Originally Posted by

**Strange**
Originally Posted by

**Janus**
In terms of pure time dilation we have the clock postulate, which states that the time dilation for such a clock can be calculated just from applying SR to the speed of the clock and that there are no additional effects due to the acceleration it experiences.

How does this relate to the equivalence of acceleration and gravity? If differences in gravity cause time dilation, then shouldn't acceleration also?

Look at it this way: Gravitational time dilation is related to the difference in gravitational potential, IOW, the amount of work needed to move from one height in a gravity field to another.

Now consider a rotating frame. In this frame, the gravitational equivalence is a gravity field that gets stronger as you move away from the axis. Now if you work out the equivalent gravitational potential between the axis and some other point in the frame, the gravitational time dilation works out to being equal to that calculated by someone in a non-rotating frame and just considering the speed that point travels around the axis.

Thus if you had two observers at the axis of the disk, one in the rotating frame of the disk and the other in an inertial frame, both would see the same time dilation for the clock on the edge. In the inertial frame it will be due to the relative motion of the clock. In the rotating frame, the observer

*could* claim no relative motion on the part of the clock, but there would be an equivalent potential gravity difference between the clock and him causing a gravitational time dilation.

So either the disk is rotating and the dilation is motion based, or considered stationary and the dilation is potential based. You don't apply both.