# Speed of light

1. Feb 17, 2010

### clm321

in physics is it possible for light to be slowed down or speed up, or is light a fixed speed?

2. Feb 17, 2010

### _wolfgang_

Yes it can the speed of light can change if hits a black hole, the speed of light cannot escape the pull of the black hole. therefore it must change

3. Feb 17, 2010

### clm321

yes but blackholes change the direction. do they change the speed also?

4. Feb 17, 2010

### Nabeshin

The answer is no, black holes do not change the speed of light.

The speed of light in a vacuum is a constant, c.

The speed of light in various materials can be different, but always lower than c. When people claim to have "stopped light", this is a claim made about the speed of light in a given material.

Note that matter is just a conglomeration of atoms. The reason the speed of light is lower in those materials is because the light is constantly being absorbed and re-emitted by the atoms. If you think microscopically, in between each absorption, the light travels in a vacuum always at c.

5. Feb 18, 2010

### Claude Bile

Nope, this is wrong, ZapperZ (and other contributors) have a nice post in the FAQ on this. To quote;

"A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations. If this is what actually occurs, then the absorption spectrum will be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. In fact, the index of refraction (which reflects the speed of light through that medium) varies continuously, rather than abruptly, with the frequency of light."

To the OP; there are two manifestations of "the speed of light". The first is the speed at which light travels. This varies, depending on the refractive index of the medium it is travelling through (so yes, light can be slowed down, in fact there is an entire field devoted to "slow light"). The other manifestation is the constant that defines the speed limit of the universe; this is a genuine constant according to standard theory.

Claude.