Earlier arguments attempted to show that coupling a classical gravitational field to quantum matter would allow violations of the uncertainty relations via measurements of the gravitational field.
Uncertainty relations(as used in physics and philosophy of science)
Rules in quantum physics stating that you cannot know certain pairs of properties (like a particle's position and speed) with perfect precision at the same time—the more accurately you measure one, the less you can know about the other.
Violations of the uncertainty relations(as used in physics and philosophy of physics)
Finding a way to measure a particle's position and speed both precisely at the same time, which would break the fundamental rule that says this is impossible.
In earlier research on quantum gravity it was often supposed that if there was at least one quantum field in the world together with the gravitational field, then given the universal coupling of the gravitational field, it must follow that the quantization of the one field somehow infects the gravitational field, implying that it must necessarily have quantum properties too. The arguments basically involve the consideration of a mass prepared in a superposition of position eigenstates. If the gr