Is the speed of light constant?

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SUMMARY

The speed of light in a vacuum is a constant 3 x 108 m/s, regardless of the observer's motion, as established by the principles of relativity. However, when light travels through different media, such as glass, its speed decreases due to the medium's refractive index; for standard glass, this speed is approximately 2 x 108 m/s. The discussion clarifies that the speed of light remains constant even if the source or observer is in motion, reinforcing the foundational postulate of relativity. Misinterpretations regarding the context of motion and light source were addressed, emphasizing the importance of understanding relativity in this topic.

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  • Understanding of the principles of relativity
  • Knowledge of the speed of light in vacuum and various media
  • Familiarity with the concept of refractive index
  • Basic grasp of physical postulates in physics
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TT0
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Hello,

Is the speed of light constant at all speeds? So will the speed of light (3 x 10^8 m/s) be the same if a person was traveling at 1000 km/hr and if they were stationary.

Thanks
 
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Yes.
 
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Why?
 
As with most fundamental physical postulates, such as the postulates of gravitation and quantum mechanics, nobody knows why.

We choose to (provisionally) believe it and adopt it as an assumption because theories built on such assumptions have proven to be useful in predicting future observations.
 
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You will find about 83 gigabazillion threads on this subject here and in the relativity subforum. A few of these are linked in the "Similar discussions" section below, but if you search you'll find many more.
 
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TT0 said:
Is the speed of light constant at all speeds? So will the speed of light (3 x 10^8 m/s) be the same if a person was traveling at 1000 km/hr and if they were stationary.
Well, no. That speed you are quoting is in vacuum. In other media, the speed will be different.
The refractory index of glass for example tells us how much slower light travels in glass than in vacuum (standard glass has a refractory index of ≈1.5, so light travels with a speed of ≈2×108m/s in glass).
 
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Svein said:
Well, no. That speed you are quoting is in vacuum. In other media, the speed will be different.
The refractory index of glass for example tells us how much slower light travels in glass than in vacuum (standard glass has a refractory index of ≈1.5, so light travels with a speed of ≈2×108m/s in glass).

you mis-interpreted the question ... was waiting for some one to do that :wink:

Andrew's answer, of YES, was correct in the context of the Q

TT0 said:
So will the speed of light (3 x 10^8 m/s) be the same if a person was traveling at 1000 km/hr and if they were stationary.

the question is assuming the light source or observer is a moving object, compared to a stationary source/observer.
eg ... so if the source is on a moving object 1000 km/hr in the example he gave is the speed of light from the source the same as if that source was stationary ... relativity says it is

c isn't 3 x 10^8 m/s + 277.78 m/s (1000 km/hr )
Dave
 
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Thank you everyone
 

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