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Photon acceleration

  1. Sep 11, 2007 #1

    PhanthomJay

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    I turn on my flashlight and I am told that instantly the photons travel at the speed of light, i.e., their speed reach 'c' in zero seconds. Infinite acceleration? Doesn't seem right. Could something be happening during the planck time that provides some acceleration during that brief period?
     
  2. jcsd
  3. Sep 11, 2007 #2
    Acceleration? What, you're claiming the photons started with zero speed?
     
  4. Sep 11, 2007 #3

    Mentz114

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    Photons are massless so the idea of acceleration as in f=ma is not applicable. Also I doubt if anyone actually believes they are created in 0 time. Applying classical concepts to photons is not useful as has been discussed in many threads in PF.
     
  5. Sep 12, 2007 #4

    PhanthomJay

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    Let me rephrase the question. I'm not a regular on this forum, so I'm not sure of what may have been previously discussed on this topic. If photon's are not created instantly, are they created within the planck time, and is their speed instantaneously 'c' at the moment of their creation?
     
  6. Sep 12, 2007 #5
    As I've said elsewhere in the forums, photons are a *model*. They have certain properties, and behave in a certain way. Unfortunately, their role in the grander model of quantum electrodynamics means that they don't behave like little billiard balls. For one thing, their speed is c, always, and cannot be changed. The specific details of creation (i.e. whether it even makes sense to talk about a "moment" that a specific photon is created) simply do not exist in this model.
     
  7. Sep 12, 2007 #6

    Mentz114

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    PhantomJay, for a non-regular you have a lot of posts. As genneth says, the models we have don't tell us what you want to know. But a photon is not a photon unless it travels at c. Therefore all photons have always had that velocity. Perhaps you are proposing that there is something that becomes a photon by acceleration. That cannot happen because the thing would have mass and nothing massy can be accelerated to light speed.
     
  8. Sep 12, 2007 #7

    pervect

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    Photons can be counted, so you'd think that the number of photons in your flashlight is an integer, and that a graph of the number of photons in your flashlight must go up in jumps.

    However, quantum mechanics is not quite that simple.

    While photons can be counted, you are not guaranteed to have an integer number of them unless you actually do count them. (Formally, you have an integer number when the field is in an eigenstate of the counting operator. I'm not quite sure exactly how the counting operator works, actually, but every quantum measurement must be associated with an operator by the formalism ).

    If you don't count them, then it's perfectly possible for there to be a non-integer number of photons in your flashlight (on the average, this is called an expectation value). This is because the electromagnetic fields in your flashlight could be in a a "quantum superposition" of states. This is one of those weird quantum phenomenom that really don't have any good classical analogue.

    Given this possibility, though, there's no need for the graph of photon number vs time to go up in jumps.

    The fact that photons don't have a number unless you count them is similar to the way they don't have a position unless you measure it, and they don't have a momentum unless you measure that. And when you measure the position, you affect the momentum, and vica-versa.

    On a practical level, one photon more or less isn't going to be measuarble anyway. So you're better off thinking of your flashlight classically, unless you design your flashlight so that it only emits a few photons.
     
  9. Sep 12, 2007 #8

    robphy

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  10. Sep 12, 2007 #9

    PhanthomJay

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    Thanks all for the responses. The 'photon bag' story summed it up quite nicely! Thanks.
     
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