Uh… yeah, he’s assuming that gravity is quantum-mechanical in nature, by some mechanism or another. That in itself is a really weak assumption. Why would you even mention the alternative?
I see no justification whatsoever for concluding that (ETI: the quantization of) gravity must therefore be detectable in the weak-field limit.
I see no justification whatsoever for concluding that gravity must therefore be detectable in the weak-field limit.
Suppose we perform an experiment where, based on the measured spin value, we move some macroscopic object with detectable gravity in opposite directions. In the Newtonian background spacetime approach there is no issue with MWI, as both branches live on the same spacetime. In a full GR case, however, the spacetime itself must decohere into different branches, or else we could detect the interaction between different branches gravitationally (I don’t know if this has been tested, but it would be extremely surprising if detected). I am not sure what would the mechanism which splits the spacetime itself be, since all current QM/QFT models are done on a fixed background (ignoring ST and LQG). So presumably this requires Quantum Gravity. Yet the whole thing happens at very low energies, slow speeds and weak spacetime curvatures, so that’s why I said that this would have to be a QG effect in the weak-field limit. Of course it would only be “detectable” in a sense that if there is no gravitational interaction between branches, then the spacetime itself must decohere by some QG mechanism.
This is essentially the standard argument for why we have to quantize gravity. If the sources of the gravitational field can be in superposition, then it must be possible to superpose two different gravitational fields. But (as I think you acknowledge) this doesn’t mean that quantum mechanical deviations from GR have to be detectable at low energies.
This is essentially the standard argument for why we have to quantize gravity.
Sort of. The problem first appears because the LHS of the EFE is a classical tensor, while the RHS is an operator, two different beasts. And using expectation value of the stress energy tensor does not work that well. The cosmological constant problem does not help, either. The MWI ontology just makes the issues starker. That’s why I am surprised that Carroll completely avoids discussing it even though GR is his specialty.
Suppose we perform an experiment where, based on the measured spin value, we move some macroscopic object with detectable gravity in opposite directions. In the Newtonian background spacetime approach there is no issue with MWI, as both branches live on the same spacetime. In a full GR case, however, the spacetime itself must decohere into different branches, or else we could detect the interaction between different branches gravitationally (I don’t know if this has been tested, but it would be extremely surprising if detected).
I’m not an expert in the field, so it may be best to take what I say with a grain of salt, but:
There was some result to some experiment that showed that there were quantum fluctuations in space-time during the formation of the universe. We don’t fully understand quantum gravity, but this shows that masses moving around and messing with spacetime isn’t going to be all that different from electrons moving around and messing with the magnetic field.
It looks to me that you’re referring to the BICEP2 results published in March. If so, this is still unconfirmed, and physicists have pointed out a possible issue with some of the data that needs to be resolved. (Its still more likely than not to end up being true but shouldn’t be taken as a definite right now).
Uh… yeah, he’s assuming that gravity is quantum-mechanical in nature, by some mechanism or another. That in itself is a really weak assumption. Why would you even mention the alternative?
I see no justification whatsoever for concluding that (ETI: the quantization of) gravity must therefore be detectable in the weak-field limit.
Suppose we perform an experiment where, based on the measured spin value, we move some macroscopic object with detectable gravity in opposite directions. In the Newtonian background spacetime approach there is no issue with MWI, as both branches live on the same spacetime. In a full GR case, however, the spacetime itself must decohere into different branches, or else we could detect the interaction between different branches gravitationally (I don’t know if this has been tested, but it would be extremely surprising if detected). I am not sure what would the mechanism which splits the spacetime itself be, since all current QM/QFT models are done on a fixed background (ignoring ST and LQG). So presumably this requires Quantum Gravity. Yet the whole thing happens at very low energies, slow speeds and weak spacetime curvatures, so that’s why I said that this would have to be a QG effect in the weak-field limit. Of course it would only be “detectable” in a sense that if there is no gravitational interaction between branches, then the spacetime itself must decohere by some QG mechanism.
This is essentially the standard argument for why we have to quantize gravity. If the sources of the gravitational field can be in superposition, then it must be possible to superpose two different gravitational fields. But (as I think you acknowledge) this doesn’t mean that quantum mechanical deviations from GR have to be detectable at low energies.
Sort of. The problem first appears because the LHS of the EFE is a classical tensor, while the RHS is an operator, two different beasts. And using expectation value of the stress energy tensor does not work that well. The cosmological constant problem does not help, either. The MWI ontology just makes the issues starker. That’s why I am surprised that Carroll completely avoids discussing it even though GR is his specialty.
I can’t get past the paywall, but I think this is what Page and Geilker do in “Indirect Evidence for Quantum Gravity”.
Oh, you mean in the trivial sense that the force actually works as normally observed instead of, well, not.
I’m not an expert in the field, so it may be best to take what I say with a grain of salt, but:
There was some result to some experiment that showed that there were quantum fluctuations in space-time during the formation of the universe. We don’t fully understand quantum gravity, but this shows that masses moving around and messing with spacetime isn’t going to be all that different from electrons moving around and messing with the magnetic field.
It looks to me that you’re referring to the BICEP2 results published in March. If so, this is still unconfirmed, and physicists have pointed out a possible issue with some of the data that needs to be resolved. (Its still more likely than not to end up being true but shouldn’t be taken as a definite right now).