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The Speed Of Light

  1. Nov 8, 2003 #1
    Before I make my post, I'd just like to introduce myself. My nickname is quantum_foam, as you can see (it's from the book Timeline by Michael Crichton). I consider myself a novice at best when it comes to the various areas of physics. I am especially interested in quantum physics, but most of it is quite confusing to me. Anyway, let me continue my post.


    I read about this experiment about a year ago, and I found it quite interesting to say the least. To me, the idea that the pulse of light existed the cesium cell BEFORE it had even entered was both defying my logic and intruiging at the same time. I have two questions for all of you:

    1. I haven't been able to find an explanation for how exactly the pulse of light exited before it entered. Do any of you know why this happens, or is it not quite known even by scientists? If you do know why this happens, I would appreciate any explanation you could give.

    2. Do you believe that this experiment actually proves that the speed of light is either infinite, or that it is a greater speed than what is generally accepted today?

    Thank you in advance for any and all information you can provide me. I'll probably be asking you questions about some points made in your responses, so please bear with my ignorance, as I am wholeheatedly interested in expanding my knowledge in physics (especially quantum physics).
  2. jcsd
  3. Nov 8, 2003 #2
    Here is a link to an article, which has a link to another good article:


    However, it focuses on superluminal group velocities, as opposed to the negative group velocity case you're talking about here, with the pulse supposedly "exiting before it enters".

    Here is a relevant Java applet:


    (That page discusses the mathematics if you choose the "More details" link, and treats the negative group velocity case.)

    They're interesting and non-intuitive, but these results do not violate the theory of relativity or any other known law of physics, so I always get kind of annoyed with the media hype over these results. It just boils down to what definition of the word "speed" you want to use. So "the speed of light barrier has been cracked" or other such claims are rather inaccurate, because there isn't any barrier like that to group velocity in the first place.
  4. Nov 8, 2003 #3
    I did my best to look up as much information on what superluminal means, and from what I can understand, it's simply a *frame of reference* which is travelling faster than the speed of light. The article then goes on to say that indeed the phase velocity and group velocity can exceed the speed of light. This is where I get confused. The phase velocity of a wave is simply the speed of the wave itself, while the group velocity is the speed of the wavepackets. Indeed the phase velocity and group velocity may break the speed of light, but it says "no energy or information actually travels faster than c."

    This is where I get confused. How can a wave travel faster than the speed of light, yet it is carrying no energy or information which travels faster than the speed of light? Does this mean the energy or information are delayed behind the head of the wave? Would the head of the wave even exist it contains no energy or information? I am thoroughly confused. Would you mind clearing this up? I have searched for an answer, but to no avail.

    Article I Used:
    Last edited by a moderator: Nov 8, 2003
  5. Nov 8, 2003 #4
    This is actually a point of contention, so don't feel bad about being confused. It's possible to prove as a general theorem that electromagnetic effects cannot causally propagate faster than c. However, it seems to be difficult to translate this into a concrete definition of the "signal velocity" of a wave, such that this "signal velocity" is always less than or equal to c. The paper remarks that one proposed definition doesn't actually work (it can lead to superluminal signal velocity). So, I'm not sure that your question has an answer right now.
  6. Nov 8, 2003 #5
    Thanks for all your input! My understanding on this experiment has been greatly improved!
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