From Feynman's Cal Tech lectures
for zero mass
Spin 1 gravitons would show anti-gravity like dark energy, i.e. likes repel.
Spin 0 violates equivalence principle for gravity binding energies as a gas gets hotter. 10/3/10
Obviously since gravity is simply the local gauge theory of the de Sitter group the real question is why do we not see zero mass spin 0 and spin 1 gravity waves in addition to spin 2 gravity waves? Why do spin 0 and spin 1 get a large rest mass for example? Also why is not the spin 0 graviton the Higgs particle since it must universally couple to everything because of the equivalence principle? Can we also have spin 1 Higgs particles? Why not?
On Zielinski's question,vI agree with Rovelli. The point is that small elements of rotating water in ii are not on timelike geodesics in the local curved spacetime. They are pushed off geodesics by the friction/viscosity. Note that in rotating superfluids quantized vortices form with normal fluid cores that have viscosity. The macro-quantum coherent phase rigidity (P.W. Anderson) acts as an effective quantum potential quasi-force to push the superfluid elements off the local geodesics.
In the hologram theory, the curved spacetime of Einstein's GR is simply a retro-causal 3D + 1 image from the future event horizon hologram computer 2D + 1 screen. Admittedly this is a really crazy idea. It may be crazy enough to be true.
The fat lady has not sung yet on this idea.