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Speed Of Light

  1. Nov 11, 2008 #1
    My teacher lectured on time dilation today and I'm confused about some things he said. How does light maintain the 2.99X108 m/s? And for that matter, what propels it to move that fast in the first place? Finally, why, as the c is approached, does the mass of an object get infinitely large?
    I know these may sound like some basic questions, but I didn't think they belonged with the Homework Forum. Thanks.
  2. jcsd
  3. Nov 11, 2008 #2


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    Have a read through the threads in this forum, these same question comes up a lot.
  4. Nov 11, 2008 #3


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    What level of school are you at? What kind of course are you taking: physics, general science, chemistry, or what? Do you have a textbook that covers this topic? How much math have you studied? The more information you can give like this, the better people are able to give answers that might help you.
  5. Nov 12, 2008 #4


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    Your questions about how light "maintains" its speed and what "propels" it indicate that you are under the impression that some kind of work must be done or force used to stay at a constant speed. That's not true. It goes back to Galileo that a body in motion stays moving at the same speed unless acted on by an external force. Light is not acted on by an external force.

    And your last question, " Finally, why, as the c is approached, does the mass of an object get infinitely large?" is simply wrong. The mass of an object does not get infinitely large. As an object goes faster, its mass gets larger. No object having non-zero mass can move AT the speed of light and never has infinite mass.
  6. Nov 12, 2008 #5
    Excellent questions. However scientists simply discover things they can't give answers as to why something is the way it is.
  7. Nov 12, 2008 #6
    Thanks for the responses!

    This is a high school Physics Course, and we are just briefly covering the topic. I was just curious about it. The textbook does not go into this depth. I am studying Multi-variable Calculus currently.

    "It goes back to Galileo that a body in motion stays moving at the same speed unless acted on by an external force. Light is not acted on by an external force."

    My mistake. I understand that it is not acted on by an external force. But what I don't understand is, what's the source of energy that allows it to gain that speed. Is it Electro- Magnetic Radiation?

    "As an object goes faster, its mass gets larger. No object having non-zero mass can move AT the speed of light and never has infinite mass."

    Why can't c be reached?
  8. Nov 12, 2008 #7

    Doc Al

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    Yes, light is electromagnetic radiation.

    A better way to describe the motion of a body with non-zero mass might be: As an object moves faster (with respect to you), it gets harder to increase its speed. As the object's speed approaches c, the tiniest increase in speed requires infinite energy. Which is another way of saying that you can't have an object reach speed c.
  9. Nov 12, 2008 #8


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    I might help you conceptualize it if you realize that anything with zero mass will move at c. If you were able to magically negate the mass of an object, that object would take off at the speed of light.
  10. Nov 12, 2008 #9
    Light IS energy.
  11. Nov 12, 2008 #10


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    The source of the light provides the energy that the light carries. In the classical (non-quantum) picture, electromagnetic radiation (which includes light, radio, etc.) is produced when electric charges accelerate. For example, when an electric current oscillates back and forth in a radio transmitter's antenna. The transmitter's power source provides the energy which gets radiated in the form of electromagnetic (radio) waves.
  12. Nov 12, 2008 #11
    False. Light has energy, but isn't energy itself.

    Just because you have a pet cat doesn't make you a cat =-/
  13. Nov 12, 2008 #12
    Then what part of light youu think is not energy?
  14. Nov 12, 2008 #13
    Its momentum.
    Last edited: Nov 12, 2008
  15. Nov 12, 2008 #14
    Momentum of light is nothing else than the "direction" of light in flat spacetime and thus the direction of energy. In curved spacetimes all momentum odds are off as momentum is not uniquely definable.
  16. Nov 12, 2008 #15
    Can you explain what "direction of energy" means if energy is always a scalar value?
  17. Nov 12, 2008 #16
    I rather abandon this discussion, it seems rather pointless.
  18. Nov 12, 2008 #17

    Jonathan Scott

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    The momentum of something is in general equal to its total energy multiplied by its velocity vector and divided by c^2. It describes the speed and direction of flow of energy. For non-relativistic speeds, it is equal to the mass times the velocity vector.
  19. Nov 12, 2008 #18
    Another way to think about where lights energy comes from is to relate the three forces we observe today (strong, weak, electromagnetic) to the early universe. When the universe made a phase transition from initially high and unstable ambient energy conditions the unified force "broke" apart and became the three "different" forces we see today.

    Many photons energy came to us via that process, called microwave background radiation; other photons appeared via fission, fusion and other reactions in stars, including sunlight. About 100 billion billion photons per second are emitted from a 100 watt light bulb.

    You could equally well ask where does the energy for any of the forces come from, including gravity. As MeJennifer posted/implied, a lot of the "real" cause is speculation/theoretical and lots remains to be learned. For example, the strong nuclear force also involves massless particles analogous to photons, yet it's behavior is quite different from electromagnetic fields. Weak force particles have mass. Why: all this may be a result of the initial conditions when the universe made it's transition to a more stable state we see today.
    Last edited: Nov 13, 2008
  20. Nov 12, 2008 #19
    Is this how the Large Hadron Collider fits in? Are they trying to recreate the phase transition from the unstable high energy to the three regular forces we see today?

    Also, why, in the early universe, did the phase transition occur?
  21. Nov 12, 2008 #20
    Is this also the case if the object has a really small mass? Like 1X10-9 nanograms? Will it move at a fraction of c?

    Intuitively, my statement (above) doesn't make sense, but neither does the statement of a mass of zero moving at the speed of light. Is there possibly a formula that explains this?
  22. Nov 12, 2008 #21


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  23. Nov 13, 2008 #22
    The answer to this question is really what I am looking for.
  24. Nov 13, 2008 #23
    How much of the responsibility for determining the answer are you willing to take on yourself? That is generally a crucial question for scientists or someone learning science. So much depends on the interpretation of the mathematics and observed behavior. Different people working on the same problems often arrive at very different interpretations.

    Case in point: Louis De Broglie answered your question nearly a hundred years ago and concluded that a photon actually does have a very tiny amount of mass. However to incorporate that as a facit of way the rest of physics works would require a major renovation or reinterpretation of very many things. Most of the physicists of his time did not follow, though a minority do still consider the value of his analysis. But that does leave your question and many related things a bit unresolved if we stick to the textbooks and the most common interpretations as you intuitively sense that some concept or observation is missing.
  25. Nov 13, 2008 #24
    I don't think that's a good analogy...the LHC is just trying to smash particles together at high enough energies to break them apart and see what's inside. The energies available experimentally fall far short of conditions shortly after the big bang. During inflation time and space did not even exist...

    It's a theory embedded in inflation theories...systems "naturally" move from more energetic to less....entropy increases. Think of a pencil balanced on it's point: its highly symmetric to say N,S,E,W but very unstable...it falls and some energy is released...now it's asymmetric (pointing,say, SSW) but quite stable on a table top. Or a hot piece of metal....it cools and becomes loses energy "naturally"...

    I think all we MAY really know that if things did not evolve this way, we would not be here...the universe could not have formed....
  26. Nov 13, 2008 #25


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    The equations

    [tex]E^2 = (pc)^2 + (mc^2)^2[/tex] ...........(1)
    [tex]pc^2 = Ev [/tex] .........................(2) ​

    ((2) being referred to in post #17) can be rearranged to eliminate momentum p and get

    [tex]E^2(1-v^2/c^2) = (mc^2)^2[/tex] ..........(3) ​

    For something that has energy but no mass, the only solution is |v| = c. Conversely, if |v| = c, m must be zero.

    The thing to realise is that massless particles always travel at the speed of light. They don't begin life at rest and then accelerate. As soon as they come into existence, they are already moving at light speed. The energy and momentum for that motion will come from the object that emitted the massless particle.

    N.B. In the above, "mass" (m) means "rest mass" (which excludes kinetic energy) not "relativistic mass" (which includes kinetic energy and is E/c2). Most modern physicists use rest mass only. The idea that mass increases with velocity applies only to relativistic mass. Light has zero rest mass, but non-zero relativistic mass.
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