We give a mathematical uniqueness theorem which in particular shows that
symmetric tensors in general relativity are uniquely determined by their
monomial functions on the light cone. Thus, for an observer to observe a tensor
at an event in general relativity is to contract with the velocity vector of
the observer, repeatedly to the rank of the tensor. Thus two symmetric tensors
observed to be equal by all observers at a specific event are necessarily equal
at that event.Comment: arXiv admin note: substantial text overlap with arXiv:0903.522