Equations Using Comma-Goto-Semicolon Rule in Curved Spacetime

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In summary, the conversation discusses two approaches for considering the wave equations of a light beam in curved spacetime. The first approach uses the paraxial approximation on the Helmholtz equation, while the second approach uses the comma-goto-semicolon rule to express the covariant derivative. The speaker believes the second approach is correct, but wonders about the conditions for using the comma-goto-semicolon rule. They also question the notation for writing covariant derivatives. It is noted that the "comma-to-semicolon" rule is not always clear-cut, as seen in the example of the Maxwell equation.
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Haorong Wu
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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!
 
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Haorong Wu said:
Also, if there are two covariant derivatives, should I write T;i;j or T;ij?
I observe ##:i:j## is used e.g. ##T_{:i:j}## in Dirac's text I have.
 
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The "comma-to-semicolon"/##\partial##-to-##\nabla## rule is not always clear-cut. To give another example, consider the Maxwell equation ##\partial_{\mu} F^{\mu \nu} = 4\pi j^{\nu}##, which in terms of the vector potential reads\begin{align*}
\partial_{\mu} \partial^{\mu} A^{\nu} - \partial_{\mu} \partial^{\nu} A^{\mu} &= 4\pi j^{\nu} \ \ \ (\dagger) \\ \overset{\mathrm{curved \ spacetime}}{\implies} \nabla_{\mu} \nabla^{\mu} A^{\nu} - \nabla_{\mu} \nabla^{\nu} A^{\mu} &= 4\pi j^{\nu} \ \ \ (\mathrm{a})
\end{align*}On the other hand, since ##\partial_{\mu} \partial^{\nu} = \partial^{\nu} \partial_{\mu}## then one can re-write ##(\dagger)## as\begin{align*}
\partial_{\mu} \partial^{\mu} A^{\nu} - \partial^{\nu} \partial_{\mu} A^{\mu} &= 4\pi j^{\nu} \\ \overset{\mathrm{curved \ spacetime}}{\implies} \nabla_{\mu} \nabla^{\mu} A^{\nu} - \nabla^{\nu} \nabla_{\mu} A^{\mu} &= 4\pi j^{\nu} \ \ \ (\mathrm{b})
\end{align*}If one defines a "curvature" operator ##\nabla_{\mu} \nabla^{\nu} - \nabla^{\nu} \nabla_{\mu} \equiv {R^{\nu}}_{\mu}## then one can re-write this last equation as\begin{align*}
\nabla_{\mu} \nabla^{\mu} A^{\nu} - \nabla_{\mu} \nabla^{\nu} A^{\mu} + {R^{\nu}}_{\mu} A^{\mu} &= 4\pi j^{\nu} \ \ \ (\mathrm{c})
\end{align*}The equations ##(\mathrm{a})## and ##(\mathrm{c})## differ by this term ##{R^{\nu}}_{\mu} A^{\mu}##.
 
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FAQ: Equations Using Comma-Goto-Semicolon Rule in Curved Spacetime

1. What is the Comma-Goto-Semicolon Rule in Curved Spacetime?

The Comma-Goto-Semicolon Rule is a mathematical rule used in the study of curved spacetime, which is a concept in the theory of general relativity. It states that when writing equations involving curved spacetime, the comma, goto, and semicolon symbols must be used in a specific way to ensure the equations are correct.

2. Why is the Comma-Goto-Semicolon Rule important in the study of curved spacetime?

The Comma-Goto-Semicolon Rule is important because it allows for the proper notation and manipulation of equations involving curved spacetime. Without this rule, the equations would not accurately represent the effects of gravity and other phenomena in curved spacetime.

3. How is the Comma-Goto-Semicolon Rule applied in equations?

The Comma-Goto-Semicolon Rule is applied by placing the comma symbol between two terms that are being multiplied, the goto symbol between two terms that are being divided, and the semicolon symbol between two terms that are being differentiated. This ensures that the correct operations are performed on the correct terms in the equation.

4. Can the Comma-Goto-Semicolon Rule be applied to any equation involving curved spacetime?

Yes, the Comma-Goto-Semicolon Rule can be applied to any equation involving curved spacetime, as long as the equation is written in a proper notation. This rule is a fundamental aspect of working with curved spacetime and is essential for accurate calculations and predictions.

5. Are there any exceptions to the Comma-Goto-Semicolon Rule in curved spacetime equations?

There are some exceptions to the Comma-Goto-Semicolon Rule, such as in certain special cases where the curvature of spacetime is negligible. In these cases, the rule may not need to be applied, but it is still important to understand and follow this rule in general to ensure accuracy in calculations.

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