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Are protons visible?

  1. May 12, 2009 #1
    If you were to have a collection of protons in vacuum with no electrons there, would they give off or absorb light and or any other spectrum?
     
  2. jcsd
  3. May 12, 2009 #2

    malawi_glenn

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    edit: I read "photon" LOL, should have my coup of coffee before reading here ;-)
     
    Last edited: May 12, 2009
  4. May 12, 2009 #3
    Any two (or more) charged particles (pRotons included) interacting with each other experience acceleration. This leads to the so-called "bremsstrahlung radiation". You can also check the "Larmor's formula" for the radiated energy. So, the answer is yes.
     
  5. May 12, 2009 #4

    ZapperZ

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    Er.. protons have charge. Make them change their velocities and they'll give off EM radiation!

    Zz.
     
  6. May 12, 2009 #5
    Electrons can be detected by Compton scattering. The cross section is about 2/3 barn. People have Compton scattered visible lasers off of 20 GeV electrons to gamma boost the photons up to several GeV. Same holds for positrons. But the proton Comprton cross section is lower by about 18362 (= 3.37 million) or so.
     
    Last edited: May 12, 2009
  7. May 13, 2009 #6
    Ok if they were accelerating. However if they were stationary how would they behave if a photon was shone at them? Do the protons have energy states akin to the Bohr theory ?
     
  8. May 13, 2009 #7

    ZapperZ

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    How is this any different than ANY other charged objects? A "stationary" electron, muon, proton, etc... (insert any charged elementary particle here) would behave the same way as well, no?

    I'm beginning to not really understand the nature of this question or what you are trying to get at.

    Zz.
     
  9. May 13, 2009 #8
    I think he is merely trying to ask if it would be possible to see a 'clump' of stationary particles, albeit protons, electrons or ... (assuming it was big enough to be observed by a normal microscope or even the naked eye?)
     
  10. May 13, 2009 #9
    I think we've been over this before. He's basically asking what the color of a neutron star would be, if it was only neutrons and they were all stationary. And maybe smaller, like, golf-ball size.
     
  11. May 14, 2009 #10
    Ok then. No reason to get so snappy. Nowhere has anyone ever said or any textbook for that matter that all charged particles interact with photons the same way. Electrons and protons despite being equal and opposite charge seem to be completely different in structure and thus to assume they would absorb and emit visible light the same would be bad on my behalf.
     
  12. May 14, 2009 #11
    Short answer, disregarding Bremsstrahlung radiation and blackbody radiation... no.

    Long answer...

    I would be surprised if the clump had some emission/absorption lines in the spectrum. If there are... then it's some sort of dynamo that is going on that I'm not aware of (or there are some external forces that are causing this.)

    It is my understanding that a clump of protons (or a clump of neutrons like in a neutron star) do NOT interact with photons the way atoms do. You can sort of think of a spring joining the proton and electron in a Hydrogen atom. When a photon is absorbed, the spring is stretched; when it is released, the spring relaxes. Just remember that the spring relaxes due to the fact that the potential energy is being transferred to the actuation of the EM-field--producing a photon. (This happens because it is the more energetically stable configuration of the system and the system is not inhibited.) The stretching and relaxing of the spring is synonymous with the emission/absorption lines you see.

    This "clump" would certainly have a probability to absorb a photon and have its temperature raised... I think--therefore--that a large enough clump would behave as a blackbody. The spectrum of the emitted radiation is determined from the clump's surface temperature.

    Please correct me where I'm wrong.
     
  13. May 14, 2009 #12
    This is correct. But photons interact with free electrons and free protons in the same way. This is Compton scattering. The cross section is inversely proportional to mass-squared, however, so although electrons can be detected using photons easily, protons cannot. The basis for the Compton scattering is charge, and since neutrons don't have any, they will be nearly invisible. except for their radioactivity.
     
  14. May 14, 2009 #13

    malawi_glenn

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    neutrons have constituents which has charge
     
  15. May 14, 2009 #14
    Oh yes, i forgot about this... it is a great example of light behaving as a particle too!

    To revise what I said above, a clump of protons (or a clump of electrons) will interact with the light via Compton scattering. The light that scatters away will have less energy. The rest of the energy is thermal and momentum energy that was imparted to the clump.

    A clump of neutrons? malawi_glenn says they have constituents that have charge... my knowledge on fractional charges and the mechanisms they take part in is next to nill.

    And when you say a clump of neutrons are invisible you mean that photons do not interact with them (ie neutron matter is transparent and non-refracting). This is truly strange that certain neutral (true neutral... no separable charges inside) would not even refract light! In other words, no optical effects at all.

    Please add/remove/correct anything here!
     
  16. May 14, 2009 #15
    Thanks for your responses. And yes ive always wondered how photons interact with neutrons, if they would pass right through as to be invisible.

    Does this give the possibility of large masses in space that can only be detected by gravity and not by anything electromagnetic (at least for a large distance away.)
     
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