That's my point - it kind of has to be, otherwise you violate conservation of momentum, energy, and gravity.
Violating those would be a bigger deal than time travel itself.
Incidentally that's why wormholes can't actually exist - they violate all of the above.
Which is a bit tongue in cheek, but I don't see how you can speak so concretely about it without having a working time machine.
Because the laws of physics are what they are. A time machine would have to fit inside those laws.
This constrains the design of the time machine - you can't just do anything you want.
Because of that, it's possible to narrow down how a time machine would work, if it could work.
That's my point - it kind of has to be, otherwise you violate conservation of momentum, energy, and gravity.
Violating those would be a bigger deal than time travel itself.
Incidentally that's why wormholes can't actually exist - they violate all of the above.