# Constansy of speed of light

1. Oct 14, 2012

### bgq

Hi,

The theory of special relativity is based on assumption that the speed of light in vacuum does not depend on the source or observer. I have two questions regarding this.

1) Which is constant, speed or velocity? Is the direction also constant?
2) Are there experimental verifications for the the constancy of speed of light in vacuum that it does not depend on the source or observer? If yes, what are these experiments?

I appreciate any replies.
Thank you very much.

2. Oct 14, 2012

### dextercioby

For the second part, I'll let the expert throw in the sources. For the first, it's the speed, that is

$$\vec{c} = c\vec{n}$$

$\vec{n}$ is a vector of unit modulus, just like the versors of axis Ox, Oy, Oz. This fact is useful in optics.

3. Oct 14, 2012

### Bill_K

The obvious place to look is the FAQ at the top of this forum, "Experimental Basis of Special Relativity".

4. Oct 14, 2012

### Staff: Mentor

I'm not sure what you mean... There has to be a source and a detector or you can't perform the experiment at all. Perhaps if you were take look at the FAQ at https://www.physicsforums.com/showthread.php?t=229034, and especially the section on "Tests of Light Speed from Moving Sources", pick out which experiments don't answer your question, post these for further discussion?

5. Oct 14, 2012

### Staff: Mentor

Just speed. One easy way to see this is to look at the Lorentz transformations, which change x but not y and z; so Lorentz-transforming a vector will change its direction unless the y and z components happen to be zero. (Note that this is the three-dimensional direction that we're talking about; I presume that this question was motivated by the discussion of angles in the light-clock thread).

6. Oct 14, 2012

### Naty1

Expanding post #5 a bit:

Spacetime is usually interpreted with space as existing in three dimensions and time playing the role of a fourth dimension..and all vary, in general, according to relative motion....related by the Lorentz transform.

the Lorentz transform shows:

1) time and space are not entirely separate entities but one frame's time gets split into another frame's space and vice versa.

2) there is a notion of "distance" called the spacetime interval which also mixes space and time together and is agreed upon by all reference frames (i.e. is invariant under the Lorentz transform).

....this means that directions in space get 'transformed' to quantities ['directions'] in time....

7. Oct 14, 2012

### bahamagreen

Yes, just speed... the translation of Einstein from German to English causes this confusion. The word from German is translated as "velocity", but in German their meaning of this is speed only; not like a velocity in English with magnitude and direction...

8. Oct 15, 2012

### bgq

Thank you all very much.