The speed of light in a vacuum, denoted by "c," is a fundamental constant of nature according to our current understanding of physics. It is defined to be approximately 299,792,458 meters per second. Importantly, the speed of light is considered constant in all inertial reference frames, meaning it does not depend on the motion or gravitational field of the source or observer.
Gravitational time dilation, as described by general relativity, affects the passage of time but does not alter the speed of light itself. While clocks in regions of different gravitational fields may tick at different rates, the speed at which light propagates through space remains constant.
It's crucial to differentiate between the effects of time dilation and the speed of light. Time dilation is the phenomenon where the passage of time is observed to vary depending on the strength of the gravitational field or the relative velocity between observers. The speed of light, on the other hand, remains constant regardless of these factors.
Regardless of an observer's location in the universe or the strength of the gravitational field they are in, the measured speed of light will always be the same. This constancy of the speed of light is a fundamental principle in modern physics and is supported by extensive experimental evidence.
Therefore, while time dilation can occur due to gravitational effects, it does not change the speed of light itself. The speed of light in a vacuum, c, remains constant in all reference frames.