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Electromagnetic energy wave (photon) questions

  1. Oct 14, 2009 #1
    some questions that have been bugging me:

    1.Is there a beginning and an end to a photon? Does it take different amounts of time for different wavelength photon energies to be absorbed? would for example a radio wave photon be absorbed at a slower rate than a higher energy/frequency gamma photon? made some pictures on paint to help explain

    Pictorial representation for lower energy photon here:
    http://www.flickr.com/photos/43566332@N06/4011200203/

    Higher energy photon picture representation here:
    http://www.flickr.com/photos/43566332@N06/4011965740/

    2.If the dispersion of a photon through space and time leads to less intensity in the emitted photon, would there be gaps in the photon? In other words in gravity free empty space would photons from the exact same origin be picked up in some spatial positions and not others? if so, at what distance from the source of the photon does this start to occur?

    Picture to help explain:
    http://www.flickr.com/photos/43566332@N06/4011336977/

    thanks ill have more questions later
     
    Last edited: Oct 14, 2009
  2. jcsd
  3. Oct 14, 2009 #2
    Your questions are little ill posed... are you asking 'How fast a photon is absorbed by an atom?' (i.e. how fast does a quantum transition occur?) or are you asking does one photon travel faster in space than another?

    Regardless, you haven't asked the most important question of them all which is "What is a photon exactly?" Becuase if you knew the answer then you would be able to answer your own questions.

    The truth is photons don't 'travel' through space at all. A photon isn't a physical thing in the sense of a particle. A photon is a means of transfering energy between a radiation field and some other field (say an atom for example). Its a very common misconception that pop culture has lead people to believe that a beam of light should be thought of a being made of photons. Its an interesting model that helps to explain for example, how many interactions you could expect between a radition field and a collection of atoms. But it doesn't even make sense to say that a beam of light has 'photons' in it. The photon doesn't exist as a physical entity, its simply a name given to the quantity of energy which is absorbed or emitted during the interaction with a radiation field. In quantum mechanics you refer to the use of creation and annihilation operators as the 'creation' and 'absorption' of photons, but this really just means you have changed the energy of the system by a discrete quantity.

    EDIT: I wanted to add as an answer to your first question though. Photons don't have a physical dimension in the sense of occupying space for the same reasons I mention above about them not being physical entities like we would think of a particle which does occupy space.
     
  4. Jan 16, 2010 #3
    when a radiation field interacts with an electron say a higher energy level electron a quantum transition occurs as i understand, the high-level electron emits a discrete quanta of energy, (known as a photon or electromagnetic wave)in the form of an equally distributed sphere expanding outward from the source at the speed of light until it is absorbed at the closest possible point, an electron, (more than one electron?) not instantaneously, but at the fastest possible known rate by another radiation field i.e. a lower energy level electron? now when the electron absorbs the photon, can the transition time of say a radio photon be compared to the transition time of a gamma photon for some difference in length of time?
     
  5. Jan 16, 2010 #4

    Born2bwire

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    When an atom drops down from an excited state and emits radiation, it emits a single photon (or photons if the transition has more than one step). This photon is not distributed across a sphere, but we do assume that the direction of its momentum is random. I do not understand what you mean by the rest of your description. We do not talk about how long it takes for a photon to be absorbed. Usually when we refer to the times of absorption or emmission we are referring to statistical numbers relating to the absorption rates of how long it takes on average for a photon traveling through a medium to be absorbed.

    As for the effects of low power, yes, we do experience granularity in photons. If you pass a wave through a highly absorptive medium say, we can end up with an effective single photon source. That is, we attenuate the wave so much that the transmitted wave has such low power that we will detect single photon events over time. This is how they essentially get the single photon sources for the double slit experiments that are built up using one photon at a time. But it is not correct to think of photons in a spatial distribution. Photons are really the interaction of the fields with our thing (detector, atom, whatever). Whenever an electromagnetic field interacts with something, it does so through a quanta of the field, the photon. This interaction occurs at a point(like) in space and so it is just like that of a particle. But that isn't to say that we consider our light source to create a photon, that the photon travels from A to B, and then interacts with our detector at B. More accurately, we consider our source to create photons at A, and we detect photons at B, and whatever happens between A and B is not something we really know anything about. We still propagate the behavior of light from A to B as a field, though this is not the typical field that you know from classical electrodynamics, it is a quantum field which has other properties.
     
  6. Jan 18, 2010 #5
    If a photon is emitted from an an atomic transition, say the hydrogen 2p->1s, with a natural lifetime of about 1.6 nanoseconds, this represents the ~maximum length of the emitted photon. At a foot per nanosecond for the speed of light, this means about ~1.6 feet long. This also relates to the photon energy uncertainty via the uncertainty principle ΔE·Δt ≥ h-bar/2.

    Noting that E = h-bar·ω, and substuting E in the above equation, we get Δω·Δt ≥ 1/2 (The engineer's equation).

    So if a radio transmitter has a transmitted photon bandwidth Δω, then the length of the broadcast photons have to have a length Δt ≥1/2Δω.

    Longer photons have a lower energy uncertainty. The 14-KeV photons from the cobalt-57 Mossbauer effect source have a Fe-57 excited state decay lifetime ~98 nanoseconds.

    Bob S
     
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