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What happens when light is brought to an absolute stop?

  1. Jun 28, 2010 #1

    According to the above article, light was brought to a complete stop during an experiment. What confuses me is that, if light has no mass, and is then manipulated to have no velocity either (which means there is no frequency and hence no energy) then what DOES it have? Does it even exist anymore? I did not understand this part of the article, and I would appreciate it if somebody could explain it to me.
  2. jcsd
  3. Jun 28, 2010 #2


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    The fiction of light slowing down is probably not an intuitively useful abstraction at this point. Really, they just found way (an apparently very good way) to store the information in the light so that they can recreate it later.
  4. Jun 28, 2010 #3
    Remeber, the light is not stopped in vacuum. What happens is actually that the state of the light is transferred to the atoms in a suitable prepared matter, which in this case is a rare-earth-ion-doped crystal.

    One can also easily do slow light in these types of materials, where the light pulse is seen to be travelling slowly through the medium. One can show however, that in this case the energy of the light is only partially stored in the medium during the travel, and returned to the light as it exits. The slower you can get it to travel the more of the energy is transfered to the material, and in the special case of stopped light, everything is in the medium. The reason it's called stopped light is probably more historical (and because it sounds cool :biggrin: ).
  5. Jun 28, 2010 #4
    :rolleyes: This is the source of a lot of confusion, sometimes even propagating among people who should know better. Sometimes such news articles even go so far I scratch my head and go: what the hell were they trying to claim? I tend to find the actual facts far more interesting, and intriguing when certain glossed over questions remain unanswered.
  6. Jun 28, 2010 #5
    Oh, so its just a not-completely-true simplification of whats actually going on, which is that energy/information is being transferred into the material?
  7. Jun 28, 2010 #6
    As light enters a transparent solid, it slows down and increases in frequency (in other words, its wavelength gets shorter).
    If light slows down to a near-standstill, therefore, it would become extremely energetic.
  8. Jun 28, 2010 #7


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    It would take a strange material to change the frequency; normally it's held constant, while velocity and wavelength vary proportionally.

    It's relevant to the topic to recognize that light slowing down is a fiction. Photons always move at the speed of light; but because they are constantly being absorbed and emitted, the progress of the (bulk of the) wavefront is delayed, and that's what is really meant by "light slows down".
  9. Jun 29, 2010 #8
    Is there any real evidence for this? I heard that how the photon slows down is actually not known.
  10. Jun 29, 2010 #9
    This is not entirely true.

    Saying photons are constantly being absorbed and reemitted suggests a resonant phenomena, which is usually not the case. By tailoring the inhomogeneous profile for a material such as the one mentioned in the OP, one can create a situation where you have a strong dispersion profile (refractive index varies greatly with frequency), but zero absorption.

    In this case the spectral components of a pulse going through the material sees different refractive indicies and when you sum up the movement of all spectral components, it is seen that the group velocity of the pulse is either slower of faster than c depending on the actualy shape of the dispersion curve.

    The refractive index itself is of course due to light interacting with the atoms in the material, but it's very offresonant interaction so one should probably not call it absorption and emission, and in addition, no light is lost in the crystal, one can make 100% go through.
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