I agree that if you allowed to screen a fixed amount of energy from GR by magic, you don’t have to have negative energy, as you can add an arbitrary and undetectable amount of it to whatever object you want. But then why stop there, GR is already broken.
GR was (apparently) broken by changes in mass; suppose instead that animagi convert between magic, energy, and mass in an analogous manner: When in a smaller form, they are more magical, and vice versa. Brooms convert magic to a combination of energy and/or something which is anti-mass but not anti-inertia, and/or something else which is not directly analogous to anything in GR.
The time turner then remains no harder than apparation combined with future prediction; future prediction is easy enough that it’s provided in novelty drinks.
I agree that if you allowed to screen a fixed amount of energy from GR by magic, you don’t have to have negative energy, as you can add an arbitrary and undetectable amount of it to whatever object you want. But then why stop there, GR is already broken.
GR was (apparently) broken by changes in mass; suppose instead that animagi convert between magic, energy, and mass in an analogous manner: When in a smaller form, they are more magical, and vice versa. Brooms convert magic to a combination of energy and/or something which is anti-mass but not anti-inertia, and/or something else which is not directly analogous to anything in GR.
The time turner then remains no harder than apparation combined with future prediction; future prediction is easy enough that it’s provided in novelty drinks.