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We know rotation period of earth (stellar day) is 86164.098 903 691 seconds of mean solar time.

At 0m time is supposed to be more dilated than at 5000m because of gravitational time dilation.

Then hypothetically we count less nanoseconds at 0m relative to 5000m of altitude.

Then assuming this is true, stellar day is shorter at 0m than at 5000m.

Doesn't it mean we have a perception of a quicker rotation of earth at 0m than at 5000m? A quicker rotation in a dilated frame? Does it make sense?


If you agree that rotation period of earth (stellar day) is the same at all altitudes, why isn't it affected by time dilation?

Copernic
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  • I don't know if this is what you have in mind, but can I suggest an interpretation of your question. If you take a rigid sphere rotatingly slowly does the variation of time dilation with $r$ cause a twisting force in it? If we assume the rotation is slow enough that the Schwarzschild interior metric is a good approximation then I thin this can be answered just by calculating the four-acceleration and then checking to see if $a^\phi$ varies with $r$. – John Rennie Dec 11 '16 at 16:08
  • Maybe it could help to see what you get. My argumentation about a twisting earth is that a proper time of ~24 hours of rotation at 5000m will lead you further (another angle relative to your starting position) than a proper time of ~24 hours of rotation at 0m by applying relativity. – Copernic Dec 11 '16 at 16:39
  • Because earth rotation period is synchronized, proper times should count both ~24 hours for stellar day. If we see a difference, it is because of the twist. – Copernic Dec 11 '16 at 16:47
  • "Albeit, it doesn't twist." would say a new Galileo. – Copernic Dec 11 '16 at 17:00
  • I disagree with that "But observers at different r will disagree on the value of the angular velocity." How can the value of angular velocity be quicker in a dilated frame??? Values of movements are supposed to be equal or slower in a dilated frame. – Copernic Dec 11 '16 at 17:23

2 Answers2

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This is a fun calculation to do so let's have a go. What we need to do is calculate the time dilation for an observer rotating with the Earth and see how it changes with height.

To do this we start with the spacetime geometry near the Earth, which is described (approximately) by the Schwarzschild metric:

$$ c^2d\tau^2 = \left(1-\frac{2GM}{c^2r}\right)c^2dt^2 - \frac{dr^2}{1 - \frac{2GM}{c^2r}} - r^2\left(d\theta^2 + \sin^2\theta d\phi^2\right) \tag{1} $$

$\tau$ is the time recorded by a clock carried by our rotating observer, and $t$ is the time recorded by a clock carried by an observer far enough from the Earth for the Earth's gravity to be negligible. The time dilation is then:

$$ \text{time dilation} = \frac{d\tau}{dt} $$

so that's what we are going to calculate.

We'll consider an observer who is stationary at the equator so $\theta=\pi/s$ and $d\theta=0$, and at a distance $r$ from the centre of the Earth. The observer isn't moving radially inwards or outwards so $dr=0$. If we put this lot into equation (1) it simplifies to:

$$ c^2d\tau^2 = \left(1-\frac{2GM}{c^2r}\right)c^2dt^2 - r^2d\phi^2 \tag{2} $$

We need to eliminate $d\phi$ and we do this by noting that if $\omega$ is the angular velocity with which the Earth rotates then:

$$ \frac{d\phi}{dt} = \omega $$

and therefore:

$$ d\phi = \omega dt $$

And we can substitute for $d\phi$ in equation (2) to get:

$$ c^2d\tau^2 = \left(1-\frac{2GM}{c^2r}\right)c^2dt^2 - r^2\omega^2dt^2 $$

And this gives us the equation we want for the time dilation:

$$ \frac{d\tau}{dt} = \sqrt{1-\left(\frac{2GM}{c^2r} + \frac{r^2\omega^2}{c^2}\right)} \tag{3} $$

And equation (3) is the result we need. As we move upwards, i.e. in the direction of increasing $r$, then the term $2GM/c^2r$ decreases and the term $r^2\omega^2/c^2$ increases, and the question is what happens to the sum:

$$ T = \frac{2GM}{c^2r} + \frac{r^2\omega^2}{c^2} $$

If $T$ increases with height (increasing $r$) then time dilation is increasing as we go up while if $T$ decreases with height then time dilation is decreasing as we go up. To find out what happens we just differentiate with respect to $r$:

$$ \frac{dT}{dr} = -\frac{2GM}{c^2r^2} + \frac{2r\omega^2}{c^2} \tag{4} $$

The radius of the Earth at the equator is $r \approx 6378000$m and the angular velocity is $2\pi$ radians in 24 hours so $\omega \approx 7.272 \times 10^{-5}$ radians per second. Put these values into equation (4) and we get:

$$ \frac{dT}{dr} \approx -2.176 \times 10^{-16} + 7.496 \times 10^{-19} \approx -2.169 \times 10^{-16} $$

And there's your answer. At the Earth's surface the time dilation for an observer rotating with the Earth decreases with height i.e. time runs faster as you move upwards. The Earth takes longer to rotate for the higher observer.

John Rennie
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Does earth rotate in a shorter time at 0m than at 5000m?

No. If it did, it would be twisting, and it isn't. The Earth's rate of rotation relative to the fixed stars is the same at all elevations. It takes a day for a point at sea level to go round full circle, and it takes the same time for a point at 5000m to go round full circle.

We know rotation period of earth (stellar day) is 86164.098 903 691 seconds of mean solar time. At 0m time is supposed to be more dilated than at 5000m because of gravitational time dilation.

It is. Clocks go slower when they're lower.

Then hypothetically we count less nanoseconds at 0m relative to 5000m of altitude.

Not just hypothetically. See the interview with David Wineland of NIST where he's talking about optical clocks: "If one clock in one lab is 30 centimeters higher than the clock in the other lab, we can see the difference in the rates they run at".

Then assuming this is true, stellar day is shorter at 0m than at 5000m.

Yes, though it's a technicality. Your clocks run slower when you're lower, so your measurement is different. Meanwhile the Earth turns in its own sweet time at the same rate at all elevations.

Doesn't it mean we have a perception of a quicker rotation of earth at 0m than at 5000m? A quicker rotation in a dilated frame?

Yes it does. But it's our perception. The Earth isn't really rotating quicker at 0m. Besides, the difference we perceive is very slight. Gravitational time dilation at the surface of the Earth is only about one part in 10-9. We hardly notice it, though we do need to take account of it in our GPS satellites. It's much more significant for a neutron star. A clock at the surface of a neutron star would be going at circa 0.8 times the rate of a clock at the surface of the Earth.

John Duffield
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  • Thank you, then you have the same view as John (follow the comments). If every event in a dilated time frame happens quicker. Time is dilated and events happen quicker relative to a non dilated frame, don't we fall in an euclidean space instead of a pseudo-euclidean one (All measures remain the same.)? – Copernic Dec 10 '16 at 18:18
  • You wrote "The Earth's RATE of rotation relative to the fixed stars is the same at all elevations". "Clocks go slower when they're lower." So, is the period the same or not? I think you mean earth rotation period is not the same, but it turns the same (by a synchronized rotation), right? – Copernic Dec 10 '16 at 18:56
  • All of this has to do with how you look at it. If time varies then so does everything else including light speed. – Bill Alsept Dec 10 '16 at 21:13
  • And what about proper time? Relativity, covariance and equivalence principles? – Copernic Dec 10 '16 at 21:39
  • @Copernic : we fall in space that is "neither homogeneous nor isotropic", our measurements don't stay the same, even if sometimes we think they do. Yes, the period of the Earth's rotation is the same at all elevations, but the period of our clocks is not. Proper time is just how much motion occurred inside your clock, see this question about time. – John Duffield Dec 11 '16 at 13:48
  • So, you are telling me twins age the same way (same cell life period), but disagree on how much time they have. – Copernic Dec 11 '16 at 15:00
  • @Copernic : I'm sorry, I don't know what you mean. If it helps at all see this about time dilation. – John Duffield Dec 11 '16 at 15:23
  • I mean stellar day is a kind of clock (like an atom clock movement is for very small things). The period of earth rotation (stellar day) show us that there is no time dilation. – Copernic Dec 11 '16 at 15:27
  • Beam a sun light every ~24 hours from stars and you have a giant "planet clock". – Copernic Dec 11 '16 at 15:33