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Photon movement

  1. Jan 29, 2009 #1
    I have a few questions about photons.

    What makes photons move at light speed?
    If you slow them down, like if you have some material that they go through that slows them down, when they escape that material, do they automatically speed back up to light speed? If so, why?

    And if you can slow down photons, does that mean you can slow them down to a complete stop and have a bucket full of photons that are stationary?

  2. jcsd
  3. Jan 29, 2009 #2
    No, you can't slow down a bucket of photons...a photon wouldn't be a photon if it was stationary since it's a moving electromagneic wave. This is often referenced as one of the ideas that got Einstein thinking about relativity - if your speed matched that of a photon, what would it look like? He realized it's impossible for something to move at the same speed of a photon...and since c is the only speed at which objects with rest mass CANNOT move at, it means it's the only speed photons CAN move at (otherwise, if they moved at any speed achievabe by other objects, it would be possible for objects to be at rest wrt photons).
    As far as I understand, the implications of this are that even in a medium which "slows down" light, the individual photons are still moving at c ... there's a link that often gets posted here by people more familiar with the forum to explanations of likely and unlikely reasons as to why light slows down when travelling through media..but i'm not sure how to access the "faq" part of the forum so i'll leave it to someone else to post the link :)
  4. Jan 29, 2009 #3
    Oh...as to "what makes them move at the speed of light"...I'm not sure if you can say that something "makes" them do that...it's more of a definition... we define a "photon" as a particle that moves at speed c in a vaccuum. if it didn't, we just wouldn't refer to it as a photon
  5. Feb 1, 2009 #4
    Thanks for the response.

    What I'm wondering is why photons move that fast to begin with? What causes them to move?
    They move at C just by their very nature?

    I've seen some materials that supposedly slow down light, but you're saying the photon themselves are still moving at C? How is what we're seeing different than what the photons are actually doing?
    Interesting. Photons are just particles of energy, right? They're not actually made of matter?

    Is there anything similar to photons that move slower than C? Like you said, if it moved slower than C, it wouldn't be a photon. What would it be if it did move slower than C?

    Thanks for the response.
  6. Feb 1, 2009 #5
    Photons are the particles that we use to model light. Since light moves at the speed of light, photons move at the speed of light. Asking yourself why they do this is often non-sensical (at least physically). They just do, because they are photons.

    Light can also be described by being an electromagnetic wave. Electromagnetic waves slow down in a medium (only in a perfect (non-existing) vaccuum do they move at c). If you still want to describe the light as photons, they are doing something complicated. What they are not doing however is moving slower than c.

    They are not made out of matter, no. They aren't "made out of" anything, just like electrons are not made out of anything (at least in our current understanding).

    It could be anything. This question doesn't really make sense...
  7. Feb 2, 2009 #6
    I think this is easier to understand if you consider the 'photon' as an EM wave. Then the question you are asking is analgous to 'why do waves on string travel at the speed they do?'

    If you think about it like this, I think it's clearer that it's not really anything to do with the photon itself. It's the medium that the wave is travelling in.

    A wave on a string's speed is determined by the properties of the string, you'll also notice a wave on a string can't not move, if it's not moving it's not a wave. Likewise the speed of a photon is due to the properties of electrical and magnetic fields.

    Does that help?
  8. Feb 2, 2009 #7
    Then I guess a better question is why light moves at the speed it does.
    So in a perfect vacuum, light moves at C, but in reality it never moves at C?
    What do you mean the photons are doing something complicated?
    What's the difference between photons and an electromagnetic wave? They move different speeds?
    Electrons don't have mass? So electrons are just energy?
    It was in response to what he said. He said if a photon is moving slower than the speed of light, it's not a photon.
    My question is if a photon DID in fact move slower than C, what would it be, if not a photon?
    A little.

    I don't want to take the string analogy too literally, but since a string requires something to begin the motion of the string's waves, is there anything that begins the motion of the light waves?
    I think what's confusing me is photons. Since they're considered particles, I imagine a small particle of something moving at the speed of light, and if it's moving at the speed of light, I would think something has to CAUSE it to move that fast.
  9. Feb 2, 2009 #8
    It just does. Light has a fixed, constant speed (as postulated by Einstein) which we happen to call c, and which we have defined to an exact value (which I can never remember).
    By defining the speed of light we have in essence changed the length of a meter to the distance light travels in 1/c seconds.

    Afaik light never moves at exactly c, that is just the theoretical value that it would travel at if it weren't in any medium.
    There's an excellent link somewhere here that explains what photons are doing in a medium that causes light (not individual photons) to move slower than c, but it's pretty complicated and I can't find the link, sorry! Maybe someone else will come along shortly.
    Photons and EM waves are two concepts that describe the same thing. Some effects can be best described by regarding light as photons, others by regarding it an EM wave. Sometimes you need both views. As far as I know, both views are consistent.

    I never said that electrons don't have mass (they do). I said they aren't made up out of anything, by which I meant to say basically that electrons (as far as we know) are elementary particles. A proton or neutron is not an elementary particle because it consists of 3 quarks. An electron does not consist of anything as far as we know.
    (Also, energy and mass are equivalent, so you could indeed say that an electron is just energy (with a charge), although I don't know if that is 100% correct...)
    (Basically, if you do certain experiments with a proton you can come to the conclusion that it must be made up out of other (3) particles. If you do similar experiments with electrons you don't need it to be build up out of other particles so as far as we know it isn't. Maybe in the future we will discover that even an electron is not elementary, but that is speculation.

    Photons do not move slower than c, so your question does not make sense. What he meant was that photons always travel at c, they cannot slow down or stop. Your question is basically the same as asking "If a car is not a car, then what is it?"...

    I think there are multiple things, such as accelerating charges or electrons falling back to a lower energy level in an atom.
    Furthermore, lightwaves are not "motion". Nothing is physically moving. EM waves are electromagnetic field strenghts oscillating, not substances (such as in a string).

    A common "mistake". I suppose by "particle" you see a solid sphere of some material floating around in space. That's not what it is at all. In fact, in quantum mechanics, even electrons, protons, or even large molecules (such as the C60 'buckyball') are actually considered "wave packets", basically a collection of many waves summed so that they are localized at a small (but not infinitely small) position in space. There is always an uncertainty in the position of any object (regardless of measuring inaccuracy) which is caused by the particle actually being a wave (packet).
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