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Speed of light

  1. Aug 17, 2004 #1
    Let me preface this question by saying that it might be silly because I might have the facts wrong, but I'll ask anyway.
    If a photon is a "little packet/bundle of energy", the photon (energy) should contain some mass, even if it is tiny, since Einstein stated mass and energy are interchangeable. Now when this mass is accelerated toward the speed of light it needs energy to "push" it to a higher speed. When the photon starts getting closer to the speed of light, the energy that it takes to get it there starts to cause the particle to gain mass until it reaches a point where the energy it takes to overcome light speed would be infinite.
    My question is that if the photon has so much energy how come when a photon hits a human it doesn't "knock him/her out cold"?

    Like I said this may be a stupid question. I think I am just missing something really easy that I should know, so feel free to rip my statement apart and add any new info I am missing.
     
  2. jcsd
  3. Aug 17, 2004 #2

    mathman

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    Photons have zero rest mass. Photons travel at the speed of light from the time they are born. They do not speed up from a slower speed. You cannot think of them as small masses being speeded up. They do not have "infinite" energy, or anything close to it.
     
  4. Aug 17, 2004 #3
    What causes them to travel at the speed of light. Are you saying that they go from zero to 180,000 miles per second in an instant?

    P.S. Im not questioning you to be stubborn, I'm just trying to get the answer :approve:
     
    Last edited: Aug 17, 2004
  5. Aug 17, 2004 #4

    pervect

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    Photons never speed up, or slow down. They travel at 'c' from the instant they are created.

    The situation in a physical media gets a bit tricky. One of the best ways of thinking about it is that the photon travels at 'c', always, even in a media, but occasionally gets absorbed and re-emitted.

    Note that the speed of energy transmission through a media thus can be slower than 'c', due to the photons being absorbed, held for a very short while, then re-emitted.
     
  6. Aug 17, 2004 #5
    CbR

    Depending on the frequency and interaction with different electromagnetic waves, photons "can knock you out cold "
     
  7. Aug 18, 2004 #6
    who said that photons dont hit the humans?!!

    photons have momentum, since they have mass.

    but the problem is in the misunderstanding of the nature of the photons mass.Photons have relativestic mass, but not a rest mass.

    even though, it still has momentum, and that was proven..
     
  8. Aug 18, 2004 #7

    Chronos

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    Photons do not have mass. They do have momentum. Only massless particles are permitted to travel at c, in fact, they are forbidden to travel at any other speed.
     
  9. Aug 18, 2004 #8
    I thought that photons do have mass since a gravitational field "bends" them when they encounter it. How does gravity attract/affect something with a zero mass?
     
  10. Aug 18, 2004 #9
    Since a photon is generated by an electron, they're both right.
     
  11. Aug 18, 2004 #10

    Chronos

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    Gravity bends space. Photons always travel the shortest distance between two points [a straight line]. In curved space, the shortest distance between two points is a geodesic [a curved line].
     
  12. Aug 18, 2004 #11
    CBR

    All in all your question is actually a brilliant question. Physicists argue if photons are EM or just a wave duality of a particle without mass.
    There is no consclusive answer as of today.
     
  13. Aug 18, 2004 #12

    selfAdjoint

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    The general relativity gravity couples to energy density, not just mass. Light does not have mass, but it does have energy, and thus it gravitates.
     
  14. Aug 18, 2004 #13
    Would the rest mass of photons actually be non zero, then the theory of quantum electro-dynamics would have to be re-written :confused:
     
    Last edited: Aug 18, 2004
  15. Aug 19, 2004 #14

    chroot

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    Virtually all of existing physics would have to be rewritten for the photon to have mass. Simply put, every experiment ever done to date would not have have worked if the photon actually had mass.

    - Warren
     
  16. Aug 19, 2004 #15
    One of my friends told me that photons would have to have mass or they couldn't exist. I told him that if they did have mass a photon would knock you on your arse and turning on a light would cost you a fortune when your power bill arrived.
     
  17. Aug 19, 2004 #16
    UNLESS, the entire light theory as we know it is reversed.
    What IF photons are constantly in the air, but connect to an electron the moment it is energized - it connects to it at the speed of light ...........when it becomes visible, or not, depending on frequency.
     
  18. Aug 19, 2004 #17

    pervect

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    Photons are affected by a gravitational field. But in GR, energy generates the gravitational field, not mass. To be more precise, it's the stress-energy-tensor that generates the gravitatioanl field - under most circumstances, however, only the component T_00 of this tensor, which represents energy density, is important.
     
  19. Aug 19, 2004 #18

    Chronos

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    Your friend is incorrect. A photon has no mass, merely momentum. He is, however, right about the power bill.
     
  20. Aug 19, 2004 #19
    I said the power bill would cost you a fortune. That'd be correct right propelling a particle with mass to the speed of light.
     
  21. Aug 19, 2004 #20

    Chronos

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    That question has already been answered. As far as I can see. The fact you have a computer that works is pretty compelling evidence.
     
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