# Which observer can detect the Gravitational Time Dilation?

We say clocks tick slowly in the strong gravitational fields. Is this stated from the perspective view of the one located in a weaker field or the one who is standing in that strong gravitational field? In Special Relativity the time dilation of clocks is said from the view of observers constrained to the other inertial frames and no observer can judge the rate of clock ticks in his own inertial frame.

• the view "clocks tick slowly in the strong gravitational fields" is true only in special cases, like our solar system. But in GR such claim might not even make any sense, f.e. you cannot compare clocks outside of black hole with clocks inside. Nov 29, 2019 at 8:26
• @Umaxo You may not be able to compare clocks outside of black hole with clocks inside, but you can certainly compare clocks at different distances from a black hole. Nov 29, 2019 at 23:40
• @D.Halsey if those distances are bound to be outside of event horizon. I am also not sure how would it work with rotating black hole - wheter the frame dragging still allows some kind of reasonable and objective comparison... Anyway, as i said, you can compare clocks in special cases, I just wanted to clarify that such statement is not general. Nov 30, 2019 at 6:29

## 2 Answers

If I've understood correctly, there is by definition nothing an observer can say about the "slowness" of their own clock. Therefore all statements about an observers clock slowing down must be relative to another observer who is not in a similar situation. In this case it means that when an observer without a gravitational field observes a clock in a strong gravitational field, they find that the clock is ticking more slowly. An observer right next to the clock never sees any slow down. More informally this can be thought to be because the observer itself is also slowed down. EDIT: As it occurs in the comments, the last sentence is not a good way go thinking about things, as it would imply an observer slowing down compared to some global clock. While this can be the case in gravitation induced slowing, the mind set cannot be applied in moving inertial frames.

• just to clarify - "More informally this can be thought to be because the observer itself is also slowed down." in STR such statement is wrong, due to relativity principle. Nov 29, 2019 at 8:12
• Although I understand that that sentence is not very scientific or nice way to think about the matter, but can you elaborate why is it wrong? Do you mean because I should not draw a distinction between an observer and the clock at the same point, or what is your point? Nov 29, 2019 at 8:28
• when you have two inertial observers moving with respect to each other, then both can claim that the other is slowed down and both views are equally valid. Your sentence tries to give some objective meaning to slowness, which can be made in GR for certain type of gravitational fields, but not in STR, no matter how informal you are trying to be. Nov 29, 2019 at 8:39
• Well yes, you are actually right! Thank you for the explanation, I think I'll edit a note into the answer based on this. Nov 29, 2019 at 8:42

In general, a Schwarzschild observer, who is away from the effects of the gravitational field, deduces that time dilates for the clocks inside a G-field. Although any other observers located at distances greater from the G-mass than the clock can detect different time dilations considering their distances from the G-mass and from the clock, a Schwarzschild observer is a more formal one.