There has been some confusion on how the time turners work and whether they are compatible with relativity.

This comment is meant to explain the simplified mechanics of it, as outlined in the User Manual.

Time turner keeps track of its world line for up to 6 hours back. When activated, it creates a branch of the whole Universe inside the past lightcone of the spacetime point on that world line and transports the wearer to that branch.

FAQ:

• Q. Why is my time turner limited to just 6 hours? A. Time turner has to keep track of your personal past well enough to spawn a completely new copy of the universe seamlessly and instantly. This is a lot of information to keep track of, a spacetime volume of roughly 13.6 billion light year^3*year. Your time turner keeps has a perfect snapshot of the current state of the Universe, and it contains a sophisticated magical firmware to extrapolate what this state had been up to 6 hours prior (proper time in the time turner's reference frame). This is done by simulating the universe backwards, which runs into the standard thermodynamical limitations. The hard limit of 6 hours was put in place to prevent the reconstructed copy of the universe from being significantly different from the user's subjective experience.

• Q. Why does my time turner create a new branch of the universe instead of modifying the existing branch? A. Unfortunately, the laws of General Relativity prohibit any true modification of the past. The GR RFC specifies a unique metric (and therefore unique matter content) for each spacetime point. Any attempt to have two or more copies of the same object at the same point in spacetime would be in violation of the RFC, and so is not supported by your time turner.

• Q. What happens to the original universe after I activate my time turner? A. The original branch still persists as if no time turner had been activated. However, since there is no known way to communicate between parallel universes, you do not need to worry about anything that happens to the copy of you and anyone you failed to save from a certain death by activating your time turner.