Question

why einstein consider speed of light as a constant quantity.

Answer 1

Answer:

The speed of light actually does vary depending upon the relative motion between a source and an observer. However, the motion of the observer always alters his perception of the varied speed of the light to make it appear to him to always be a constant.

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Answer 2

Answer:

Since Einstein talks to the velocity ( a vector quantity: - speed with direction) rather than speed alone, it is not clear that he meant the speed will change, but the reference to the special relativity suggest that he did mean so.......In special relativity, the speed of light is constant when measured in any inertial frame..

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