Physics Asked by Rahul S. on January 29, 2021
So I was reading through Becker, Becker, Schwarz and there is a line in the second chapter that states that $h^{alpha beta} = (h_{alpha beta})^{-1}$ where $h_{alpha beta}$ is defined as:
$$h_{alpha beta} = frac{partial X^{mu}}{partial sigma^{alpha}} frac{partial X^{nu}}{partial sigma^{beta}} g_{mu nu}$$
where $X^{mu}$ is our coordinates on our spacetime manifold, $sigma^{alpha}$ is our coordinates on our worldsheet, and $g_{mu nu}$ is our spacetime metric. This seems very natural given that $h_{alpha beta}$ is precisely the induced metric on the worldsheet and for metrics on our spacetime $g_{mu nu}^{-1} = g^{mu nu}$. However, I am having a hard time proving this. Namely,
$$h_{alpha beta}h^{alpha gamma} = frac{partial X^{mu}}{partial sigma^{alpha}} frac{partial X^{nu}}{partial sigma^{beta}} g_{mu nu}frac{partial X^{mu’}}{partial sigma_{alpha}} frac{partial X^{nu’}}{partial sigma_{gamma}} g_{mu’ nu’}$$
which doesn’t look like it will yield $delta_{beta}^{gamma}$. I have tried fiddling with the algebra with no avail…
As you have stated, $h_{alphabeta}$ is the induced metric on the worldsheet, which we obtain by acting with what are analogous to projection operators on the metric.
This is a metric in its own right, and by definition the inverse in the matrix sense is $h^{alphabeta}$, such that, in $d$ dimensions, $h^{alphabeta}h_{alphabeta} = mathrm{Tr} , mathbb I = d$.
It should be noted that $h$ is also often used for the fundamental form in most differential geometry literature which whilst related to the induced metric, carries spacetime indices still. The definition most useful to physicists, but not the most general, is that it is given by,
$$h_{munu} = g_{munu} pm n_mu n_nu$$
where $n_mu$ are the unit normals and the sign depends on if they are spacelike or timelike. This one is not as often used in string theory, but it is in general relativity, and I am pointing it out here so you don't get confused between the two.
Answered by JamalS on January 29, 2021
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