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Questions puzzling me about light

  1. May 31, 2005 #1
    i have some questions puzzling me about light. the answers are probably simple but i dont now the answers to them. so here i go

    Why is it that light slows down in dense transparent materials, such as perspex, and regains its speed once it leaves the material?
    N.B. is it an energy loss/gain or just illusion or something else.

    furthermore i read that light has infact beened slowed significantly and even stopped by passing it through a cold sodium gas. My question is that if light has been stopped then wouldnt time also be stopped?

    Which leads me to my next question if a transparent material slows time (not sure if its true or not) does this mean aquatic life living underwater experience a slightly slower time elapse than what we surface dwelling being experience?

    this may sound farout and i could very well be wrong. Its just something that popped into my head the other day whilst thinking about light. Unfortunately i dont have a deep understanding so i need some help.

  2. jcsd
  3. May 31, 2005 #2


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    {We really need an FAQ for this kind of questions}

    At the simplest level, the photons do not slow down in a material. The scattering and abosorption/reemission by the material (i.e. the lattice) are what is reponsible for the apparent "slowing" down of light. But in between such events, they still travel at c.

    Nope.. Lena Hau can hold that stopped light as long as she wants. I still see the clock ticking on my wall when that happens.

    You need to consider what is meant by "stopping" light. Shine light onto a black piece of paper. Look at the other side. No light! You have just stopped light! Nothing interesting happened. Time didn't stop, no blackholes, and no Nobel Prize. What's the big deal?

    What Lena Hau and company did is "hold" information of that light indefinitely. This means you have to hang on to the energy AND phase of the light without losing coherence, and THEN, show that you can retransmit those at a later time. This is what's difficult to achieve and differ considerably than shinning light onto a black surface.

    In physics, we ALWAYS encounter situation like this where the words and questions we use have to be reexamine to make sure we understand what they mean and how limited they are.

  4. May 31, 2005 #3
    but if information is "held" would time essentially stop for the reference frame of the photon?
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