# Faster then light..-ve index...

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How it so happens that a light beam traveling through a material with -ve index moves faster then 'c'?I read that 'it emerges before it enters'.What exactly does that mean?And finally,why it does not violate relativity or will a massless particle moving faster then 'c' violate relativity?Why?Whynot?also,what are the consequences(or assosiated phenomenans) of this faster then 'c' travel.

Also,if someone is intrested,here is the news link

http://news.bbc.co.uk/1/hi/sci/tech/841690.stm

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the light doesn't actually travel faster than c, what happens is that the leading edge of the wave gets turned into a peak, so the detector on the other side picks up the peak alot sooner than it should of.

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How it so happens that a light beam traveling through a material with -ve index moves faster then 'c'?I read that 'it emerges before it enters'.What exactly does that mean?And finally' date='why it does not violate relativity or will a massless particle moving faster then 'c' violate relativity?Why?Whynot?also,what are the consequences(or assosiated phenomenans) of this faster then 'c' travel.

Also,if someone is intrested,here is the news link

When you use punctuation, like a period or a question mark or a comma, you put a space after it. Its very irritating to read if you dont.

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the light doesn't actually travel faster than c, what happens is that the leading edge of the wave gets turned into a peak, so the detector on the other side picks up the peak alot sooner than it should of.

What you wrote there says: "The light doesn't travel faster than c, it just gets detected as if it were moving faster than c." I'd be interested in hearing you justify the two statements. I assume you're discussing the group velocity, but I know of nothing about group velocity that makes your statement make sense.

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Keneshin,

The article you linked to is from 2000. There has been a lot of research in the area of negative index (NI) materials since then. It is a fascinating subject. The phase velocity is faster than c, but interestingly, the group velocity travels the wrong direction, opposite to the direction of the energy. The doppler shift is also reversed. The diffraction limit is no longer a limit. Truly, some amazing things happen.

There has always been a great deal of hand-waving concerning how light operates in materials as people try to reconcile the strange behavior with the simple rules the public has been fed for decades. The fact that NI materials cannot be described by those cute one-liners (e.g., light is slowed because it is being absorbed and emitted, or is taking a longer path, etc.) suggests they probably were poor descriptions to begin with.

Evidence of what I'm saying here can be found in the publications of scientists who believed NI materials wouldn't result in negative refraction, wouldn't result in phase velocity greater than c, and wouldn't result in sub-diffraction limit imaging. I can send you to a paper that argues all of those things, and does so via causation. All of those things have been experimentally shown to occur.

Probably the most correct answer to your question - although undoubtedly the least fun - is that claiming a "speed of light" in a material whose index of refraction is wavelength dependent simply doesn't have any meaning. The assumption is still that light travels at c, but somehow some really complicated things are going on that obscure determining this simply. This is one of those assumptions that makes everyone feel better, and is nice to say to people outside science, but has no measurable evidence that I'm aware of. When actually working with the materials, the subject is just more complicated than that.

So the short answer is that some very funny and very weird things are going on here, but that there is no reason to think they violate causation or relativity.

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'']When you use punctuation, like a period or a question mark or a comma, you put a space after it. Its very irritating to read if you dont.

Thanks for advice. I will try to take proper care.

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CPL.Luke is right, the position of the peak changed making it appear like it was faster than light velocity.

Suppose in optical information transfer, you send in a pulse from one side and wait for the pulse to emerge on the other side. In such a case, most people keep track of the peak of the pulse because it is the easiest to observe. As the leading edge goes into the material, it becomes the peak because of the 'special' group velocity dispersion. So while the input peak hasn't even gone into the sample, already there is a peak on the other side. This is the illusion of 'faster than light velocity'.

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CPL.Luke is right, the position of the peak changed making it appear like it was faster than light velocity.

Right. With the small exception that the statement is physically inaccurate and logically inconsistent. A more correct response would have taken into account that the cesium trap and NI materials mentioned in the article have refractive indices that depend on frequency, and therefore the measurement of the group velocity isn't an acceptable measure of the speed the light is traveling.

Saying something "appears" or "looks" to be traveling faster than light is ineffectual without giving a good reason for why it does.

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Right. With the small exception that the statement is physically inaccurate and logically inconsistent. A more correct response would have taken into account that the cesium trap and NI materials mentioned in the article have refractive indices that depend on frequency' date=' and therefore the measurement of the group velocity isn't an acceptable measure of the speed the light is traveling.

Saying something "appears" or "looks" to be traveling faster than light is ineffectual without giving a good reason for [i']why[/i] it does.

Every material in the universe has a RI which varies with frequency, so it goes without saying. And the group velocity is the only important value in determining the speed of optical information...phase velocity is useless.

I thought CPL.Luke provided a good enough reason of why it 'appears' to be travelling faster. On a side note, I think that article is meant to mislead (for amazement factor).

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Excuse me, but doesn't the pulse coming out the other end constitute at least one bit of information? If nothing else, it says that the pulse is about to be sent. Doesn't that mean that dozens or millions of bits of information can be sent?

Writing in the journal Nature, Dr Lijun Wang and colleagues say their light beam raced through the atom trap so quickly that the leading edge of the pulse's peak actually exited before it had entered.

If the leading edge can be sent far enough in time, could it be used to warn us about an impending tsunami? Certainly that would qualify as information.

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Excuse me, but doesn't the pulse coming out the other end constitute at least one bit of information? If nothing else, it says that the pulse is about to be sent. Doesn't that mean that dozens or millions of bits of information can be sent?

I don't understand what you mean here. Can you clarify?

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It is a fascinating subject. The phase velocity is faster than c' date=' but interestingly, the group velocity travels the wrong direction, opposite to the direction of the energy. The doppler shift is also reversed. The diffraction limit is no longer a limit. Truly, some amazing things happen.

[/quote']

Some of these amazing things are remarkably similar to the effects of supersonic motion on sound waves. See the attachments.

I can send you to a paper that argues all of those things' date=' and does so via causation. All of those things have been experimentally shown to occur.

[/quote']

Could you send me the paper please?

So the short answer is that some very funny and very weird things are going on here, but that there is no reason to think they violate causation or relativity.

Causation, may be not.

Relativity, I wouldn't bet on it

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I haven't read all of the above. I assume someone has pointed out this is anomalous dispersion.

Mowgli: the article was from 2000, a bit out of date. Relativity is not violated, there's been 6 years of research on negative index materials, scientists are quite sure that no information travels faster than c and therefore relativity is not violated.

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scientists are quite sure that no information travels faster than c and therefore relativity is not violated.

This interpretation of SR always confused me. I thought SR dealt with objects (possibly with zero mass, as light) in motion, not with abstract concpets like information. The word "information" implies mind, consciousness, perception, intelligence, and whatnot. Why rope in information and all this baggage? Okay, I understand that the motivation is to safeguard causality - so that the "information" about the effect is prevented from reaching the observer before that about the cause. Still, the abstract nature of the word "information" takes the discussion down a slippery slope - into the usefulness of information, possiblity (or lack thereof) of using it for communication etc. Not physics, in my humble opinion.

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Check out this applet that shows the group velocity faster than c. Click on the applet to see the apparent 'signal' travelling backwards - seemingly from the destination to the source Check out the other applets too - quite interesting.

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We've had several discussions on anomalous dispersion. I suggest availing one's self of the search function.

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That's one of the better animations I've seen guardian, thanks.

This interpretation of SR always confused me.
It's a small technicality that means nothing can (and I'm just making this up, so don't try and pick holes in it!) directly travel faster than light.

What I mean is that no signal, no particle, no wave and nothing that could be used to transmit data could ever travel faster than c.

However things can "indirectly" travel faster than c. I'll give an example: if you hold a laser beam up at a ceiling (say it's pointing north) and then rotate the laser so it's point, say, north-east. Due to the distance between the laser source and the dot on the ceiling the dot on the ceiling will move quicker than the laser source. If you moved the laser source quick enough then the dot on the ceiling would move quicker than c.

Another technicality. SR does not say nothing can travel faster than c. It actually says that nothing can cross the "c-barrier". Meaning that if something came into existence travelling faster than c then that could be possible, it could not however slow down to sub-c speeds. However from other theories and experimental data the majority of scientists agree that no such particle exists.

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If you moved the laser source quick enough then the dot on the ceiling would move quicker than c.

Actually' date=' it wouldn't, if you think about it

But I will let that pass.

Another technicality. SR does not say nothing can travel faster than c. It actually says that nothing can cross the "c-barrier".

In my view, SR mapped the speed of light to infinity. In fact, Einstein pointed out this mapping in his 1905 paper. The mapping is most obvious in its consequences - nothing can travel faster than c (because c = infinity), and it takes an infinite energy to get to c (because you are trying to get to an infinite speed), time stands still if you travel at c (because it takes you no time to get any where if you are traveling at infinite speed) and so on. And, of course, adding or subtracting a finite speed to the speed of light wouldn't change it just like adding anything to infinity still yields infinity.

This mapping of c to infinity is philosophically sound because our sense of space is based on the light inputs to our eyes. Any distortions in our perception of space can be thought of as "real" distortions while still thinking of c as infinity, which is what SR does.

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mowgli the dot can move faster than light, and no information is transfered between the two points faster than c because they are really getting therre information from the laser source which is bound by travel at c.

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Actually' date=' it wouldn't, if you think about it

But I will let that pass.[/quote']Actually, if you think about it, it would!

In my view' date=' SR mapped the speed of light to infinity. In fact, Einstein pointed out this mapping in his 1905 paper. The mapping is most obvious in its consequences - nothing can travel faster than c (because c = infinity), and it takes an infinite energy to get to c (because you are trying to get to an infinite speed), time stands still if you travel at c (because it takes you no time to get any where if you are traveling at infinite speed) and so on. And, of course, adding or subtracting a finite speed to the speed of light wouldn't change it just like adding anything to infinity still yields infinity.

This mapping of c to infinity is philosophically sound because our sense of space is based on the light inputs to our eyes. Any distortions in our perception of space can be thought of as "real" distortions while still thinking of c as infinity, which is what SR does.[/quote']I don't like the way you say c is infinite here. It might be a maximum possible speed, but with infinity if you add one then you still have infinity, whereas with c if you add one you just get an even bigger number. This number is a finite number. Sure, you could never reach it as a velocity, but it is still a finite number.

Do you see where I am coming from?

It is a limit. That does not make it infinite.

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Could you send me the paper please?

How would you like it?

If you have access, here is the reference:

Valanju et. al.,Phys Rev Lett, 88, p 187401. "Wave Refraction in Negative Index Media: Always positive and Very Inhomogenous.”

Some choice quotes:

We show that ng is positive in NIM, and hence all interpretations of phase refraction as “negative light refraction” and “light focusing by plane slabs” [1 –6] are incorrect. Further, we argue that, in general, causality and ﬁnite signal speed would be violated if any physically realizable wave (signal) suffered “negative refraction.”

In conclusion, we have shown that causality and ﬁnite signal speed preclude negative refraction for any waves incident on any material, including NIM. The NIM dispersion implies positive group refraction even when phase refraction is negative, and causes large angles upg between phase and signal fronts and creates inhomogeneous waves that rapidly decay, during and after passage through NIM. The strong distortion of the signal puts severe bounds on the bandwidth of the information that can be transmitted through NIM devices. Negative refraction ray diagrams in all earlier literature do not represent the correct positive wave (i.e., signal) refraction by NIM.

Experimental confirmation of the things they claim to have proven impossible in the paper came about... well, about the same time the paper did.

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I thought CPL.Luke provided a good enough reason of why it 'appears' to be travelling faster. On a side note, I think that article is meant to mislead (for amazement factor).

And, for the record, I will concede that if you twist yourself into knots and ignore the need to actually be able to predict what is going on in these systems efficiently, you can write off all of these interesting effects in such ways.

However, it isn't useful, or even particularly logical. In materials with a positive index of refraction, varying phase and group velocities are trivial to write off. In anomolous dispersive materials, it requires more acrobatics, and begins to look very strange. In left handed materials, the explanations just sound absurd, and lead to terribly illogical conclusions.

Such as the paper I cite above. I can post a couple more if anyone finds that of value.

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mowgli the dot can move faster than light, and no information is transfered between the two points faster than c because they are really getting therre information from the laser source which is bound by travel at c.

I was thinking about the fact that it takes time for the laser to hit the dot, not so much the relativistic limit. Kind of like, if you take a garden hose and turn around fast enough (say inside cylindrical room), does the spot where the water hits the wall move faster than the speed of light? I think as you turn faster and faster, you only make more and more water spirals. (But trying to work this out mathematically, I don't seem to be able to demostrate it, so I'm not so sure... )

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I don't like the way you say c is infinite here. It might be a maximum possible speed' date=' but with infinity if you add one then you still have infinity, whereas with c if you add one you just get an even bigger number. This number is a finite number. Sure, you could never reach it as a velocity, but it is still a finite number.

Do you see where I am coming from?

It is a limit. That does not make it infinite.[/quote']

I meant c is mapped to infinity, not that c is infinity. In other words, if you add two velocities, you can never get a velocity with magnitude greater than c. To make the mapping more explicit, let's say you are travelling at v towards a source of light. You would expect the speed of light as you measure it would be c+v, but you would be wrong and the speed of light would still be c. Now, if the speed of light was really infinite, you would expect the speed as measured by you to be infinity+v, which is still infinity. This is what I meant by c is mapped to infinity.

May be it is better stated in AE's own words, “For velocities greater than that of light our deliberations become meaningless; we shall, however, find in what follows, that the velocity of light in our theory plays the part, physically, of an infinitely great velocity.”

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Actually' date=' if [i']you[/i] think about it, it would!

Okay, since I was challenged to think about it, I did. Here is what I came up with:

If you take your laser pointer, shine it on your ceiling and turn it so fast that the dot should be moving faster than the speed of light, you would see something quite weird. The dot will first appear on the ceiling almost directly above your head, then it will split and become two dots and going in opposite directions.

Would you take my word for it? I didn't think so!

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