How come gravity is $\mathcal{N}=8$? Why is graviton spin 2

Question

I never understood why supergravity $\mathcal N=8$, nor why the spin of a graviton is 2.

I've been reading around but I still don't have a back-of-the-envolope understanding.

This post imported from StackExchange Physics at 2014-08-10 19:49 (UCT), posted by SE-user user47548

Comments

Hi, welcome to this site. I don't think these two questions are related. If so, could you please split them up in two separate questions? Also, the question about the graviton has been asked before, see for instance this question and this in general.

This post imported from StackExchange Physics at 2014-08-10 19:49 (UCT), posted by SE-user Hunter

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Actually they are related - it takes 8 supersymmetry transformations to go from the spin=+2 graviton to the spin=-2 graviton.

This post imported from StackExchange Physics at 2014-08-10 19:49 (UCT), posted by SE-user Mitchell Porter

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Regarding the graviton, see my answer physics.stackexchange.com/q/108230

This post imported from StackExchange Physics at 2014-08-10 19:49 (UCT), posted by SE-user JamalS

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On the $\mathcal N = 8$ part: This is just the maximal supergravity. There can be no more than 32 supercharges, because more supercharges would necessarily introduce higher spins than 2 into the theory - and we don't know how to deal with spins larger than 2. Smaller supergravities exist, though, see e.g. mSuGra

This post imported from StackExchange Physics at 2014-08-10 19:49 (UCT), posted by SE-user Neuneck