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Faster Than Speed Of Light

  1. Feb 24, 2004 #1
    Alright so were in physics class and we are covering relativity and learning that nothing can travel faster then the speed of light. So then a couple weeks later we find out something can. I believe it is called the U228 electron that can travel faster than the speed of light that is realized in Nuclear Power Plants. If anyone has any more information it would be greatly appreciated.
  2. jcsd
  3. Feb 24, 2004 #2
    As you approach the speed of light your mass will approach zero. You can not have mass and travel faster than the speed of light. The 2 are inversely related to each other. The the U228 electron actually an electron or is it a photon released from an electron returning to its original orbital???

  4. Feb 24, 2004 #3


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    I think you're talking about Cerenkov radiation, a blue glow that occurs when a particle (like an electron) exceeds the speed of light in its local medium -- like water. Particles can certainly can go faster than speed of light in a medium. No particles can go faster than the speed of light in a vacuum, however.

    - Warren
  5. Feb 24, 2004 #4
    Sorry, then.

    Last edited: Feb 24, 2004
  6. Feb 24, 2004 #5
    A tachyon can travel faster than the speed of light where the E-L equation of motion for a spacetime metric is equal to -1 (I think).

    In a nuclear reactor you see Cherenkov radiation as a result of the emitted particle travelling faster than the speed of light in the medium.
  7. Feb 24, 2004 #6
    Thanks alot you got what I was talking about... Cerenkov radiation.
  8. Feb 24, 2004 #7


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    But tachyons are still purely theoretical entities.

    When physicists talk about the speed of light, they usually mean c, the universal constant speed of light in a vacuum. When light goes through an optically dense medium, it still travels at c. The apparent loss of speed is due to repeated scattering by electrons in the medium. The amount of energy required to reach c is theoretically infinite, for any massive object.
  9. Feb 24, 2004 #8
    I’m trying to remember, does Cerenkov radiation have similarities with sound travel on breaking sound barrier, and after it ?

    I know this equation, but what happens when you have zero rest mass, and your speed is c:
    [tex]m = \frac{m_0}{\sqrt{1-v^2/c^2}} = \frac{0}{\{0}[/tex]
    Last edited: Feb 24, 2004
  10. Feb 24, 2004 #9


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    Yes, you can draw some parallels between sonic booms and Cerenkov radiation.

    And the equation you've listed, the definition of "relativistic mass," does not actually apply to photons. That equation is part of an old (and confusing) pedagogy in which mass gets larger with increasing relative velocity. Most physicists today would prefer to speak of only one kind of mass: rest mass.

    - Warren
  11. Feb 24, 2004 #10
    Mass and velocity are not related at all. There is no such equation like the one you have erroneously derived from the expression for relativistic momentum.

    Mass is invariant. Please let's not start this whole relativistic mass business again.
  12. Feb 24, 2004 #11
    Wow GRQC, can you please explain?! I have seen that formula everywhere!
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