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Why photon never stop?

  1. Feb 1, 2005 #1
    I know photon has no mass, and have energy 0/0, which mean photon can have any energy. But photon can interact with matter. It also can be absorbed by electron. But these interaction, and the fraction never stop photon, nor do photon lose it energy. Why?
     
  2. jcsd
  3. Feb 1, 2005 #2
    A photon cannot be absorbed by an electron, this would violate the consevation of energy. Electrons can only scatter photons, they can only be absorbed by electrons bound to quantum systems(Atoms).

    Have look at the Compton effect.
     
  4. Feb 1, 2005 #3

    mathman

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    Photoelectric effect is essentially an electron (within an atom) absorbing a photon.
     
  5. Feb 1, 2005 #4

    dextercioby

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    I think the most important aspect is still the EMISSION OF THE ELECTRON...

    What else would Einstein's rudimentary theory refer to??And besides,one usually computes [itex] \frac{d\sigma}{d\Omega} [/tex] for the emission...Pages 339-341 of 1994 (copyright) edition of Sakurai will confirm my statements...

    Daniel.
     
  6. Feb 2, 2005 #5

    mathman

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    Photoelectric effect is the result of a bound electron absorbing a photon and thus having enough energy to escape from its atom.
     
  7. Feb 2, 2005 #6

    dextercioby

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    I'm glad you finally figured it out... :smile:

    Daniel.
     
  8. Feb 3, 2005 #7
    How can they have any energy if the amount of energy in the universe is fixed?
     
  9. Feb 3, 2005 #8

    Astronuc

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    Conservation of mass-energy.

    And, we do not know that mass-energy in the universe if fixed. Most theories probably assume that - but we haven't 'seen' the entire universe.

    Energy (mass-energy) is transformed.
     
  10. Feb 3, 2005 #9

    ZapperZ

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    I did not originally intend to jump into this thread, but I think at this point, I should. There are several issues that need to be addressed or corrected here.

    1. If you think about it carefully, it is more UNLIKELY that anything is "stopped", meaning it is actually unusual for anything in the universe to be in the same inertial frame as we are. In spite of our terrestial experience, most things and objects are NOT in our inertial frame. Therefore, what should be asked, really, is why is that object stopped, and not why an objected is not stopped, because the former is the more unusual, highly specific event.

    2. Based on the rudimentary Newton's Law, to stop anything, one needs to make that object interacts with some external force or field. And object with mass m and charge q can interact with either a gravitational field or an electric field. Thus, to "stop" it, one apply either a gravitational force or an electric force in the opposite direction to its direction of motion. You can't do this with photons (no charge and no rest mass). While they do follow the spacetime curvature of gravitational fields, they don't really interact with gravity the same way as we know classically. So you have no mechanism to stop them!

    3. One needs to define the word "stop". If I shoot an object into a blackbox, and the object never leaves that blackbox over a period of time, can I then say that I've "stopped" that object? If so, then one can say that one has stopped light when it never makes it out of an object. So anything that absorbes this light can be thought of as having stopped light.

    4. However, in physics, #3 isn't usually defined as stopping light. This is because photon numbers are usually not conserved. Photons can be converted easily into other forms of energy, so they are really not stopped, but rather destroyed. Physics makes a distinction between these two.

    5. So can light be stopped in the "physics" sense? Yes! What it means here is that the energy and phase coherence of light is preserved somewhere and then, at a later time, retransmitted without any loss. Lene Hau at Harvard has achived this several years ago.[1] For physicists, this is what is meant by stopping light.

    6. In a typical photoelectric effect, the photon is NOT absorbed by an atom or an electron, resulting in the emission of that electron. Remember that a photoelectric experiment is done on SOLIDS, or more specficially, on metals as cathodes. In a metal, the conduction electrons are the ones being emitted in this effect. Conduction electrons DO NOT belong to a particular atom of the metal. The overlapping of the valence shells in a metal causes the formation of "bands" in which the valence electrons loses it's confinment to individual atoms and are now part of the whole bulk solid. So it is incorrect to think that the individual atom's atomic structure are still valid within such energy scale, especially in the usual photoelectric effect experiments.

    Zz.

    [1] C. Liu et al., Nature v.409, p.490 (2001).
     
    Last edited: Feb 3, 2005
  11. Feb 4, 2005 #10
    Thanks a lot. So, what Zapper trying to say is the photon is defined as stopped if it turn into energy. When an electron transfrom from a higher energy state to a lower one, isn't the electron emit photons? Where do the photons come from?
     
  12. Feb 4, 2005 #11
    Why is the speed of light a universal constant ?

    Why is the speed of light a universal constant ?

    This is actually the same question as asking "Why photon never stop? " .
    Particles mainly do not have a properties like "their speed". Their speed may change. Photons have a given speed: c.
    Other massless particles also have the same speed c. This is a necessity for massless particles if SR is to be consistent.

    Seems to me it is a very good question !
     
  13. Feb 4, 2005 #12

    ZapperZ

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    That's not what I said! Pay attention to the #5! You don't just "absorb" a photon's energy and call that stopping light.

    Secondly, electrons do NOT emit photons! It is the whole atom (nucleus + electrons) or the whole solid that emit photons upon a transition! Free electrons cannot emit and absorb photons! There are violations of conservation rules if that happens!

    Thirdly, if photon number is not conserved, then not only can photon just "disappear" upon absorption, it can certainly also "appear" upon creation via an appropriate transition. Why do you think that there are selection rules for an atomic transition, for example? Not only do you need an energy transition to create a photon, but that transition must also be between approriate orbital to conserve angular momentum quantum number.

    Zz.
     
  14. Feb 4, 2005 #13
    What is the source of the photons in a synchroton?
     
  15. Feb 4, 2005 #14

    ZapperZ

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    1. The bending of the path of the electrons

    2. The wiggler

    3. The undulator

    In all three cases, they require the electrons interacting with some field to cause an acceleration. In QFT language, they couple to the virtual photon fields. In none of these do they emit photons by themselves spontaneously. Free electrons, by themselves, cannot "absorb" nor "emit" photons.

    Zz.
     
  16. Feb 4, 2005 #15
    iunno, does electrons have a fixed amount of rest mass energy? or something like that.
    the way i see it is that if it does then electrons emitting photons means that it is losing energy... which i does not make sense to me if it has a fixed amount of energy. no idea, tired and probably not sure what i am writing makes any sense.
     
  17. Feb 4, 2005 #16
    Basically, as soon as photons reach the free electron, they act as a field, so the electron isn't free anymore.

    And of course, a free electron won't emit because it has momentum : uniform rectilinear motion. An electron in a synchrotron is not "free".
     
  18. Feb 4, 2005 #17

    ZapperZ

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    Still, if you draw a feynman diagram, can you still terminate the photon line right at that "free" electron without the assistance of a nearby "massive" particle?

    Now this is different than "beam loading" by bunches of electrons in an EM field. This phenomenon is treated classically because the EM field wavelength is considerably larger than the electron's deBroglie wavelength, and also because it involves a continuous stream of electrons and EM field. One never sees any beam-loading of a single photon by a single free electron.

    Zz.
     
  19. Feb 4, 2005 #18
    I'm trying to reword what your saying according to what I know from EM. I suspect Feynman diagrams and QFT are not necessary to explain this, though I would appreciate recommendations for introductory readings on the subject.
     
  20. Feb 4, 2005 #19

    ZapperZ

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    Yes it does! A "photon" being absorbed or emitted by an electron requires such a treatment. Classical E&M are utterly incapable of dealing with this phenomenon at this scale.

    Zz.
     
  21. Feb 4, 2005 #20
    My reference (Nuclear and Particle Physics Source Book by McGraw) states than an electron under linear acceleration does indeed emit any EM radiation, albeit not that much. What causes this to happen?
     
    Last edited by a moderator: Feb 4, 2005
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