- #1
Haorong Wu
- 418
- 90
- TL;DR Summary
- What conditions should a physical equation satisfy so that the comma-goto-semicolon rule can be applied to it?
Recently, I am considering the wave equations of a light beam in curved spacetime. Here I have two approaches. Both start from the Helmholtz equation ##\psi^{,\mu}_{~~,\mu}=\eta^{\mu\nu}\psi_{,\mu,\nu}=0## in the Minkowski spacetime, and ##\psi## is assumed to be ##T(x,y,z)e^{ik(z-t)}##.
In the first approach, I could impose the paraxial approximation on the Helmholtz equation yielding the scalar wave equation ##2ik T_{,3}=\eta^{ij}T_{,i,j} ## where ##i## and ## j## run in the spatial coordinates. Then I write its counterpart in curved spacetime according to the comma-goto-semicolon rule yielding ##2ik T_{;3}=g^{ij}T_{;i;j} .##
In the other approach, I would first use the comma-goto-semicolon rule on the Helmholtz equation to have ##g^{\mu\nu}\psi_{;\mu;\nu}=0##. Then I express the covariant derivative by partial derivative. Along the process, the paraxial approximation is used to eliminate ##T_{,3,3}## term.
Now if I subscribe some metric to these two results, I will have inconsistent equations. In the second approach, ##g^{33}\psi_{;3;3}##, which does not appear in the first approach, will introduce some new terms. I think the second approach is correct since the Helmholtz equation is more symmetric than the scalar wave equation. This makes me wonder what condition should a equation satisfy so that I could use the comma-goto-semicolon rule to turn it into a covariant form?
BTW, when I write covariant derivative, should I write ##T(x,y,z)_{,i}## or ##T_{,i}(x,y,z)##? Also, if there are two covariant derivatives, should I write ##T_{;i;j}## or ##T_{;ij}##?
Thanks!
In the first approach, I could impose the paraxial approximation on the Helmholtz equation yielding the scalar wave equation ##2ik T_{,3}=\eta^{ij}T_{,i,j} ## where ##i## and ## j## run in the spatial coordinates. Then I write its counterpart in curved spacetime according to the comma-goto-semicolon rule yielding ##2ik T_{;3}=g^{ij}T_{;i;j} .##
In the other approach, I would first use the comma-goto-semicolon rule on the Helmholtz equation to have ##g^{\mu\nu}\psi_{;\mu;\nu}=0##. Then I express the covariant derivative by partial derivative. Along the process, the paraxial approximation is used to eliminate ##T_{,3,3}## term.
Now if I subscribe some metric to these two results, I will have inconsistent equations. In the second approach, ##g^{33}\psi_{;3;3}##, which does not appear in the first approach, will introduce some new terms. I think the second approach is correct since the Helmholtz equation is more symmetric than the scalar wave equation. This makes me wonder what condition should a equation satisfy so that I could use the comma-goto-semicolon rule to turn it into a covariant form?
BTW, when I write covariant derivative, should I write ##T(x,y,z)_{,i}## or ##T_{,i}(x,y,z)##? Also, if there are two covariant derivatives, should I write ##T_{;i;j}## or ##T_{;ij}##?
Thanks!