If a theory of quantum gravity were found and it implied a purely virtual particle mediating gravity violates microcausality, it would have significant implications for our understanding of fundamental physics. Microcausality is a principle that states that local physical interactions should not lead to effects that propagate faster than the speed of light, ensuring that cause and effect relationships are preserved.
If a theory violated microcausality, it would challenge our fundamental understanding of causality and the structure of spacetime. However, it would not necessarily imply that causality as a whole is completely invalidated. It would suggest that our current notions of causality and the way we understand the propagation of interactions need to be revised or refined to accommodate the new insights provided by the theory.
The consequences of violating microcausality in a theory of quantum gravity would depend on the specific details of the theory and the nature of the virtual particles mediating gravity. It could potentially have implications for the stability of physical systems, the predictability of phenomena, and the behavior of particles in extreme conditions such as black holes or the early universe.
It is important to note that the search for a theory of quantum gravity is an ongoing endeavor, and at present, we do not have a complete and experimentally confirmed theory that unifies quantum mechanics and general relativity. Any discussion regarding the consequences of violating microcausality in such a theory would be highly speculative and subject to further research and investigation.