Is the speed of light constant?

In summary: I was waiting for some one to do that :wink:In summary, the speed of light is not the same in all media.
  • #1
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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  • #3
Why?
 
  • #4
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
 

1. Is the speed of light really constant?

Yes, according to the current understanding of physics, the speed of light is constant and does not change in a vacuum. This is known as the "constant speed of light postulate" and is a fundamental principle in Einstein's theory of relativity.

2. Can the speed of light be exceeded?

No, the speed of light is considered the maximum speed in the universe. It is approximately 299,792,458 meters per second and cannot be exceeded by any object or particle.

3. Why is the speed of light important in physics?

The speed of light plays a crucial role in many areas of physics, including relativity, electromagnetism, and quantum mechanics. It is used to define the basic units of length and time, and is a fundamental constant in many equations and theories.

4. Has the speed of light always been constant?

There is evidence that the speed of light has been constant throughout the history of the universe. However, some theories, such as the "variable speed of light" theory, propose that the speed of light may have been different in the past.

5. Can the speed of light vary in different mediums?

Yes, the speed of light can vary depending on the medium it is traveling through. For example, light travels slower in water or glass than it does in a vacuum. This is due to the interaction between light and the particles in the medium.

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