# Slowing down light

1. Jul 13, 2012

### sirchick

Hey

I was reading about an experiment that slows light down to 1 mile an hour:

Source:
http://news.bbc.co.uk/1/hi/sci/tech/655518.stm

What i wanted to ask was, if light slows down, does it gain mass whilst it is moving slower than its maximum speed limit in a vacuum, in effect building "potential energy" for when it exits an object that slows it down, then shoots off to maximum speed in a vacuum again ? ?

If light escapes an object that slowed it down, where is the force that speeds it back up once its left that object, instead of just staying at the speed it was slowed down to.

Shouldn't there be some kind of force that accelerates when it leaves the object, perhaps by expelling mass or something ?

2. Jul 13, 2012

### cragar

light is moving at c when it is in between electrons in the material. There is just a lot of absorption and re emission thats causes this time lag.

3. Jul 14, 2012

### Staff: Mentor

You can describe light in a medium as an effective slow particle, too.

A force is required only if the momentum changes, not the velocity. As far as I know, the momentum of slow light is an ongoing discussion (for example, see this pdf). In any case, light does interact with a medium, therefore it can exchange momentum.

4. Jul 15, 2012

### PaulS1950

A photo cannot exist at less than C. it is emitted at C and only exists because at C it doesn't experience time as passing. as soon as its velocity drops below C it "ages" and is gone. When traveling through some materials it is constantly deflected or absorbed and re-emitted so it seems to be travelling slower but it always travels at C.
Paul

5. Jul 15, 2012

### AbsoluteZer0

The speed of light is a constant in a vacuum, as you have said. As for the slowing down of light, light appears to travel slower within different mediums as the photon is constantly being absorbed and remitted from the atoms of that medium. Once the photon leaves one of the atoms of the medium, it travels at a constant speed until it is reabsorbed by another atom. Light never travels below c, it is only that the absorption and reemission of the photons causes a slight delay which gives the appearance of a slower travelling speed.

6. Jul 15, 2012

### PaulS1950

I believe I just said that......?!
Paul

7. Jul 15, 2012