The fact that GPS satellite clocks run slower is actually evidence for both time dilation and the effects of gravity on the speed of light. However, the prevailing explanation is based on the principle of time dilation, which is a fundamental concept in the theory of relativity.
According to the theory of relativity, time dilation occurs due to the relative motion between two observers or the influence of gravitational fields. In the case of GPS satellites, both effects play a role.
The satellites in the GPS system orbit the Earth at a high altitude where the gravitational field is slightly weaker compared to the surface. However, the difference in gravitational potential between the satellites and the Earth's surface still has a significant impact on time dilation.
Time dilation causes the clocks in the GPS satellites to run slower relative to clocks on the Earth's surface. This effect arises from the satellites' higher velocities and the weaker gravitational field they experience. The time dilation effect has been experimentally confirmed through various measurements and is a well-established aspect of the theory of relativity.
On the other hand, the speed of light is influenced by gravitational fields, as predicted by general relativity. According to this theory, gravity can cause light to experience gravitational redshift or blueshift, altering its frequency as it moves through varying gravitational potentials.
In the case of GPS satellites, the gravitational redshift does affect the speed of light to a certain extent. However, the impact of this effect on the time difference observed in the clocks is relatively small compared to the contribution from time dilation.
The primary reason why time dilation is considered the dominant factor is that the observed time difference in the GPS system aligns closely with the predictions of time dilation based on the theory of relativity. This agreement, along with the wealth of experimental evidence supporting time dilation in other contexts, reinforces the notion that time dilation is the underlying mechanism at play.
In summary, while both time dilation and the effects of gravity on the speed of light contribute to the behavior observed in the GPS satellite clocks, the preponderance of evidence and the consistency with the theory of relativity support the conclusion that time dilation is the primary explanation for the observed time differences.