# Is the speed of light constant?

1. Oct 6, 2015

### TT0

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 travelling at 1000 km/hr and if they were stationary.

Thanks

2. Oct 6, 2015

Yes.

3. Oct 6, 2015

Why?

4. Oct 6, 2015

### andrewkirk

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.

5. Oct 6, 2015

### Staff: Mentor

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.

Last edited by a moderator: Oct 6, 2015
6. Oct 7, 2015

### Svein

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).

7. Oct 7, 2015

### davenn

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

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

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

8. Oct 8, 2015

### TT0

Thank you everyone