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The variables used in general relativity to describe the shape of spacetime. If your question is about metric units, use the tag "units", and/or "si-units" if it is about the SI system specifically.

3 votes
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85 views

Connection between the metric tensor and mass

The general expression of a line element in a space with metric tensor $g_{\mu \nu}$ is $$ds = \sqrt{ g_{\mu \nu} dX^{\mu} dX^{\nu} }$$ If we consider a curve $X^{\mu}(\tau)$ parametrised by $\tau$, t …
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0 votes
0 answers
40 views

Why does the line element expression contain only second order differential terms? [duplicate]

The general expression of the line element $ds^2$ is $$ds^2 = g_{ij}dX^{i}dX^{j},$$ where $g_{ij}$ is an element of the metric tensor. Is there a rigorous proof of why there are no terms in the expres …
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1 vote
3 answers
884 views

Difference between upper and lower indices in Einstein notation

Consider a $(2,0)$ tensor $X^{\mu \nu}$ that can be represented in matrix form by: $$X^{\mu \nu} = \pmatrix{ a & b & c & d \\ e & f & g & h \\ i & j & k & l \\ m & n & o & p}\tag{1}$$ Here $a, b, c, …
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