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Anomalous refractive index? How doesnt this defie relativity

  1. Jun 29, 2007 #1
    I was reading this article

    I dont understand how this doesnt defie relativity. It sais something about you can see the light exit the caesium before it even enters, thus having went faster then the speed of light but the article sais it doesnt defie relativity because you cant use it to transfer infromation faster then the speed of light. So how do we know it went faster then the speed of light if no infromation is transfered before the light gets to us. I think im way off on this one...
  2. jcsd
  3. Jun 29, 2007 #2
    In these situations, no signal goes faster than light, it's just that you can sometimes arrange for the light coming out to reach a peak before the light that's going in peaks.
  4. Jun 30, 2007 #3
    isnt the light going in the same light thats going out?
  5. Jun 30, 2007 #4


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    This is a phase velocity vs group velocity issue. (Read the original bbc article - it even mentions group velocity by name. This is encouraging, sometimes journalists omit critical tecchnical details like this).

    Nothing physical is moving faster than 'c'. To see this visually, you might try the applet at

    http://galileo.phys.virginia.edu/classes/109N/more_stuff/Applets/sines/GroupVelocity.html [Broken]

    Some other online treatments:

    Last edited by a moderator: May 2, 2017
  6. Jun 30, 2007 #5
    I think I understand this, if I do it makes this BBC articale look pritty decieving.

    "The end result was a beam of light that moved at 300 times the theoretical limit for the speed of light. "

    But from what I understand from reading those articles is that the lights velocity is actualy still C, and group velocity is just how fast it would be if you mesured the total distance moving up and down through the curves devided by time?

    I dont know if I interprited that right but if so is it suprising to see group velocity go that fast? Does it ushaly go the same speed the light does, I guess it couldnt if your mesuring the hole wave compared a straight line.
  7. Jul 2, 2007 #6


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    Yup. But what else did you expect from the mass-media--it's like an excited little school-girl babbling incoherently about things she doesn't understand in order to excite and surprise her silly friends. Science, on the other hand, marches on, maintaining his sang-froid.
  8. Jul 2, 2007 #7


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    :rolleyes: I don't think that's a fair comparison. The journalist isn't someone who knows absolutely nothing, just believing everything he is told, and writing it in the press. He has a PhD (a quick google search shows he's an astronomer) and is trying to write scientific articles for the general public-- something that is not very easy to do!
  9. Jul 2, 2007 #8


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    Mass-media as a metaphorical school-girl? It's as fair as life.
  10. Jul 2, 2007 #9
    Shool boys aren't babbling incoherently too? LOL
  11. Jul 2, 2007 #10


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    okay, school boys babble too...
  12. Jul 14, 2007 #11
    This text is from the Wikipedia entry - Speed of Light

    I had just read it earlier today, and thought of it when I read your post.

    "Faster-than-light" observations and experiments

    Main article: Faster-than-light

    The blue glow in this "swimming pool" nuclear reactor is Cherenkov radiation, emitted as a result of electrons traveling faster than the speed of light in water.
    The blue glow in this "swimming pool" nuclear reactor is Cherenkov radiation, emitted as a result of electrons traveling faster than the speed of light in water.

    It has long been known theoretically that it is possible for the "group velocity" of light to exceed c.[5] One recent experiment made the group velocity of laser beams travel for extremely short distances through caesium atoms at 300 times c. In 2002, at the Université de Moncton, physicist Alain Haché made history by sending pulses at a group velocity of three times light speed over a long distance for the first time, transmitted through a 120-metre cable made from a coaxial photonic crystal.[6] However, it is not possible to use this technique to transfer information faster than c: the velocity of information transfer depends on the front velocity (the speed at which the first rise of a pulse above zero moves forward) and the product of the group velocity and the front velocity is equal to the square of the normal speed of light in the material.

    Exceeding the group velocity of light in this manner is comparable to exceeding the speed of sound by arranging people distantly spaced in a line, and asking them all to shout "I'm here!", one after another with short intervals, each one timing it by looking at their own wristwatch so they don't have to wait until they hear the previous person shouting. Another example can be seen when watching ocean waves washing up on shore. With a narrow enough angle between the wave and the shoreline, the breakers travel along the wave's length much faster than the wave's movement inland.

    The speed of light may also appear to be exceeded in some phenomena involving evanescent waves, such as tunnelling. Experiments indicate that the phase velocity and the group velocity of evanescent waves may exceed c; however, it would appear that the front velocity does not exceed c, so, again, it is not possible for information to be transmitted faster than c.

    In quantum mechanics, certain quantum effects may be transmitted at speeds greater than c (indeed, action at a distance has long been perceived by some as a problem with quantum mechanics: see EPR paradox, interpretations of quantum mechanics). For example, the quantum states of two particles can be entangled, so the state of one particle fixes the state of the other particle (say, one must have spin +½ and the other must have spin −½). Until the particles are observed, they exist in a superposition of two quantum states, (+½, −½) and (−½, +½). If the particles are separated and one of them is observed to determine its quantum state then the quantum state of the second particle is determined automatically. If, as in some interpretations of quantum mechanics, one presumes that the information about the quantum state is local to one particle, then one must conclude that second particle takes up its quantum state instantaneously, as soon as the first observation is carried out. However, it is impossible to control which quantum state the first particle will take on when it is observed, so no information can be transmitted in this manner. The laws of physics also appear to prevent information from being transferred through more clever ways and this has led to the formulation of rules such as the no-cloning theorem and the no-communication theorem.

    So-called superluminal motion is also seen in certain astronomical objects, such as the jets of radio galaxies and quasars. However, these jets are not actually moving at speeds in excess of the speed of light: the apparent superluminal motion is a projection effect caused by objects moving near the speed of light and at a small angle to the line of sight.

    Although it may sound paradoxical, it is possible for shock waves to be formed with electromagnetic radiation. As a charged particle travels through an insulating medium, it disrupts the local electromagnetic field in the medium. Electrons in the atoms of the medium will be displaced and polarised by the passing field of the charged particle, and photons are emitted as the electrons in the medium restore themselves to equilibrium after the disruption has passed. (In a conductor, the equilibrium can be restored without emitting a photon.) In normal circumstances, these photons destructively interfere with each other and no radiation is detected. However, if the disruption travels faster than the photons themselves travel, as when a charged particle exceeds the speed of light in that medium, the photons constructively interfere and intensify the observed radiation. The result (analogous to a sonic boom) is known as Cherenkov radiation.

    The ability to communicate or travel faster-than-light is a popular topic in science fiction. Particles that travel faster than light, dubbed tachyons, have been proposed by particle physicists but have yet to be observed, and would potentially violate causality if they were.

    Some physicists, notably João Magueijo and John Moffat, have proposed that in the past light travelled much faster than the current speed of light. This theory is called variable speed of light (VSL) and its supporters claim that it has the ability to explain many cosmological puzzles better than its rival, the inflation model of the universe. However, it has not gained wide acceptance.
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