# Conserved quantity of Spactime Diffeomorphism Group

So it's my understanding that the underlying symmetry of GR is the Diffeomorphism Group of 3+1 spacetime.

It's also my understanding that a symmetry implies a corresponding conserved quantity in a physical theory.

Then, if my understanding is correct, there should be a conserved quantity associated with the Diffeomorphism Group of 3+1 spacetime. What is it?

• What do you mean across reference frames? The energy of given geodesic is something that will be a geometric invariant--all observers will agree on its value. Oct 1, 2014 at 15:53
• Charge is conserved, so is angular momentum, isn't it? Oct 1, 2014 at 15:54
• Re question v2: diffeomorphism invariance of the action integral is the subject of the 2nd Noether theorem. The conservation statements of this theorem are the twice contracted Bianchi identities. See this PDF pages 14 and 15 for the gory details. Oct 1, 2014 at 16:29
• I think John Reannie answered my question. Oct 1, 2014 at 17:11
• It seems like this question attracted 2 answers, and then the question was completely rewritten...?
– user4552
Oct 24, 2019 at 20:20

A conservation law says that something is neither created nor destroyed but only moves around as the system evolves. Since time is just a part of spacetime, in relativity this amounts to the vanishing of a covariant divergence: $\nabla_\mu Q^\mu = 0$ for some $Q$ (which may have other components). All the conservation laws you know and love are conserved in this local sense in GR: particle number density in a fluid ($Q^\mu = nu^\mu$ with $u^\mu$ the components of the fluid's 4-velocity, assuming there is no annihilation/production), rest mass density ($Q^\mu = \rho u^\mu$, again under that assumption), energy and momentum ($Q^\mu = T^{\mu\nu}$, with no further assumptions needed), etc.
$${T^{\mu\nu}}_{;\nu} =0\ .$$