# It's confusing

1. Mar 8, 2005

### thomate1

Why light itself cannot travel faster than c = 300000 km/s ?

2. Mar 9, 2005

### Andrew Mason

You are really asking why the speed of light is constant, because if the speed of light is constant, then it cannot travel faster than itself.

The speed of light is determined by two physical constants that are properties of empty space:

$$c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}$$

Since these two constants are invariant properties of magnetic and electric fields in empty space, the speed of light in a vaccum is constant.

AM

3. Mar 9, 2005

### witze

The speed of light can easily be derived from Maxwell's equations. But I believe in relativity the invariance of the speed of light is considered to be a fundamental postulate, proved only by experiments. If I'm not mistaking, this postulate is more general than what can be derived from Maxwell's equations. BTW, c=299,792,458 m/s EXACTLY.

4. Mar 9, 2005

### Integral

Staff Emeritus
Because Maxwell derived the constant speed of light, which was then shown by Michelson and Morley to not to motion relative to some fixed medium, Einstein was able to simply postulate the constancy of the speed of light. He did not need to verify or reprove that which was already well known by the Physicists of the day. Somewhere around 1980 the meter was redefined in terms of the wave length of light so now the speed of light is an integer number in meters/sec, since it has been defined that way.

5. Mar 9, 2005

### dextercioby

"c" is defined as the speed of light in VACUUM.Classical vacuum.Empty spave.The photon doesn't interact with anything and both his velocity "c" and his apparent velocity:
$$v_{app}=:\frac{c}{n}$$

are both "c",becuse the vacuum has refraction index +1.

This last assertion (mentioning of an apparent velocity in a medium) is a consequence of the fact that the photon always travels at "c"...

Daniel.

6. Mar 10, 2005

### sueanne

Basicly hun, light is constant, that speed is as fast as it can get. but u can slow it down using prisims and mirrors.

7. Mar 10, 2005

### Andrew Mason

Prisms and mirrors don't slow down light. Light always travels at c. It is just that when it encounters matter, it keeps getting absorbed and reemitted, so it takes longer to get through.

AM