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Superluminal velocity

  1. Jan 12, 2007 #1
    there are some reports on superluminal velocity

    www.space.com/scienceastronomy/generalscience/faster_than_c_000719.html

    news.bbc.co.uk/1/hi/sci/tech/841690.stm

    I just want to know what you might think of this.

    I personally think that this might lead us to something practical.
     
  2. jcsd
  3. Jan 13, 2007 #2

    Jorrie

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    Wrong Wang simultaneity?

    I suspect Wang got his definition of simultaneity wrong!

    Jorrie
     
  4. Jan 13, 2007 #3
    This is a common mistake: taking the group velocity instead of phase velocity as the speed of light. For a very good explanation of the difference, see here
     
  5. Jan 13, 2007 #4

    ZapperZ

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    Y'know, my expectation is obviously too high. I was expecting that something THIS old and have been discussed pretty extensively, would by now be common knowledge.

    The NEC experiment has been discussed so many times, I'm surprised that this thing is still propping up. I love the way this website is claiming that it is "controversial". It isn't! Even the authors of that paper did not claim that any part of the wave traveled faster than c. Read it if you don't believe me.

    The whole effect is due to a rather anomalous dispersion of the medium, in which the pulse is severely attenuated. In effect, if you have a gaussian pulse, the front foot of the pulse was amplified, while the back foot was attenuated, causing the PEAK position of the gaussian to be shifted further forward. This, however, requires the "coorporation" of the medium. Nothing in here traveled faster than c, if that is still unclear.

    I expect this to repeat with the recent paper that was published in App. Phys. Lett. on sound waves supposedly having "superluminal" velocity

    http://physicsweb.org/articles/news/11/1/8/1

    So who wants to bet on when we'll first see someone in here claiming such a thing?

    Zz.
     
  6. Jan 13, 2007 #5

    pervect

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    Usually people who perform such experiments are reasonably careful to point out that they don't violate relativity in any way, shape, or form. In fact, relativity was used to predict the results of the experiments. But this caution doesn't come through in the popular media. Hence - controversy - generated by bad reporting, where the expirement proves one thing, and the media makes grandiose claims. The claims being made by the experimenters may not be controversial, but the claims being made by the media certainly are.

    I was especially amused by the alleged "practical applications".
     
  7. Jan 15, 2007 #6
    ups, I think I have much to learn. At least for now I learned not to trust the media. If there is nothing travelling faster than c, then what seems to be travelling faster than c?
     
  8. Jan 15, 2007 #7
    They have actually transmitted the whole Mozart music at a speed faster than c.
    Information has broken the speed barrier causing some violation of the causality principle.
    At least this is what they claim.
     
  9. Jan 15, 2007 #8
    then? is it true? so the information has travelled faster than c?
     
  10. Jan 15, 2007 #9

    pervect

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    There is no convincing evidence that a Mozart symphony has actually been transmitted faster than 'c'.

    Suppose you have a psychic, who claims to recieve messages faster than light.

    You decide to put her to the test. So you send her the following sequence:

    1,2,3,4,5,6,7,8,9

    and ask her what the next number is. She correctly predicts 10, and does so at such a time that she couldn't have recived the number '10' by any possible light signal.

    Are you convinced that she received psychic messages "faster than light"? I'm not.

    The problem is that predicting the next element of the sequence {1,2,3,4,5,6,7,8,9,x} is not "hard" - you can do it by linear interpolation.

    Draw a waveform of a Mozart symphony, record it, and plot the value of the signal at 1 nanosecond intervals. You will see that it is essentially a straight line. Thus sending a Mozart symphony is not a good test for whether one has sent a signal unless one sends it quite a long distance. It's no harder to predict the value of a Mozart symphony correctly 1 ns in advance than it is to predict that 10 was the next number in the above sequence. In both cases, one uses simple linear interpolation.

    To provide convincing evidence, you'd have to send a random sequence, not an easily predicted one. A Mozart symphony is simply not random, and not a good test.

    The technique in question fails the test if one attempts to send random information (such as white noise) rather than a Mozart symphony.
     
  11. Jan 15, 2007 #10

    Chris Hillman

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    And be very wary of that claim!

    The entire rest of the world says that they are wrong. I wouldn't take these claims very seriously.
     
  12. Jan 16, 2007 #11

    ZapperZ

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    There have been whole threads on this. I know because I've responded to such claims several times.

    First of all, the phenomenon involved is completely different than the one in the OP. So essentially this is a thread hijack.

    Secondly, what was claimed was that the information can travel faster than c during tunneling (different mechanism than the NEC experiment). Again, there have been a lot of stuff written to dispute this. One should no longer make such claim without pointing out all the papers that contradicts such claim. Not doing that is just plain irresponsible and an outright misinformation.

    Zz.
     
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