# action

7. ### A Landau Lifshitz Gravitational field equation

Book: Landau Lifshitz, The Classical Theorey of Fields, chapter 11, section 95. I have gone through the derivation of Einstein field equations but not without holes to fill and fix in my understanding. Lets start with the action for the grtavitational field $S_g$ which after some explanation...
8. ### I What is the motivation for principle of stationary action

Is the motivation for the action principle purely from empirical evidence, or theoretical arguments, or a mixture of the two? As I understand it, there was some empirical evidence from Fermat's observations in optics, i.e. that light follows the path of least time, notions of virtual work and...
9. ### B Pushing wall

Dear PF Forum, I have a problem with calculating energy which I should have learnt long ago in my high school time. If I push (accelerate) a 1 kg object 1 meter/sec2 for 10 seconds, I spend energy like this. Distance = 1/2 at2. So I would have pushed this object for 50 metres. So I'm using...
10. ### A Invariance of Wess Zumino Action under SUSY

Hi guys, I have a very basic question about the WZ model. I want to show that it is invariant under SUSY transformations. The action is \int{d^4 x} \partial^\mu \phi* \partial_\mu \phi +i\psi^† \bar{\sigma}^\mu \partial_\mu \psi The SUSY transformations are \delta\phi = \epsilon \psi ...
11. ### B All actions are reactions

My grandson asked me this question: If in fact,as Newton said,all actions are accompanied by an equal and opposite reaction, then there can be no single actions in the universe;all actions must therefor be part of a reaction between two opposite and equal actions,which are in fact also...
12. ### Lagrangian and dimension

Dear all, If we consider the lagrangian to have both geometric parts (Ricci scalar) and also a field, the action would take the form below: $$S=\frac{1}{2\kappa}\int{\sqrt{-g} (\ R + \frac{1}{2} g^{\mu\nu} \partial_\mu \phi \partial_\nu \phi -V(\phi)\ )}$$ which are...
13. ### Connection between Planck's Constant and Action?

Hello, I've noticed that Planck's constant h and the action of a particle S both share the units of Joule-second. I was wondering if there were a connection between the two, but my Modern Physics textbook (Harris) doesn't say anything about it. Wikipedia's definition of Planck's constant says...
14. ### Capillary action at different temperatures

Hi guys, I'm doing a project at the moment revolving around capillary action and surface tension. Today I conducted an experiment to observe capillary action in a capillary tube and paper towel at different temperatures of water. I don't understand why I've got the following results: As you...
15. ### Equation of motion from the action

Hello Physics Forums! Supposing that we have an action that says: $$L=\frac{1}{2}R-g_{C\bar{D}}\partial_{\mu}z^C\partial^{\mu}\bar{z}^D+\frac{1}{4} + \frac{1}{4}ImM_{IJ}F^I_{\mu\nu}\cdot F^{J\mu\nu} +\frac{1}{4}ReM_{IJ}F^I_{\mu\nu}\cdot \tilde{F}^{J\mu\nu}$$ where...
16. ### What does it mean to "minimise" an action ?

This is with regards to the principle of least action. I understand that an action is a functional (a function where functions have values assigned to them, I believe?) When trying to figure this out, I understood minimising as finding the true path of the principle of least action. But that...
17. ### Varying the action with respect to metric

Homework Statement i want to find the variation of this action with respect to $g^{\mu\nu}$ , where $N_\mu(x^\nu)$ is unit time like four velocity and $\phi$ is scalar field. $I_{total}=I_{BD}+I_{N}$ ## I_{BD}=\frac{1}{16\pi}\int dx^4\sqrt{g}\left\{\phi...
18. ### Attempts to explain quantum entaglement

So I tried googling it but unfortunately i haven't found anything, so I though i'll take my quarries here. I would like to know, how scientists have tried to explain how quantum entanglement works. Thanks for the answers. :)
19. ### Principle of least action?

hi, please if somebody could explain why anybody would consider the "action" and is there any proof that the minimal action actually gives the correct route of a problem?