- #1
kawillzocken
- 2
- 0
i have a mathematical question which is quite similar to one asked before, still a bit different
https://www.physicsforums.com/threa...agrangian-density-for-real-k-g-theory.781472/the first term of KG-Lagrangian is: [itex]\frac{1}{2}(\partial^{\mu} \phi)(\partial_{\mu} \phi)[/itex]
when i try do find [itex]\frac{\partial L}{\partial(\partial^{\mu}\phi)}[/itex], i have two different options, where of course only one can be right.
1) [itex]\partial^{\mu} \phi[/itex] and [itex]\partial_{\mu} \phi[/itex] are different things, so one gets: [itex]\frac{\partial L}{\partial(\partial^{\mu}\phi)} = \frac{1}{2}\partial_{\mu} \phi[/itex]
2) [itex]\partial_{\mu} \phi = \eta_{\mu \nu} \partial^{\nu} \phi \rightarrow \frac{\partial L}{\partial(\partial^{\mu}\phi)} = \frac{\partial}{\partial(\partial^{\mu}\phi)}(\frac{1}{2}\eta_{\mu \nu}(\partial^{\nu} \phi)(\partial^{\mu} \phi)) = \partial_{\mu} \phi[/itex]
i am confused. several sources tell me that 2) is right, but my understanding of partial derivative tells me to do 1).
as i think of it, one may not simply put equations into others when doing the partial derivative, because it then changes. like z(x, y(x)) = 2x + y(x). partial derivative with respect to x gives 2, but it changes when i put in y(x).
help please :)
thank you
https://www.physicsforums.com/threa...agrangian-density-for-real-k-g-theory.781472/the first term of KG-Lagrangian is: [itex]\frac{1}{2}(\partial^{\mu} \phi)(\partial_{\mu} \phi)[/itex]
when i try do find [itex]\frac{\partial L}{\partial(\partial^{\mu}\phi)}[/itex], i have two different options, where of course only one can be right.
1) [itex]\partial^{\mu} \phi[/itex] and [itex]\partial_{\mu} \phi[/itex] are different things, so one gets: [itex]\frac{\partial L}{\partial(\partial^{\mu}\phi)} = \frac{1}{2}\partial_{\mu} \phi[/itex]
2) [itex]\partial_{\mu} \phi = \eta_{\mu \nu} \partial^{\nu} \phi \rightarrow \frac{\partial L}{\partial(\partial^{\mu}\phi)} = \frac{\partial}{\partial(\partial^{\mu}\phi)}(\frac{1}{2}\eta_{\mu \nu}(\partial^{\nu} \phi)(\partial^{\mu} \phi)) = \partial_{\mu} \phi[/itex]
i am confused. several sources tell me that 2) is right, but my understanding of partial derivative tells me to do 1).
as i think of it, one may not simply put equations into others when doing the partial derivative, because it then changes. like z(x, y(x)) = 2x + y(x). partial derivative with respect to x gives 2, but it changes when i put in y(x).
help please :)
thank you