On the Origin of Gravitational Lorentz Covariance
Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley(Submitted on 3 May 2013)
We provide evidence that general relativity is the unique spatially covariant effective field theory of the transverse, traceless graviton degrees of freedom. The Lorentz covariance of general relativity, having not been assumed in our analysis, is thus plausibly interpreted as an accidental or emergent symmetry of the gravitational sector. . . . Lorentz covariance is a central pillar of the modern field-theoretic interpretation of general relativity (GR). From this point of view, GR is no more and no less than the unique Lorentz covariant theory of an interacting massless spin-2 particle. In this paper, we show that GR can be derived without assuming Lorentz covariance. Our approach relies on the weaker assumption of spatial covariance, within the context of the effective field theory of the transverse, traceless graviton degrees of freedom[.]
The massless spin-2 graviton, while hypothetical and never directly observed, has strong theoretical support for this reason. No other hypothetical particle is so widely believed to really exist.