar 8, 2022 at 9:59 | comment | added | m4r35n357 | My point is that newbies, by definition, do not understand the limitations of those tools! | |
| Mar 8, 2022 at 9:58 | comment | added | Prahar | I don't think so. I think it is perfectly acceptable to use crude tools for specific purposes as long as you understand the limitations of those tools. It is in the same vein that the images are used. | |
| Mar 8, 2022 at 9:57 | comment | added | m4r35n357 | Then they should be ashamed ;) If they are not "reading too much into such pictures", then what sense are they "using" it? | |
| Mar 8, 2022 at 9:55 | comment | added | Prahar | Actually, those who understand GR properly actually do us it quite often as it is quite an informative and succinct way to represent geometries. The difference is that those who understand GR properly know not to read too much into such pictures. | |
| Mar 8, 2022 at 9:24 | history | answered | m4r35n357 | CC BY-SA 4.0 |