Solution

ID: past-exam-of-the-mathematical-physics-course-of-the-university-of-oxford/2007/b/8/i/solution

Write and restore for the numerical estimate. A stationary clock at radius has . Let radii and denote the upper and lower computers. The metric is stationary and the same two fixed spatial points exchange both signals, so each signal has the same coordinate propagation time. Therefore the interval between reception events equals the interval between emission events in coordinate time: the common delay cancels.
For upper proper running time , the lower computer has run for
In the weak gravitational field near Earth's surface, and
The lower clock runs more slowly, so its remaining work is
Thus about 660 micro-operations remain; the proper-time shortfall is only about seconds.
Successful comparison tests the gravitational dependence of stationary proper-clock rates, specifically the radial variation of and its weak-field gravitational redshift prediction, assuming the computers execute equally per unit proper time. It also checks that this clock effect is independent of the chosen physical clock mechanism. It does not separately verify the Schwarzschild radial metric coefficient: although light propagation uses the null metric, the identical start and finish propagation delays cancel from this particular observable. This is Gravitational redshift measured by a computation counter.

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