Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A Does the superluminal transport of energy violate causality?

  1. Jan 19, 2016 #1
    In superluminal light pulse experiments, as shown in Fig. 4 of the Letter [Nature 406 (2000) 277], the whole pulse intensity profile observed is advanced by 62 ns nearly without any distortions, and the light pulse energy must be transported faster than the speed of light in vacuum (superluminally). The superluminal energy transport without any losses, observed in their experiments, does not violate Einstein’s special theory of relativity?

    http://www.nature.com/nature/journal/v406/n6793/full/406277a0.html
     
    Last edited: Jan 19, 2016
  2. jcsd
  3. Jan 19, 2016 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    It is not. The material passes energy to the front of the pulse and absorbs energy from the back.
     
  4. Jan 19, 2016 #3
    Please give references to support your theory.

    According to the Letter [Nature 406 (2000) 277], the probe light pulse is placed in the middle of two gain lines spectrally and it contains no spectral components to be amplified. In other words, the light pulse go through a LOSSLESS anomalous dispersion medium, and there is no energy exchange between the pulse and the medium. From this I don't think your argument:
    "The material passes energy to the front of the pulse and absorbs energy from the back."
    works.
     
  5. Jan 19, 2016 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    There is no net gain, but if there would be no energy exchange the pulse would behave like in a vacuum. The reference is the paper you quoted. It is called "gain-assisted" for a good reason and the pulse is not monochromatic (a monochromatic pulse does not have a meaningful group velocity).
     
  6. Jan 19, 2016 #5
    The Letter [Nature 406 (2000) 277], which I cited, never said "The material passes energy to the front of the pulse and absorbs energy from the back." The authors of the Letter are all experts, but the argument "the material passes energy to the front of the pulse and absorbs energy from the back" does not seem like an expert opinion; I never heard of such theory; so I judge they never said.

    "gain-assisted" ----- means using two gain lines to create a lossless anomalous dispersion region. You did not read the Letter.
     
  7. Jan 19, 2016 #6
    It doesn't says it with the same words but it seems that's how it works. Due to different energetic states of the atoms the media can absorb and emit energy. In order to get the described result it needs to emit energy into the front of the pulse and absorb energy from the tail. The experiment is designed to keep energy and shape of the pulse unchanged. In the result the observed group velocity exceeds the speed of light or is even negative but there is no superluminal transfer of energy.
     
  8. Jan 19, 2016 #7

    Nugatory

    User Avatar

    Staff: Mentor

    Nonetheless, mfb's explanation is consistent with the mathematical analysis in this paper.
    The paper doesn't use those words because it's written for a technical audience not laypeople.

    As this thread started with your misunderstanding of the the physics involved, and you have been given the best explanation possible without walking you through the analysis in the paper, this thread is closed.
     
  9. Jan 19, 2016 #8

    Nugatory

    User Avatar

    Staff: Mentor

    Not according to the authors:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Does the superluminal transport of energy violate causality?
  1. Violation of causality (Replies: 7)

Loading...