Speed of one photon vs. another photon

[Genecks]

Is there a limit to how fast a photon can travel in relation to another photon?

If so, can it be considered that a photon is never at its maximal speed?

 

I'm looking for a layman explanation.

[Cap'n Refsmmat]

Assuming we don't mix reference frames, all photons should travel at exactly the same speed. (Also assuming they're traveling through the same medium.)

[Mr Skeptic]

Well, if you have two photons traveling in opposite directions, you just add their velocities together like so:http://en.wikipedia.org/wiki/Velocity-addition_formula

 

So, they travel at c in relation to each other, even though from our reference frame it naively looks like it should be 2c. This formula seems to work, despite the fact that photons aren't supposed to have a reference frame. Hmm.

[Sisyphus]

This formula seems to work, despite the fact that photons aren't supposed to have a reference frame. Hmm.

 

Adding C to anything will get you an answer of C, though. So from the "photon's frame," everything would be moving at C relative to it, meaning objects and photons in the same direction would have zero relative velocity, etc. In other words, it breaks down.

Edited March 1, 2010 by Sisyphus

[Mr Skeptic]

That's what it's supposed to do though. However with speeds larger than c it does give weird results, but then we don't actually observe speeds larger than c.

[Genecks]

Ok, so to go faster than light, an object would need to be encapsulated by a medium that allows it to go faster than light and travel through/within said medium?

 

Hmm. As such, scientists currently consider the best medium to be a vacuum?

Alright. But isn't a vacuum hypothetical?

 

I think I see why scientists say it's impractical for something to go faster than the speed of light. But I don't think it's impractical to not enable an object to go the same speed as light. I think I'm starting to see why a lot of people have been arguing about in the past few decades.

Edited March 3, 2010 by Genecks

[michel123456]

Motion includes 2 concepts: space & time. As much as i know, a photon do not experience time, so the "moving" concept is unknown to a photon.

[Bob_for_short]

If we speak of a transparent medium, glass, for example, two photons (waves) of different frequencies ω1 and ω2 may propagate with different velocities due to n(ω) < 1. The light velocity in vacuum c is the maximum possible for a wave.

 

In a tarnsparent medium with a variable n(r,ω) the wave may accelerate and decelerate - its wavelength λ(r,ω) may vary with r.

 

This is well known in the optical waveguides.