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1) Time slows-down in strong gravity field. But I cannot understand for whom (in which reference frame)

What about the paradox that time stops for an observer at earth who thus would never see the traveller disappear behind blackhole horizon. But I think that strong gravity happens around the traveller, so it is the traveller whose time shall stop! And for earth observer time is never changed b/x gravity around the observer never changes.

2) And is there time inside black-hole if time stops at the horizon - also in which reference frame we have this problem? If time stops at horizon in one reference frame than we have this problem only in this reference frame.

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  • $\begingroup$ First. I added a "general-relativity" tag and deleted "relativity". Yours is a GR question if there is one. Second. You'd better wait instead of accepting an answer too fast. You could regret later. Third. I've in mind to write an answer but don't know when I'll find time. Wait and see. $\endgroup$
    – Elio Fabri
    Mar 31, 2019 at 19:31
  • $\begingroup$ Not sure why this was down voted.... $\endgroup$
    – Rick
    Mar 31, 2019 at 21:45
  • $\begingroup$ @Rick I hope that before question is voted for closing time will be left for me to finish the answer I'm writing. $\endgroup$
    – Elio Fabri
    Apr 1, 2019 at 14:20
  • $\begingroup$ I can't understand why do they find the question unclear. To me is perfectly clear. The OP has several doubts and confusion on a subject anything but easy to understand, as answer by Arpad Szendrei shows. If question is closed that will remain the only answer. An excellent result, IMO. $\endgroup$
    – Elio Fabri
    Apr 4, 2019 at 10:12

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This is called gravitational time dilation, and is not caused by mass (contrary to popular belief) but by stress-energy. Even massless photons have gravitational effect, because they have stress-energy. The black hole has a strong gravitational field (stress-energy), but only compared to the stress-energy of Earth. The observer is on Earth, in a much weaker gravitational field (less stress0energy).

Now it is the difference between the stress-energy of the black hole and Earth that causes this time dilation.

Now the traveler sees his own clock as ticking normally. The observer on Earth sees his own clock tick normally too. It is when they try to compare their clocks that they see that the traveler's clock is ticking slower when it closes up to the black hole.

Now the traveler will not see anything unusual happen. He will cross the event horizon, just like normally, he won't even notice the event horizon (other then being spaghettified).

Now the observer will see the traveler's clock tick slower (relative to his own clock), and at the point where the traveler reaches the horizon, the observer will see the traveler's clock stopped, meaning, that the tick on the traveler's clock would be equal to infinite time on the observer's clock.

There, the traveler seems to have frozen on the event horizon (from the observer's view).

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