gravitational forces are created by objects with mass moving through a 3 dimensional plane over the 4th dimension which is time ie "space/time" objects with more gravitational forces affect time more so than others. space/time is proven to exist as when we send satellites into our atmosphere time is a few ms different in speed on those satellites which in order to get a correct gps location people here on earth have to calculate for the time variance between the two.

that being said time travel is theorized in one way using this method. even though it deals with a black hole and impossible situations one way time travel has been theorized would be to get so close as to be within orbit of the black hole for a slight amount of time rotating around it at near the speed of light thus increasing your own gravitational forces and increasing the speed of time for all objects surrounding you.

(they've already covered the fact that center of gravity exists in virtually every object known to man)

where physics start to break down is on the sub molecular level, particles don't seem to react the way that physics would suggest which means physicists have to come up with a new method of physics that explains the behavior of particles on that level.