The constancy of the speed of light is a fundamental principle of physics, established through numerous experiments and supported by the theory of relativity. It states that the speed of light in a vacuum is the same for all observers, regardless of their relative motion.
The Michelson-Morley experiment, conducted in 1887, was indeed designed to detect the hypothetical "ether" medium through which light was thought to propagate. The experiment aimed to measure any variations in the speed of light due to the motion of the Earth through the ether. However, the experiment yielded null results, failing to detect any significant variation in the speed of light as the Earth moved in different directions. This unexpected outcome posed a challenge to the prevailing theories at the time and eventually led to the development of the theory of special relativity by Albert Einstein.
According to the theory of special relativity, the constancy of the speed of light is a fundamental postulate. It states that the speed of light in vacuum, denoted by 'c', is the same for all observers, regardless of their relative motion. This means that the speed of light does not add up to the velocity of the source in the way you described.
The theory of relativity provides a new understanding of space and time, introducing concepts such as time dilation and length contraction. It shows that as an object's speed approaches the speed of light, the classical notions of space and time break down, and new relativistic effects come into play. This theory has been tested and verified by numerous experiments over the years, providing strong evidence for the constancy of the speed of light.
In summary, the constancy of the speed of light is a well-established principle in physics, supported by experimental evidence and the theory of relativity. It means that the speed of light is not additive to the velocity of its source in the way classical objects might be.