Travelling at the speed of light

[Art]

I read somewhere that Einstein's interest in the properties of light stemmed from a question he posed to himself "What would light look like if you matched it for speed?" This then led to his theories of relativity. The question though is still valid, because although he determined that the speed of light in a vacuum is a universal constant, light moves slower through other mediums depending on the refractive index of the medium. In fact it is even possible to travel faster than light for example electrons can through water which has a refractive index of 1.33. So is it possible to answer Einstein's original question and if not why not ie is there a law in physics preventing such knowledge, is it only an apparent reduction in the speed of light as a consequence of it taking a longer path due to ricochets off molecules or is it just a matter of limitations in our technology preventing us from conducting such an experiment? IMHO I imagine it is only an apparent reduction due to a longer path taken. Can anyone confirm this?

 

[Dr.Brain]

Please can you elaborate how can electrons travel faster than light? ... 'c' is the limit which relativity imposes on speed , and talking of technology , we have done experiments at speeds close to 'c' and all have always abided by the rules of relativity.

 

[El Hombre Invisible]

He never said faster than light in a vacuum (c). He said faster than light, e.g. in water.

 

[JesseM]

But the inspiration for his thought-experiment was the fact that Maxwell's laws say light always travels through a vacuum at the same speed, so implicitly I think it's clear he wasn't talking about traveling alongside light moving through a medium.

 

[Art]

But the inspiration for his thought-experiment was the fact that Maxwell's laws say light always travels through a vacuum at the same speed, so implicitly I think it's clear he wasn't talking about traveling alongside light moving through a medium.

Apologies, I thought my post was pretty unambiguous but to clarify I was talking about light traveling through a medium. Electrons can travel faster than light when both are traveling through a medium such as water.

 

[JesseM]

Apologies, I thought my post was pretty unambiguous but to clarify I was talking about light traveling through a medium. Electrons can travel faster than light when both are traveling through a medium such as water.

I understood that you were talking about light traveling through a medium, my point was that the context of Einstein's thought-experiment had to do specifically to do with light traveling through a vacuum, light traveling through a medium isn't really relevant to this thought-experiment. So, when you asked "So is it possible to answer Einstein's original question", the answer is still that it's impossible to travel alongside light traveling through a vacuum, which is all his question was about.

 

[JesseM]

is it only an apparent reduction in the speed of light as a consequence of it taking a longer path due to ricochets off molecules

I think the classical explanation would not be that the incoming electromagnetic wave just slows down when it enters the medium, instead it'd be that the medium responds to the incoming wave with its own electromagnetic waves, and the sum of the original wave and the new waves is a wave moving slower than c. See the explanations on this page, for example, or on this sci.physics thread, where one poster gives a simplified picture of what's going on:

Light makes the *charged* particles in the medium oscillate. They reradiate this energy with a retarded phase, and the interference between the incident wave and all of the reradiated waves combine to produce a retardation which increases with propagation distance in the medium. Since all this oscillating is going on at the same frequency (that of the incident light), a spatially progressive phase retardation is equivalent to a slower wave.

Having given that simple picture, let me point out that the wave velocity of electromagnetic radiation is not always decreased by interaction with a medium. While the explanation has (I think) some physical value, it does not explain the phenomena in a universally applicable manner. That is often the case with simple explanations, and you should keep a little yellow light flashing in your brain alongside this simple explanation.

At a quantum level, my guess would be that the individual photons are still traveling at c between the particles of the medium, but that they are periodically being absorbed and re-emitted, which slows their progress. That's just a guess though, I don't know much about quantum electrodynamics.

 

[Symbreak]

If you were a light ray in a vacuum, you can definitely say that 1. There would be no concept of mass 2. You would not 'interact' with anything: an interaction would either consist of a transition into a particle or a change in wavelegnth. 3. There would be no concept of space because you could not observe light emmitted by other matter.

So we can conclude that mass, velocity along with spatial dimension is relative only to 'inertial observers' (ones with mass). They do not exist for any bosons, or entities that travel at c.

 

[Art]

I understood that you were talking about light traveling through a medium, my point was that the context of Einstein's thought-experiment had to do specifically to do with light traveling through a vacuum, light traveling through a medium isn't really relevant to this thought-experiment. So, when you asked "So is it possible to answer Einstein's original question", the answer is still that it's impossible to travel alongside light traveling through a vacuum, which is all his question was about.

In the article I read, Einstein wondered what a light wave/particle would look like if you traveled alongside it at the same speed. Would it look like a frozen wave or a particle or whatever. Relativity then showed you could never achieve this situation of equal speeds in a vacuum however it is theoretically possible if light travels at less than c in other mediums and so my question is the same as Einstein's original with the proviso in a medium where the speed of light is less than c and so no longer a constraint on the experiment.

However it appears from the first article you referenced light does not slow down in a medium with a refractive index greater than 1 but rather the photons that exit are not the same as the ones that entered and the 'swapping' times are what gives the appearance of the slowing of the speed of light. However the 2nd reference talks of a retarded wave which suggests it does slow down so now I'm confused again.

 

[Aether]

Relativity then showed you could never achieve this situation of equal speeds in a vacuum however it is theoretically possible if light travels at less than c in other mediums and so my question is the same as Einstein's original with the proviso in a medium where the speed of light is less than c and so no longer a constraint on the experiment.

However it appears from the first article you referenced light does not slow down in a medium with a refractive index greater than 1 but rather the photons that exit are not the same as the ones that entered and the 'swapping' times are what gives the appearance of the slowing of the speed of light.

How about in the case of gravitational refraction? Doesn't light actually slow down in a vacuum when the refractive index of the vacuum is greater than 1, and speed up when it is less than 1? And aren't the photons that exit an experiment the very same as the ones that entered?

How to pace a photon: 1) Cause the photon to orbit within the event horizon of a black hole; 2) Orbit outside the event horizon of the black hole; 3) Send me a post card when you have some results, but before you have released them to the general public.

 

[Integral]

How about in the case of gravitational refraction? Doesn't light actually slow down in a vacuum when the refractive index of the vacuum is greater than 1, and speed up when it is less than 1?

No, the speed remains the same, the velocity is effected by the presence of a massive object. This means that we observe a direction change of light passing near a star or other massive object. So the photon does not speed up or slow down.. ever.

And aren't the photons that exit an experiment the very same as the ones that entered?

Not necessarily, in fact probably not.

How to pace a photon: 1) Cause the photon to orbit within the event horizon of a black hole; 2) Orbit outside the event horizon of the black hole; 3) Send me a post card when you have some results, but before you have released them to the general public.

There are many troubles with this though experiment. Not the least of is how do you know you are pacing a photon? It is impossible to get such information out of a black hole.

 

[Aether]

No, the speed remains the same, the velocity is effected by the presence of a massive object. This means that we observe a direction change of light passing near a star or other massive object. So the photon does not speed up or slow down.. ever.

The travel time of light varies as well as direction, so if the speed does not vary then it is the distance that varies?

Not necessarily, in fact probably not.

I meant in the case of gravitational refraction experiment, are photons discontinuous in free flight?