time

[jerryyu]

a long time ago i read something about the fact that the North/South poles are a second slower or faster(?) than lands located about the equator. It suddenly just re-occurred to me and i am just wondering if it's true or not and (if it's true) why is it slower or faster?

[swansont]

a long time ago i read something about the fact that the North/South poles are a second slower or faster(?) than lands located about the equator. It suddenly just re-occurred to me and i am just wondering if it's true or not and (if it's true) why is it slower or faster?

 

 

Turns out not to be true. If the earth were a rigid sphere, the poles would run slightly faster than the equator, as the equator would run slow owing to the dilation from its motion. This is a prediction in Einstein's 1905 paper on relativity. However, the Earth is not a rigid sphere — it's oblate from the rotation, and on the geoid (the idealized sea-level surface) the deformation is exactly the right amount for the gravitational time dilation to compensate. So time on the geoid runs at the same rate. If you are stationary, you only have to compensate for gravitational effects for not being on the geoid, i.e. elevation, or, if you have a very good clock, earth tide effects.

[imatfaal]

Swansont - could you answer a follow up? Is the fact that the reduction of the gravitation potential dilation balances the increased relative velocity dilation a 'lucky' coincidence or is there some deeper connexion that causes this to be so?

[swansont]

Swansont - could you answer a follow up? Is the fact that the reduction of the gravitation potential dilation balances the increased relative velocity dilation a 'lucky' coincidence or is there some deeper connexion that causes this to be so?

 

It's one of those tempting coincidences. I think it only holds for small deformations, where the shape is still approximately spherical.