Timeline for The electromagnetic Lagrangian $L=(8\pi)^{-1}(E^2-B^2)$ vanishes for an EM plane wave. Why doesn't the energy-momentum tensor also vanish?
Current License: CC BY-SA 4.0
5 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jul 26 at 1:32 | comment | added | Ján Lalinský | Think about what $\frac{\partial L}{\partial g_{\mu\nu}}$ means. It does not mean a derivative of 0; $L$ here is meant to be a function of $g$, so it can change, when $g$ changes. | |
| Jul 25 at 22:25 | answer | added | Andrew | timeline score: 0 | |
| Jul 25 at 22:02 | answer | added | Lenard Kasselmann | timeline score: 1 | |
| Jul 25 at 17:32 | comment | added | Lenard Kasselmann | The Lagrangian itself might be zero for a free field, but that doesn't imply that its derivative with respect to the metric tensor is also zero. | |
| Jul 25 at 17:25 | history | asked | Khun Chang | CC BY-SA 4.0 |