It means we don't know how to quantize GR. Unlike the standard model of particle physics, it's a classical theory, i.e. a set of equations which tell you determ

It means we don't know how to quantize GR. Unlike the standard model of particle physics, it's a classical theory, i.e. a set of equations which tell you deterministically, without any of the inherent uncertainty of QM, how a given initial configuration will evolve. This becomes a problem when you try to include GR in even simple experimental setups which expose QM's non-classical behavior, like the Stern-Gerlach experiment [1]. What does gravity do before the path chosen by the particle (which is gravitationally attractive) is measured? [1] https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experime...