the old school paradigm tried to make everything work out by letting time slip. You can ride on a photon, in your imagination, and look at the wavelengths of other photons. Another person can ride on another photon and look at you. Supposedly. What you see, or measure, in the other photon depends on you and your photon. But what if a third person is standing still and looking at both of you. . . . . you end up having to theorize that time is changing at different rates in different systems. But the stuff inside the said systems doesn't have any way or knowing who's watching, or where from, and how "time" has to be stretched or compressed to comply with the observer's notions of physical laws.
the conclusion is that whatever the speeds of systems, the speeds of other systems, or the the presence or absence of observers taking measurements. the time is the same everywhere. No such thing as time travel. No path dependency, no way to navigate around the universe and get to some other "time".
