The constancy of the speed of light, as postulated by Einstein's theory of special relativity, is a fundamental principle of the theory. It states that the speed of light in a vacuum (denoted as 'c') is the same for all observers, regardless of their position, movement, or velocity relative to the source of light. This principle has been extensively tested and confirmed by numerous experiments and observations.
The constancy of the speed of light has important implications for our understanding of the nature of space and time. It leads to phenomena such as time dilation and length contraction, which describe how time and space appear to change for observers in relative motion.
Time dilation refers to the effect of time appearing to pass slower for an object in motion relative to a stationary observer. This effect is a consequence of the constancy of the speed of light. As an object approaches the speed of light, the passage of time for that object slows down relative to a stationary observer. However, it is important to note that this slowing of time is experienced by the moving object itself and is not an instantaneous or abrupt process.
Instantaneous movement or instant quantum collapse, as you mentioned, are unrelated concepts to the constancy of the speed of light and time dilation. The constancy of the speed of light is not affected by instantaneous movements or quantum phenomena such as quantum collapse. It is a fundamental property of the universe that holds true in a wide range of physical scenarios.
In summary, the constancy of the speed of light means that light always travels at the same speed in a vacuum, regardless of the position, movement, or velocity of the observer. This principle is a foundational aspect of special relativity and has been well-supported by experimental evidence. Time dilation, as a consequence of the constancy of the speed of light, describes how time appears to pass differently for objects in relative motion.