- #1
Emilie.Jung
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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 $$\tilde{F}^I_{\mu\nu}=\frac{1}{2}\epsilon_{\mu\nu}^{\mu'\nu'}F^I_{\mu'\nu'}$$
If I want to find the equation of motion of this, I know I can start from action and use the Euler Lagrange equation or directly use d*F=0 (Maxwell's equation).
This is one of the very rare times where I encounter general relativity as I am still a second year physics student. I have seen this sort of action (but way simpler) in my classical mechanics course) where we almost always use the Euler-Lagrange equation or vary the action with respect to the functional term and set it equal to zero and VOILA! you get the equation of motion you need. Here though I am finding a little of difficulty. The thing is that what was found for this action was the following answer:
$$d(ReM_{IJ}F^J+ImM_{IJ}\tilde{F}^J)=0.$$
I have tried so manyy ways to derive this but failed at it.
First, I tried to do it in the general way thinking I would avoid mistakes so I started by transforming the ##\tilde{F}^{\mu\nu}## to ##\frac{1}{2}\epsilon^{\rho\sigma\mu\nu}F_{\rho\sigma}## to get rid of the tilde and then deriving the action with respect to ##F^{\kappa\lambda}## and then set it equal to zero. BUT the procedure seems to take forever and I am not getting the answer above(It is too long I couldn't write it down, but if it matters I could type few of the details). I guess there is a better track to take, but I have no idea what it might be. On some websites, I found that instead of ##\partial^{\mu}F_{\mu\nu}## that we usually use in Classical Mechanics, there is now a G term, for example they write the maxwell equation as ##\partial^{\mu}G_{\mu\nu}## or something like that. You might relate to this better than I am. I am only trying to tell you what I have searched for before finally creating an account on this forum.
I am trying to practice those from the web in a spontaneous manner as my professor said she will bring us a bonus part in the coming exam that has to do with general relativity after she taught us a little of einstein indices operations. I know she might not bring this tough level but I am curious to know how things work in General Relativity and why if I followed the very classical way, I am not getting the answer above for the equation of motion? Any advice will be useful.
Yours,
Emilie.
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 $$\tilde{F}^I_{\mu\nu}=\frac{1}{2}\epsilon_{\mu\nu}^{\mu'\nu'}F^I_{\mu'\nu'}$$
If I want to find the equation of motion of this, I know I can start from action and use the Euler Lagrange equation or directly use d*F=0 (Maxwell's equation).
This is one of the very rare times where I encounter general relativity as I am still a second year physics student. I have seen this sort of action (but way simpler) in my classical mechanics course) where we almost always use the Euler-Lagrange equation or vary the action with respect to the functional term and set it equal to zero and VOILA! you get the equation of motion you need. Here though I am finding a little of difficulty. The thing is that what was found for this action was the following answer:
$$d(ReM_{IJ}F^J+ImM_{IJ}\tilde{F}^J)=0.$$
I have tried so manyy ways to derive this but failed at it.
First, I tried to do it in the general way thinking I would avoid mistakes so I started by transforming the ##\tilde{F}^{\mu\nu}## to ##\frac{1}{2}\epsilon^{\rho\sigma\mu\nu}F_{\rho\sigma}## to get rid of the tilde and then deriving the action with respect to ##F^{\kappa\lambda}## and then set it equal to zero. BUT the procedure seems to take forever and I am not getting the answer above(It is too long I couldn't write it down, but if it matters I could type few of the details). I guess there is a better track to take, but I have no idea what it might be. On some websites, I found that instead of ##\partial^{\mu}F_{\mu\nu}## that we usually use in Classical Mechanics, there is now a G term, for example they write the maxwell equation as ##\partial^{\mu}G_{\mu\nu}## or something like that. You might relate to this better than I am. I am only trying to tell you what I have searched for before finally creating an account on this forum.
I am trying to practice those from the web in a spontaneous manner as my professor said she will bring us a bonus part in the coming exam that has to do with general relativity after she taught us a little of einstein indices operations. I know she might not bring this tough level but I am curious to know how things work in General Relativity and why if I followed the very classical way, I am not getting the answer above for the equation of motion? Any advice will be useful.
Yours,
Emilie.