The Mystery of the Leaning Timber
        
To understand static equilibrium in a gravitational field, we consider a thought experiment in which a strut under longitudinal compression is disposed obliquely relative to a gravitational field. The strut is whimsically referred to as a "leaning timber". Figure TRG2 denotes such a strut.
 
        
The gravitational field g is directed downwardly in the figure, and the strut is disposed obliquely relative to g. F1 and F2 denote horizontal forces on the ends of the strut, and F3 and F4 denote vertical forces on the ends of the strut. The latter do not interest us. We are interested only in the balance of horizontal forces, F1 and F2, or the corresponding momentum balance.
        
The gravitational field we contemplate is to be precisely unidirectional. The gravitational field vector anywhere in the region of interest is to be precisely parallel to the gravitational field vector anywhere else in the region. We would not use Earth's field because, even above a featureless prairie, the gravitational field vectors are not precisely parallel, they point towards the center of the Earth.
        
The field we require would be found, however in Einstein's elevator. The acceleration (or gravitational) field seen by an observer who accelerates in field-free space sees a field with precisely parallel field vectors. This is because if acceleration is in the Z direction, the X and Y coordinates are not involved, and that is because the field can be thought of as being due to an ongoing Lorentz transformation involving Z and T, but not involving X and Y. The unidirectional field we require could also be approximated in a small region above the Earth provided rings of dense material were placed below and around the experimental region.
        
It is well established, from general relativity, that, due to the gravitational field, g, clocks at low elevations run more slowly than clocks at higher elevations. This effect is seen in the spectra of stars such as the sun, in which known spectral lines are shifted to longer wavelengths, which indicates light of lower frequency. The effect has been observed in terrestrial laboratories, for example, in the Harvard Tower Experiment by Pound and Rebka. (See Link 1 below.) Gamma rays sent from the top to the bottom of the tower showed an increase in frequency, and those sent from the bottom to the top showed a decrease in frequency. It is understood that for any pair of identical clocks that properly measure time at their own locations, a clock at a low elevation will run more slowly that its duplicate at a high elevation. The fractional change in rate, based on general relativity and confirmed by experiment, is as follows.
ΔZ * g / c2
        
In the preceding expression, ΔZ is the difference in elevation between the clocks, g is the magnitude of the acceleration of gravity and c is the velocity of light. We consider a thought experiment lasting a finite time, Tbot seconds as indicated by the lower clock. The time interval indicated by the upper clock is then:
Ttop = Tbot * ( 1 + ΔZ * g / c2 )          Equation TRG1
        
To be valid, Equation TRG1 requires that the magnitude of (ΔZ * g) be much less than c2. To be precise, we should say that Equation TRG1 is valid in the limiting case wherein the magnitude of (ΔZ * g) is vanishingly small compared to c2. This condition was met in the Harvard Tower Experiment, and we assume it for our thought experiment. We note that the appearance of the clocks in Figure TRG1 is inconsistent with this limiting case, since the clocks indicate a very large difference in elapsed time. The figure is drawn as it is to emphasize that the clock at the top indicates more elapsed time than the clock at the bottom. Ttop is greater than Tbot.
        
The term "impulse", as used in mechanics, is used to indicate the product of force and time. The impulse applied by force F1 acting for the time Ttop is ( F1 * Ttop ). The impulse applied by force F2 acting for the time
Tbot is - ( F2 * Tbot ) . It is noted that F1 and Ttop are measured by instruments placed at the upper end of the strut. Likewise, F2 and Tbot are measured by instruments placed at the lower end of the strut.
        
We now ask for the conditions for equilibrium. We look for the case in which the strut does not accelerate either toward the right, nor toward the left. From our freshman physics class, we would expect that equilibrium would prevail if F2 = F1.
        
But, now we consider momentum balance. We consider a thought experiment having a duration of Tbot seconds as seen by an observer at the lower end of the strut, and Ttop seconds as seen by an observer at the upper end. The time interval, Ttop at the top end of the strut begins at the same instant as Tbot at the bottom end of the strut. Likewise, the time interval Ttop at the top end of the strut ends at the same instant as Tbot at the bottom end of the strut. This is possible, even though Ttop does not equal Tbot because space-time is curved due to the gravitational field. If F1 = F2, then the net impulse applied to the strut during the duration of the thought experiment is given by:
        
net impulse = ( F1 * Ttop - F2 * Tbot ) = F2 * ( Ttop - Tbot )          Equation TRG2
        
By applying Equation TRG1 to Equation TRG2, we obtain:
        
net impulse = F2 * ( Ttop - Tbot ) = - F2 * ( ΔZ * g / c2 )          Equation TRG3
        
With considerable consternation, we note that, having assumed force balance, that is F1 = F2, we are left with a finite net impulse. Thus, if the forces F1 and F2 are balanced, then, during the duration of our thought experiment, the strut is given a finite momentum, which causes it to begin moving toward the right. If we had assumed a momentum balance at the outset, then we would have found that the forces F1 and F2 were not equal. Something has to give. Either we abandon force balance, or we abandon momentum balance. We suspect that, for the strut to be in static equilibrium, then there must be momentum balance. In the following section we will apply that guess to explain how the weight of a uniform, vertical electric field is supported.