varying light speed and speed of information

[Mordred]

http://en.wikipedia.org/wiki/Speed_of_light

I was going through this article and was having trouble with the statement where they describe the group pulses of light can exceed c, however the speed of information is still constant at c
http://news.bbc.co.uk/hi/english/sci/te ... 841690.stm
this is the article itself.

I'm wondering if it would not be better after reading research such as this to refer to c as the speed of information rather than the constant speed of light through a vacuum or space time contstant as you often read articles where they describe radiation as to moving backward in time which we know effect cannot preceeds cause,
One other interesting point I was thinking about. Einstein essentially worked with pre exixting mathematics developed through emperical evidents developed by other scientists, He merely was the one to get accepted the thought that light is a constant speed. However we know that his theory of general relativity has been confirmed on numerous occassions such as the time dilation effects. I would be curious to know what those scientist would think the speed of light is if they were travelling at 50% the speed of light. Would they have a different value? Did the value we accept today take into consideration our own relative velocity and how this would affect our perspective? granted compared to the speed of light were moving slow

view points?

 

[alphachapmtl]

Light also slow down while traversing glass (or any refractive material).

 

[SteveCNC]

That kind of experiment makes me wonder about light speed . Most speed is measured relative to some other object but light I can't help but wonder if there is a way to assess our actual speed relative to true zero . To me it seems like depending on the direction of travel and the direction of the light being measured as to what you might see . Do they take into account a red or blue shift when measuring things like that ? is that how you can judge the actual speed vs the relative speed ?

 

[Mordred]

Makes one wonder what our perspective would be if we were actually staionary after all we are on a moving planet, in a moving solar system with a moving galaxy with an expanding universe

 

[ramparts]

It's a bit hard to explain but the group velocity is just an apparent velocity, rather than the velocity of any individual light rays. If you have a group of photons you'd see, from a distance, the group would appear to travel faster than c, but none of the individual components making it up are actually traveling that quickly.

Steve - actually, the fact that the speed of light is constant means there can't be any "true zero." Why? Let's assume true zero exists. If light moves at a certain, constant speed relative to true zero, then you should be able to detect your motion relative to that true zero by measuring light having a different speed, the same way that if you measure a car moving at 40 mph when it's really moving at 100 mph relative to the ground, you know that you're moving at 60 mph relative to the ground.

The problem is that when you measure the speed of light, no matter what your motion is, and where the light is coming from relative to you, you always measure the same speed of light. Pretty freaky. This means there can't possibly be a true zero, or an objective reference frame; if there were, we'd measure different speeds of light depending on our motion relative to it.

 

[SteveCNC]

While I'm sure your right ramparts , but what your saying sounds to me like light speed is also relative to it's source but I was under the impression that it was a constant , or could it have something to do with your motion causing elongation along the path of travel . If I remember right it's a bell curve that shows elongation as you approach light speed which means at any speed there is some elongation just maybe not very much . Possibly since light speed is so fast that we don't have the significant figures accuracy to see any difference . But from what your saying I would hazard a guess it has more to do with elongation than anything as to why we may never know what true zero is if there is one . Stuck in our relative world

 

[ramparts]

No, in fact I'm saying the exact opposite The speed of light doesn't care about your motion or the motion of its source. It is 2.99*10^8 meters per second measured in every possible inertial reference frame. Not a matter of how much accuracy we have - no matter how accurate your measuring tool, we expect to see the speed of light the same no matter your motion.

 

[Mordred]

Lets say that our understanding of light is correct in that it is a constant speed along with all the other massless particles.
However when we measured that speed we ourselves are under several dilation effects. Dilation is probably a bit inaccurate. One our time reference is influeced by our motion. the space time is around us has been influenced by the mass of our planet and the mass of our sun etc. Now to what you were saying our motion itself is not going to influence the actual speed of light but the apparent speed. However what we percieve as time 1 second etc which is one part of the equation to derive the value of velocity is influenced. If you were to remove our gravity well and place us at a zero time reference point (still aloowing for time changes etc)
What would we percieve the speed of light to be then? Would it still be the same?
One reason I was thinking of this is I was curious as to what our perspective would be if we were near the gravity well of something the strength of a black hole assuming you can maintain a stable position with out being drawn into it.

 

[SteveCNC]

Yeah I listened to a University lecture last night about Einstein's 1905 papers and it makes more sense now how light would measure the same no matter what direction it's coming from . That was really botherring me due to what seemed like relative motion not causing a change but it's due to the time factor that we see it as a constant speed .

 

[ahook12]

Her is a mind experiment that I have posted before but never got a good answer;

If you have perfectly rigid rod(can not be bent, stretched or compressed) and you move one end the other end will move at the same time, Does this exceed the speed of light? If it does then how? if it does not then why not?

 

[SteveCNC]

If my understanding of motion is correct , at the atomic level all motion can be described as a wave . I am certain that wave propagates at less than the speed of light . I would also bet that you could verify that with laser measurements on an accelerating object . I am pretty sure different materials will react differently but even a rod of Carbide would still follow the same laws of motion .

 

[origin]

Her is a mind experiment that I have posted before but never got a good answer;

If you have perfectly rigid rod(can not be bent, stretched or compressed) and you move one end the other end will move at the same time, Does this exceed the speed of light? If it does then how? if it does not then why not?

If you move the rod the other end will not move at the same time. The reason that it seems that this scenario could exceed the speed of light is because you put unrealistic boundry conditions on the it. The bar would compress, bend or stretch. It is not possible to have the bar made of the material you describe. It is really just that simple.

 

[ahook12]

This is interesting that no one is willing to say that the rod will exceed the speed of or will be the same as the speed of light, it must be one or the other. So which is it?

 

[origin]

This is interesting that no one is willing to say that the rod will exceed the speed of or will be the same as the speed of light, it must be one or the other. So which is it?

None of the above.

 

[Mee_n_Mac]

This is interesting that no one is willing to say that the rod will exceed the speed of or will be the same as the speed of light, it must be one or the other. So which is it?

No one is willing to limit their answer to your set because you've set up an unrealistic scenario. It's impossible to have rod that won't compress, thus the whack at one end will travel at less than C[sub]0[/sub] through any real rod. IF you had your magic rod then the whack would travel, by definition, instantaneously to the other end thus exceeding lightspeed. Your thought experiment is one of a many that fall into this type ...

http://en.wikipedia.org/wiki/Ehrenfest_paradox

http://en.wikipedia.org/wiki/Born_rigidity

 

[ahook12]

Thank you Mee_n_mac your references were very helpful.