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A great question. First--It isn't always constant. Light can travel slower than "c" ("the speed of light"). In a perfect vacuum light travels at "c". It's slower in glass, air, water, etc., light can even stop! But "c" is the upper speed limit. Second--Not only light but ANY disturbance in the universe has a speed limit of "c". Even the influence of gravity travels at the speed of light. Third--Einstein didn't just wake up one day with a good idea. The constancy of the speed of light (and relativity in general) explained many curious observations that had been made, and solved many physics problems. Fourth--There have been competing theories that allow a variable "c" but all the experiments indicate that it is indeed a constant value.

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Q: Why is light speed always constant?
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