# Question on definition of form-invariant

## Main Question or Discussion Point

Hello everyone. I'm reading Weinberg's 'Gravitation and Cosmology' and I'm having some problems understanding the definition of a 'form-invariat function'. He says:
A metric $g_{\mu\nu}$ is said to be form-invariant under a given coordinate transformation $x\to x^\prime$, when the transformed metric $g^\prime_{\mu\nu}(x^\prime)$ is the same function of its argument $x^{\prime\mu}$ as the original metric $g_{\mu\nu}(x)$ was of its argument $x^\mu$, that is,
\begin{equation}
g^\prime_{\mu\nu}(y)=g_{\mu\nu}(y) \; \text{ for all }y.
\end{equation}
If the previous condition was true doesn't this simply mean that $g_{\mu\nu}^\prime$ is the same function as $g_{\mu\nu}$? Should we ask for:
\begin{equation}
g^\prime_{\mu\nu}(x^\prime)=g_{\mu\nu}(x)
\end{equation}
?

Thanks a lot!

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If the previous condition was true doesn't this simply mean that $g_{\mu\nu}^\prime$ is the same function as $g_{\mu\nu}$?
That is precisely what he means, that they are the same function.
I think that's why he used the symbol "y" instead of x's, so that it wouldn't be confused as having something to do with the coordinates x and x'.

Last edited:
ShayanJ
Gold Member
The problem with $g'_{\mu\nu}(x')=g_{\mu\nu}(x)$ is that it means the two functions have same value in a single point. But because $x'$ associates different numbers to that point compared to $x$, $g'_{\mu\nu}(x')=g_{\mu\nu}(x)$ necessarily means $g$ and $g'$ are different functions and don't have the same form.
But $g'_{\mu\nu}(y)=g_{\mu\nu}(y)$ means that if we give the two functions, the same numbers, they will give equal results which means they're the same function. Note that y in the right refers to a different point than y in the left because they are same numbers in different coordinate systems.

• Einj
Note that y in the right refers to a different point than y in the left because they are same numbers in different coordinate systems.
Oh I see. That is clear. Thanks!