Sunday, 27 December 2015

Demostrating possible equivalence of two tensors


Is there anyway to see by inspection that a form like a(x2)3(gμσxρxν+gμρxσxν+gνσxρxμ+gνρxσxμ) may be equivalent to (i.e reduced down to or reexpressed) b(gμνxρxσ+gρσxμxν)(x2)3? where gμν is the metric tensor (diagonal).


I have tried to put in various permutations of μνρσ and from 1111 and 2222 for example, I obtained the constraint that a/b=2 but I am not really sure what this means. If I try the combination 1221 e.g then it implies b=0, which seems to contradict my first result.


Does this mean that the two forms are not equivalent?



Answer




Indeed, if no values of a and b work for across different sets of indices, then the forms are not equivalent.


In fact, these two forms are not equivalent even under the restriction of the metric being diagonal (and thus are not equivalent under a general metric). The diagonal case is easy to analyze, and you gave a good set of indices to do it: μνρσ=1221. Then the large parenthesized part of the first expression becomes g11x2x2+g22x1x1. The middle terms drop out because they involve off-diagonal parts of the metric. However, both terms in the second expression also involve off-diagonal metric coefficients, so the second expression is identically 0.


Given tensor components Tμνρσ and Sμνρσ, we have T1221=g11x2x2+g22x1x1 while S1221=0, so clearly T∝̸.


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