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DanielLC comments on What false beliefs have you held and why were you wrong? - Less Wrong Discussion
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What does that mean? You can have MWI without Quantum Gravity. It just won't have any gravity.
If I had to guess, I'd say that you mean that you won't be able to get general relativity working just by doing quantum physics on a non-flat spacetime. You have to have the spacetime metric itself vary along different universes. This is true, and it seems pretty obvious. If you didn't do that, then gravity would have to be the same in all universes. But there's another universe where Earth is somewhere else, so the gravitational field obviously has to be moved.
I don't understand. GR describes the metric tensor through the Einstein's equations, relating (the) energy (tensor) and the metric tensor. If you grab yourself an empty universe, then put some stuff in it, then do the incredibly hard math (this step usually goes wrong) out you get a metric tensor. In QM the energy is given in terms of the wave-function. You claim that the observation that the earth's gravity pulls us in the general direction of the earth is inconsistent with the idea of putting the full wavefunction's energy into this equation?
If you look at Schroedinger's equation for one particle, it's easy to generalize it so that the particle is in curved spacetime. The problem is when you get entanglement involved. Normally, for n particles, you do Schroedinger's equation in R^3n, and each triplet corresponds to the coordinates of one particle. You could generalize that to M^n where M is an arbitrary manifold, but that means you'd have to use that manifold for every universe.
You could try running Einstein's field equations on the 3n+1-dimensional configuration space (+1 being time, not an extra space dimension) and running Schroedinger's equation on that. I don't know if that would work. If it does, you didn't get MWI without quantum gravity. You discovered quantum gravity.