Photon interactions

  1. A completely positively ionized material would be practically invisible?
    Normally it interacts with electrons, witch tell them what to do next (reflect, pass trough..), but a completely positively ionized matter wouldn't have any electrons to interact with. Of course they can still interact with the nuclei, but the chances they actually hit the nuclei would be extremely low, like neutrino (?), because of the sheer size of nuclei and because photon doesn't interact with protons electric field.
  2. jcsd
  3. Vanadium 50

    Vanadium 50 18,244
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    I'm not sure what to say other than "no, that's not correct".
  4. Ok, than what would make it visible? What would the photon interact with (and reflect, get absorbed)?
  5. Astronuc

    Staff: Mentor

    A fully ionized material is a plasma, but stable plasma have equal amounts of + nuclei and - electrons in order to maintain charge neutrality. Otherwise, a fully + ionized material would disperse by virtue of the strong Coulomb forces (+ charges would repel). The only way to maintain the mass would be to impose a magnetic field. A large magnetic bottle would be rather visible, so confining a large invisible positively ionized mass would seem rather impractical.

    What is 'they' in "Of couse they can still interact . . . ."? Photons?

    A fully positively (or negatively) charge mass will not remain together. Photons scatter off free (unbound) charges.

    How does one conclude "a completely positively ionized matter wouldn't have any electrons to interact with"? This is rather impractical, since 1) the positively charged matter would disperse without off-setting negative charges, and 2) the free electrons would not be far way, and 3) the excessive + charge would attract electrons from neutral atoms.
  6. It depends on the material density, temperature and the photon wave-length.

    Even normal material is nearly transparent for X-rays, for example.

    Photons do not practically interact with electric fields of any nature. They interact with charges. If you compare the cross sections for scattering from charges of different masses, the heavier charges scatter less (in a single photon - single charge interaction). But there are collective effects at low frequencies so a dense and large system can reflect the incident EM wave even though the material consists only of nuclei.

  7. So it is invisible.
    Se, i'm talking about a perfect situation here - it is well separated from electrons and contained.
  8. Astronuc

    Staff: Mentor

    Perfect - but fictional (i.e. ignoring the real-world physics). Besides, if one were to have a huge positive charge, it then affects the behavior of neutral (normal) matter around it.

    If one is looking for something that is 'non-detectable' as opposed to 'invisible', then it is impractical to have a positively charged matter.

    Without electrons, a positively charged nucleus only scatters photons, and of low energy they would not interact. Nuclear primarily interact with (absorb) gamma rays, i.e. photons of energies in the keV-MeV range. One can look at isomeric transitions, and gammas associated with n-capture to see the typical gamma energies.
  9. Vanadium 50

    Vanadium 50 18,244
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    Let's ignore the very salient point that the sort of material you are describing does not and cannot exist, and go back to your original message:

    None of this is correct. Nuclei are larger than electrons, not smaller. Light does interact with protons.
  10. Yes, of course, i know that! But electron orbits the nuclei and is wave-like, so the photon interacts, because electron is not really a point particle. wave-particle duality anyone?

    The material is not solid, of course, and is confined with electromagnetic fields. The field generators are far enough and strong enough so the large positive mass doesn't steal any electrons.

    "Without electrons, a positively charged nucleus only scatters photons, and of low energy they would not interact."

    Why? What would photons interact with? The chance of hitting a nuclei is extremely small, why then neutrinos can easily fly trough matter with no or minimal interactions?
  11. The three ways photons interact with ordinary matter are the photoelectric effect (including deep core photoejection), Compton scattering, and pair production. With no electrons, the photoelectric effect cross section would be zero. Photons can compton scatter off of protons, but the cross section is lower by about 18362 (proton-to-electron mass ratio squared). The pair production cross section would be nearly the same. The photon cross section for interacting with nuclei would be unchanged. This includes gamma-neutron interactions, for example O16(γ,n)O15, which peaks between 20 and 30 MeV.

    α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ ς σ τ υ φ χ ψ ω
  12. So, for photoelectric effect photon needs a electron - we can rule out this in this case.
    For pair production photon needs to hit a nuclei (with photon energies >1,22 MeV) - we can rule out this for this case. (Because of low chance that photon will actually hit the nuclei)
    For Compton scattering it needs either electron or nuclei - rule out the electron case and the nuclei case for the same reason as pair production, low chance.
    Almost every photon would go through unaffected, so the thing would be invisible to EM radiation.

    Or explain me this: why neutrino can travel through entire planets without hitting anything? Because of the low chance of hitting nuclei.

    Ah, i'm going crazy!
  13. malawi_glenn

    malawi_glenn 4,725
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    particles do not interact with "collision" as if they were tiny balls, they interact via quantum fields.

    The neutrino only interact with the "weak force", which is weaker than the electromagnetic force. The weakest force is gravity.
  14. As I said, if the material is dense and large, and you photon or EM wave is long, the material can reflect the EM wave. It is a collective effect. You may consider the material as a conductor. Conductors reflect long waves. So your material can be visible.

    Yes, the interaction cross section for neutrino is very small so it makes a long way between two successive collisions.

    Last edited: Jun 26, 2009
  15. For Compton scattering, the photon has to transfer some momentum to the proton, so the equation is the same as for Compton scattering off electrons, so the proton mass has to be substituted for the electron mass in the equation. So instead of a cross section of about 0.66 barns, it is about 0.2 microbarns. So it is very small. But as I said in my earlier post, photo-nuclear reactions can have cross sections of 10's of millibarns, depending on specific nucleus. The photonuclear cross section threshold in beryllium is about 1.67 MeV.

    Unlike photons, the neutrino can only interact via the weak force, not electromagnetic.
  16. mheslep

    mheslep 3,326
    Gold Member

    I think the basic Feynman diagram describes the situation. The electron shown is of course charged matter. The photon (green gamma line in the diagram) can either be absorbed or emitted. If a photon detector is inserted so that it views the emitted photon, then the electron is 'seen'. Or conversely if the photon has an external source and is absorbed, the lack of its detection also is evidence of the electron, i.e. it is 'seen'. I suppose the same holds true for nuclei.
  17. But, since when photons interact with electromagnetic field? I mean, you cant bend a photon (change direction of motion) using a powerful electro magnet!
    Yes, the neutrino interacts with week force and because week force has low range, the neutrino has to come close to the nuclei - the range in witch week force acts. But the photon... does it interact with the electromagnetic field around the nuclei? :uhh: I don't think so.
    If thats the case, why cant one bend a photon with powerful electromagnet? ::confused:
  18. malawi_glenn

    malawi_glenn 4,725
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  19. You serious or you just making fun of me? One can bend photons with powerful enough electro magnet? Of course, to do it on macro scale would require extreme amounts of energy and it would be kinda hard to make the magnet, but still - photons can be bent by by a powerful enough electro magnet??

    wow, thats something completely new. I must have died and changed universe i live/d in... :noface:
    Of course i knew taht photon has magnetic and electric field, but... yeah :shy:
    Last edited: Jun 26, 2009
  20. malawi_glenn

    malawi_glenn 4,725
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    Well, the photon electric field is oscillating, so it would just be averaged out.

    Since you are asking this in the particle physics thread, the EM field's quanta are photons, and photons do not couple directly to photons.

    To lowest order, the pair prodcution Feynman diagrams are:

    (Z = the nucleus with atom number Z)

    Now recall that this is not what happens in reality, Feynman diagrams are just mathematical tools and representations of terms in an perturbative expansion of the interaction.
  21. We are discussing photon scattering off of matter without electrons. Your diagram is for electron-electron scattering with a photon mediator.
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