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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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Related questions

Can the speed of light be changed?

no. speed of light is always constant . as long as the light stays in the same medium. the speed is generally slightly different in different media.


Why constant is used?

Constant is the speed of light and as the speed of light cannot change it is 'constant'


What is the average constant speed of light in vacuum?

The speed of light in vacuum is 299,792,458 meters per second, and that's not an average. That's what it always is.


Why use constants?

Constant is the speed of light and as the speed of light cannot change it is 'constant'


Does lightning go at the speed of light?

The speed of light is always constant. In air or vacuum, it is 299,792,458 m/s.


What does special relativity say about the speed of light?

The speed of light is constant.


Can light exceed its normal speed?

The speed of light is constant and can not be changed


Does time always move at a constant speed?

The speed of light is always the same as long as it's traveling through the same medium. But its speed is different in different media, and those are all less than its speed in vacuum.


What is speed that does not change - called?

constant


What is the constant speed at which light and other forms of radiation travel?

The constant speed at which light travels is 300M m/s.


What is the different between constant velocity and constant speed?

constant velocity is when you maintain speed and direction, this usually is in a straight line, and constant speed means that your speed is always constant at all times.


Is the speed of light a constant?

Yes