I was trying to understand Witten's proof of the Positive Energy Theorem in General Relativity by reading the original argument given by Witten. I am comfortable with the overall argument, but I would like to understand the following statement, made on the second paragraph of page 394 in the previous link:
"The only invariants that can be formed from the $1/r$ term in the metric tensor are the total energy and the total momentum."
These invariants (which I will call "$1/r$-built" for short) refer to an asymptotically flat initial value (spacelike smooth) 3-surface in spacetime.
Is there an obvious reason why this should be true? Looking at the definition of the ADM-energy and momentum, it seems plausible because the only data we have are the first and second fundamental form, and it should be possible to write any invariant as a combination of these, and consequently, any $1/r$-built invariant as a function of ADM energy and momentum. However, this reasoning is too hand-wavy, so I wonder if there is a clear cut explanation.
My interest in this fact is that although it is not really a logical step in the proof, if true it is probably one of the best ways to motivate a spinorial proof (I was thinking of something along the lines of "if we can construct a manifestly non-negative $1/r$-built invariant by means of the asymptotic behavior of spinors, then it should be possible to prove that energy is non-negative by writing this invariant as a function of ADM energy and momentum").
From a purely physical perspective, if it were true that ADM energy and momentum suffice to specify the system (to order $1/r$), then they should be the only independent invariants. However, this suggests that an asymptotic observer who knew the total energy and momentum could reconstruct the metric up to order $1/r$. I was thinking if it is not possible to construct a counterexample by defining an axisymmetric spacetime in which the Killing field of azimuthal symmetry is build up only with $1/r$ terms and showing that it is asymptotically distinguishable from its non-rotating counterpart. In this spirit, I think it is worth asking a broader version of my question:
What kinds of "physically interesting" boundary terms can appear in a Hamiltonian formulation for an asymptotically flat spacetime manifold in General Relativity?
