Gravity is the missing link between QM and GR. Matter Waves do not have gravity, do not age, and are not physical. Spacetime(gravity/age/physicality/local/phase velocity) is assigned via decoherence.
The quantum/classical boundary is the mass of 0.3 micrometers because gravity can't be automatically assigned below that (objects above this line are automatically decohered) and because that is the width it takes light to travel in one femtosecond. 0.3 micrometers isn't a unit of mass, but it is the width an object would be that has the right amount of mass.
Duality at the same instance is not a thing. A particle/wave will be one or the other for its path from A to B. What matters is if a particle will decohere in its flight. A dead stop isn't decoherence, that is wave collapse. Wave collapse does not influence what a particle will be in its flight. It's possible for a wave to make it from point A to B without being measured before the final screen. That's why it shows fringes. You don't get quantum weirdness (Superposition (not talking about superposition of states), Entanglement, Tunneling) events when it's a physical particle. They don't experience weirdness after decoherence. Only cohered waves are allowed weirdness events.
Entangled waves become physical particles at the same momment with decoherence. If they are to be physical in flight, they will be so from the beginning, no midair swaps. A measurement far after the double slit experiment shows this. Future observed matter-waves decohere before they start moving because their momentum direction triggers decoherence. (Decay of coherence)
The quantum field doesn't use the full dimension of time (or gravity) from spacetime. It doesn't have a forward time limit for being influenced by physical states (spacetime). This is the core of what measurement/observation is.
You don't use a wave function for a physical particle
A particle gets reflected by a potential well if I can describe the particle mathematically without the wave function. Duality has been assumed during physical particle flights because they can take the path of diffraction ..but that path is guided/influenced by the quantum field ..not that the particle itself is a wave.
This is the gateway to the Unified Theory. Physical particles go with GR, Unobserved Quantum Waves go with the Quantum Field. Spacetime is separate from the Quantum Field.
The Quantum Field and Spacetime are two separate realms. The Schrödinger equation is assuming the coordinates are in spacetime. The quantum field has all the properties needed to propagate a wave without spacetime. Space isn't expanding in cosmic voids beyond the Local Group, the quantum field is. Spacetime doesn't stretch/expand ..it only bends. Spacetime is everywhere, but it is not enacted everywhere. Decohered mass enacts it. We know it is not enacted everywhere because unobserved quantum waves can complete their journeys without being observed(fringes). It seems when space doesn't have mass in a region ..spacetimes' influence diminishes. Spacetime isn't bent in voids.
Nothing is physical without spacetime.