# Is this correct

1. Oct 20, 2005

### Cosmo16

I saw this awhile ago, I don't remember where though.

Supposdly some scientists accelereted the beam of a laser faster then the speed of light, by shooting it through a high density gas. The opinion being that Relativity only states that nothing can go faster then the speed of light in a vaccum. is this a vaild conclusion? I don't know enough about relativity to know for myself, but it seems like a valid conclusion to me.

2. Oct 20, 2005

### Jimmy Snyder

I'm just guessing, but I think what happened was that a particle, perhaps an electron, was accelerated in a gas to a velocity that was less than the speed of light in vacuum, but greater than the speed of light in that gas. Such speeds are achieved routinely and are the source of a phenomenon known as Chernenkov radiation.

However as you have worded it, it sounds like it was a laser beam and not a particle that was shot. I can't guess what is meant by light travelling faster than light.

3. Oct 20, 2005

### George Jones

Staff Emeritus
A superposition (sum) of waves was sent through a medium such that the group velocity (Yes, group velocity!) of the resulting wave packet moved faster than the speed of light in a vacuum. Greg Egan has a nice animation of this http://gregegan.customer.netspace.net.au/APPLETS/20/20.html" [Broken].

From Egan's webpage: "Nothing is actually travelling with these peaks, though; they're just an artifact of the way the different frequencies are slipping in and out of phase."

Don't hesitate to come back if you have questions.

Regards,
George

Last edited by a moderator: May 2, 2017
4. Oct 21, 2005

### Cosmo16

What does he mean by "the way the different frequencies are slipping in and out of phase"

P.S. Thanks for the link, the animation helped.

5. Oct 22, 2005

### George Jones

Staff Emeritus
Two waves are completely in phase if their crests line up or if their troughs line up, and completely out of phase if the crests in one wave line up with the troughs in the other. When waves are in phase, they tend to reinforce each other; when waves are out of phase, they tend to cancel each other.

In the animation, two effects have to be taken into account: 1) different colours have different wavelengths; 2) different colours move at different speeds.

The reason for 2) is as follows. The index of refraction of a medium is defined as the ratio of the speed of light in a vacuum to the (effective) speed of light in the medium, i.e., n = c/v. Thus, v = c/n. In what is called a dispersive medium, the index of refraction is slightly different for different colours of light. Since n depends on colour and c is constant, v depends on colour.

The combination of 1) and 2) above cause the different colours to slip in and out of phase.

Regards,
George