# Photons at less than speed of light?

1. Dec 13, 2005

### rushil

According to Special Relativity, A particle with rest mass = 0 can exist if and only if it is moving at the speed of light. A photon is a particle which comes into this category. However, when light travels in a medium it's speed is always reduced.
Does this contradict relativity? What I want to ask is that when light travels at a lower speed in some medium. do photons exist and why? Why do photons 'seem' to exist at speeds less than that of the speed of light.

2. Dec 13, 2005

### Pengwuino

Photons can't go slower then the speed of light because photons ARE light. It's like asking can a truck go slower then the speed of a truck.

3. Dec 13, 2005

### rushil

So what's happening when light is going through a medium where its speed is less than 3 * 10^8 m/s ??? Doesn't relativity say ( without making any reference to 'light'!) that photons can exist ONLY at 3 *10^8 m/s ??? What;s happening here? That's my question!

4. Dec 13, 2005

### Pengwuino

No, it says that photons can only exist at the speed of light. 300,000,000m/s is simply the speed of light in a certain medium (a vacuum).

5. Dec 13, 2005

### Tom Mattson

Staff Emeritus
Light never travels with a speed that is anything less than c.

When light travels through a medium, it is constantly absorbed and then re-emitted by the atoms of that medium. The photons may excite the atoms into a metastable state which has a finite duration. The average speed of the light is decreased from c by an amount that depends on how long the photons are "held captive" by the atoms, but the instantaneous value of their speed is always c.

6. Dec 13, 2005

### rushil

That was exactly what I thought!!! It was collaborated by what I read in Hecht's Optics... and now by you, Tom!!!! Well, all this is in contradiction to what Indian books say!!!! You can understand the true phenomenon only by reading more! :tongue2:

7. Dec 14, 2005

Staff Emeritus
"Speed of light" almost always means "speed in a vacuum", to a sufficient approximation speed between astronomical bodies. Density between local stars is around 1 proton per cubic kilometer, and it's much lower between galaxies.

So travel in media is excluded from the definition. What happens in media? There are two kinds of media; normal ones and specially prepared nonlinear ones.

In the normal media the photons travel until they are captured by an atom, then they are absorbed and the atom is driven by this to a higher energy state ("quantum leap"). After a moment it "relaxes" down to its prior state by emitting a new photon. Conservation of momentum means that the new photon has the same momentum as the old one, and it continues along the original path,... until it meets another atom and the process repeats. In this way the photons always travel at c (which, since they are massless, relativity requires them to do), but the time of passage, due to the successive atomic interactions, is longer and the effective speed from point to point is lower.

In the specially prepared nonlinear media, the group velocity and phase velocity are jiggered deliberately to make some wave form travel faster than the photons (held up by those interactions) can travel. That a wave form (but not a signal) can travel faster can be illustrated by swinging a flashlight while pointing it at a wall. Obviously the photons aren't travelling any faster, but if the wall was a light-year away, you could make the spot from the flashlight move faster than light. The clever experimenters can set up special media to make this happen in the laboratory, and reprters are always delighted to hear and report that "Einstein has been refuted".

8. Dec 14, 2005

### Jimmy Snyder

9. Dec 14, 2005

### Staff: Mentor

Just an editorial comment: that's a common oversimplification made by American high school textbooks as well. Because they never tell you that that speed through a medium is only an average or apparent speed due to the interaction with particles, the first time just about anyone hears about Relativity, they get confused.

It annoys me.

10. Dec 15, 2005

### Jimmy Snyder

Then we should see 'lucky' photons squirt through more rapidly than the bulk of their cohort and 'unlucky' ones that arrive late. Do we? Also, how do you explain coherent light in this picture?

Last edited: Dec 15, 2005
11. Dec 15, 2005