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Why light travels

  1. Jul 12, 2005 #1
    How does light travel? What gives the energy to the photons to travel at c? And if in order for something to travel at c you need an infintie amount of energy, how come photon travels at c?
  2. jcsd
  3. Jul 12, 2005 #2
    no, you don't. photons are massless. [tex]E=hf[/tex]
  4. Jul 12, 2005 #3
    To elaborate, it only takes an infinite amount of energy for something to travel the speed of light if it has a rest mass (a mass in its own frame of reference) because relativistic mass increases as speed increases. But for something that doesn't have a rest mass (such as a photon), it has to move at the speed of light in order to even exist at all. (Or atleast, that's what the equations seem to suggest to me. I posted my reasoning behind this here: https://www.physicsforums.com/showthread.php?t=79637 and nobody told me I was wrong so I assume it was correct.)
    Last edited: Jul 12, 2005
  5. Jul 12, 2005 #4


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    Here is a link to get you started:

    Special Relativity as a Physical Theory

    And here:
  6. Jul 12, 2005 #5
    I think modern interpretation is that mass doesn't change (it's the same in all inertial systems), but energy does according to formula:

    [tex]E = \frac{mc^2}{\sqrt{1-(v/c)^2}}[/tex].

    So mass is the same in the eyes of every intertial observer (the term "invariant" is commonly used).
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