Astronomy Fundamentals

Matter and Energy

What does it mean when we say the speed of light is constant?

Saying that the speed of light is constant means that any observer watching any particular beam of light will measure that beam to be moving at the same speed. It does not matter whether the observer is moving toward, away, or not at all relative to the beam; it also does not matter how fast the observer is moving. This property of light is a key concept of the special theory of relativity, which was articulated by Albert Einstein in 1905.


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