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Question about photons

  1. Dec 2, 2005 #1
    hi there,
    This may seem like a stupid question, but I'm having a lot of trouble finding out anything just from reading.
    Ok, so photons are described as behaving both like waves and particles. They have no mass, like other forms of electromagnetic radiation, otherwise they could not travel at C, right? Ok. SO, if nothing WITH mass can be accelerated to the speed of light, because it would take infinite energy and become infinitely massive, can something WITHOUT mass exist as a non-moving wave/particle, and then be accelerated to C? In other words, Is radiation something that is created from something (since it is not massive this would not violate thermodynamics, right?) or is it there to begin with and then accelerated, or what?
    Any help would be appreciated
    Thanks
     
  2. jcsd
  3. Dec 2, 2005 #2

    selfAdjoint

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    In quantum theory the are described as something we can describe by mathematical state vectors in a Hilbert space, which is capable of behaving in a wavelike or pointlike way depending on how it is observed/interacts with the environment.

    ALL forms of electromagnetic energy are described by photons in relativistic quantum theory.

    Right.

    Relativity says that a massless particle does not have a rest frame, which is just other language for not being able to exist with v = 0.

    A photon is emitted from some particle interaction. Its energy is depleted from the pre-existing particle states whatever they are, so that energy is conserved. But it is not correct to think of the photon as pre-existing inside the previous states; rather it emerges out of the whole interaction picture. There are also quantum issues, which I don't want to go into here, over virtual photons, which are very important for carrying electric chanrge, but which don't have real energies.
     
  4. Dec 2, 2005 #3
    I have a follow up question. This is only intended as a question, and absolutely not intended to be argumentative, so please do not take it as that. I cannot come up with an answer for it in any book or other research material I have found yet.

    If some subatomic particles which travel at less than C, but still have mass (and certain physical dimensions), can travel deep through the Earth, but photons, which would have no mass (and assumably even smaller physical dimensions), and travel at C, cannot even pass through dark construction paper in light form?

    (yes, I know that radio waves and many other higher frequency waveforms can pass through mass, but even then, not to a very deep distance through Earth.)

    Is there something I am missing?
     
  5. Dec 2, 2005 #4

    JesseM

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    Being stopped by travelling through a medium isn't primarily a question of mass, it's a question of the forces by which the object interacts with the medium--photons interact with matter via the electromagnetic force (and normal matter is full of positive and negative charges), while particles that travel through matter easily have a weak or nonexistent electromagnetic interaction. The electromagnetic force is responsible for most forces you experience in everyday life--for example, when you push against a wall and the wall pushes back, that's an electromagnetic interaction between the atoms in your hand and the atoms in the wall. In theory even a very massive object that didn't interact via the electromagnetic force could pass easily through solid matter (and since it wouldn't radiate light either, us normal-matter beings wouldn't even notice).
     
  6. Dec 2, 2005 #5
    Isn't it possible to dismiss the notion that photons "travel" at all? We interpret light as a traveling object by reference to macroscopic ones such as water waves and golf balls. But if instead we interpret light as a quantum influence between an emitting body and a receiving one, doesn't this eliminate a few problems? For example, instead of considering the "speed" of light as if it were a moving object, it may be simpler to consider the time it takes for an atomic change in one body to show its influence on another body located at a given distance. The math would work out the same way, but the preception would be more like entanglement: spooky action at a distance, but not instantaneous. It also makes it easier to grasp that different radiations penetrate different substrates: they are just poorly tuned to influence these particular molecules so they entangle with something located behind them instead.

    Too weird?
     
  7. Dec 2, 2005 #6
    I suposse you were trying to say <<of considering light as if it were a moving object>> -cause i dont think someone consider the speed of light as a moving object -.

    The concept of "action at a distance is directly related with the "instantaneous".

    Even more, if you don't take the light as a moving object, you would be saying that if in the tame elapsed between both events (emiting and receiving the signal) another object, say an electron, interposes exactly between the receiver and emitter, exactly in the direction supossed for the wave, there will still the receiver have the signal. If you don't take into account different posible scenarios, is not physics. You supose the systems being isolated, but you can add more and more interactions. Thats actually a reason why we do consider light as a moving object.
     
  8. Dec 2, 2005 #7
    Sorry, I meant: instead of considering the "speed" of light as if light [it] were a moving object...
    I was suggesting something new.
    I see your point. If a producing atom pre-selects a corresponding consuming atom then the photon would have to swerve around any obstacle interposed in the way. If no pre-selection is involved then the atom that consumes a produced photon must be selected at a later time according to the movement of other bodies. At least some probability wave would have to propagate in order to make this selection on the way, so nothing much is gained by the idea.

    Oh well. :rolleyes: Thanks for the reply!
     
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