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Questions on Light

  1. Jan 5, 2004 #1
    I stumbled onto your question about why photons move at C in the Quantum Theory Forum:

    Why is it that massive objects can not travel the speed of light but massless objects can?

    I assume it's going to be difficult to get a real scientific answer outside of the usual mathematical explanations. I suppose know one really knows why massless particles travel at C.

    It's been suggested that massive particles travel at C through time as massless particles travel at C through space. I'm know authority on physics but it is interesting to think about. Do you have any ideas about this or are you just searching for your own answers right now?
    Last edited by a moderator: Jan 5, 2004
  2. jcsd
  3. Jan 5, 2004 #2
    I may not fully understand your question, so please excuse me.

    Energy must be put behind an object with mass to give it acceleration. The faster an object comes to c the more energy it needs, until it needs an infinite amount of energy to reach c. And because the universe works on real numbers, we could never get an infinite amount of energy behind a massive object.

    The possibility behind a massless particle always traveling c is becuase (you must excuse me on this, I won't be able to get it out correctly) any number put into zero is still zero. So any amount of force from a massive object could relativly be considered infinite to the massless particle, giving it the speed of c.

    Now the question is, Why does it only go c, instead of having an infinitly fast velocity?
  4. Jan 7, 2004 #3
    Consider ...

    Photons do not move.

    They are stationary bits sewn into the fabric of spacetime.

    Electromagnetic signals are transmitted from photon to photon in waveform.

    The massless photon does not travel at the speed of light - it is the transferred energy wave that moves at c.

    We live in a Universe that is a photon soup.

  5. Jan 11, 2004 #4


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    There are several ways to describe this, but I'm not sure that anything would be "deep enough" for you. Everything in physics has axioms at some point that you just have to deal with.

    If you accept that a geometric spacetime makes sense, then nothing could travel faster than light. Say that you pick two points A and B on a superluminal (spacelike) trajectory. Some (subluminal) observers will see A before B, while others will see the reverse. That obviously makes no sense if you believe in causality. So all objects travel at or below the speed of light (null or timelike trajectories).

    A particle at rest travels along a timelike trajectory. You could accelerate the particle, but its maximum speed can't go above c from the causality argument. Now the particle can either be accelerated up to c (exactly), or not. If that could be done in finite time, it would be very strange for there to be a sudden limit to where the object couldn't be pushed anymore. Aesthetically, a smooth transition should occur. Therefore massive particles can only reach c asymptotically.

    From the usual meaning of mass as it relates to inertia, a massless particle should have no inertia - the slightest touch would "infinitely" accelerate it. This would mean that massless particles should always move at the limiting velocity of a massive particle, which is of course the speed of light.

    Also, if the massless particle where moving at any less than the speed of light, someone could theoretically be in its rest frame where it would have zero energy. This violates energy (-momentum) conservation.
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