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Faster than light

  1. May 5, 2005 #1

    This raised a few questions. If light has no mass, can i assume it is then energy and could thus be transferred back into mass?

    And if energy can travel at 310 times the speed of light, why couldnt its equivalent in mass.
  2. jcsd
  3. May 5, 2005 #2
    Yes, light energy can sometimes change into mass. For example, a photon may become a particle-antipartical pair. However, the antiparticle willl usually soon annihilate with another particle, making that particle and the antiparticle disappear, and making another photon.
    Or, a photon may strike some atom or molecule and bump it into a higher-energy state, which will increase the effective mass of that item. (or that's my vague understanding, anyway).

    Now, as to the Wang, Kuzmich and Dogariu experiment, I remember much discussion about it at the time (five years ago), and seemto recall that while it's an interesting effect, it doesn't actually translate to energy travelling at greater than light speed.

    I think it's kind of like this:
    Imagine a huge stadium encompassing the whole solar system. Now, give everyone a timer, and ask them to stand up and sit down when the timer goes off. If you set up your timers properly, you could have a Mexican Wave that goes around the stadium at faster than light speed...
  4. May 5, 2005 #3

    surely you could do this, but i dont see the relation.
  5. May 5, 2005 #4


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  6. May 5, 2005 #5


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    He is demonstrating what is called superluminal motion. If you shine a flashlight at a distant star, and then quickly point it at another star, you can make the spotlight move faster than the speed of light. There isn't really any THING that exceeds c, so no violation.

    But I don't know how superluminal motion relates to your question either.
  7. May 6, 2005 #6
    Well, that's pretty much what they did in the experiment. The energy was already in place, they just made each bit jump at the right time to look like a superluminal pulse. (Although "Just" might not be the right word!)
  8. May 6, 2005 #7
    Shadows and light spots can go faster than light but they can't carry physical information. This is confirmed by many experiments, including the spot of a laser which is pointed at the surface of the moon.

    This can also be confirmed using logic. Distance and direction are abstract and therefore there is no real communication taking place between the physical objects.
    Last edited: May 6, 2005
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